Name: measurement: si units and conversions
Uploaded: 2020-09-03T13:38:25.000Z
Duration: 3 min 38 s
Channel: JoVE

K12 Standards

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Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Structure And Properties Of Matter

Quantifying matter

Measurement: Standard Units

В науке измерение это присвоение числового значения такой характеристике, как масса, размер и температура вещества. Каждое измерение предоставляет два вида информации. Первый это число размер или величина измерения.Второй единица измерений сравнительный эталон для измерения. Итак, когда вода кипит при температуре 100 градусов Цельсия, число 100 представляет величину измерения, а градус Цельсия представляет собой единицу измерения. Вместе они передают относительную величину рассматриваемого свойства вещества температуру кипения.Без единиц измерения число может быть бессмысленным или сбивать с толку. Например, если высота Эйфелевой башни составляет 300, без указания единиц измерения, контекст этого числа 300 неясен. Это означает, что Эйфелева башня может быть относительно высокой или очень низкой.но без единиц измерения для сравнения это затрудняет оценку. Иногда также предоставляется третья форма информации погрешность. Эти данные указывают на неточность измерения.Таким образом, для точного измерения и определения различных свойств вещества были разработаны общие системы измерения. Международная система единиц или система СИ используется учеными повсеместно. В системе СИ установлены единицы измерения для 7 основных свойств длины, массы, времени, температуры, электрического тока, количества вещества и силы света.Они называются стандартными или базовыми единицами измерения. Из них в химии чаще всего используются базовые единицы измерения для первых четырех свойств. Стандартной единицей длины в системе СИ является метр, представленный символом единицы измерения м"Большие расстояния часто указываются в километрах или км"где 1 километр равен 1000 метрам.И наоборот, более короткие расстояния можно указывать в сантиметрах, см"где 1 сантиметр равен 0, 01 метра;или миллиметрах, мм"где 1 миллиметр равен 0, 001 метра. Точно так же стандартной единицей массы в системе СИ является килограмм, представленный символом единицы измерения кг"Килограмм равен 1000 граммов. Еще одним фундаментальным свойством является температура, для которой единица СИ кельвин, а символ единицы измерения К"в верхнем регистре.Его пишут без слова градус"или символа градуса, чтобы отличить его от градусов Цельсия и градусов Фаренгейта. Например, вода закипает при 373, 15 К, а нормальная температура человеческого тела составляет примерно 310 К.Четвертая наиболее часто используемая базовая единица измерения секунда, единица времени СИ, символ единицы измерения с"В то время как более короткие временные интервалы могут быть выражены в миллисекундах, где 1 миллисекунда равна 0, 001 секунды, более длинные временные интервалы могут быть выражены в мегасекундах, где 1 мегасекунда равна 1 миллиону секунд. В качестве альтернативы также используются единицы, не относящиеся к системе СИ, такие как часы, дни и годы.

Измерения: стандартные единицы

Каждое измерение предоставляет три вида информации: Размер или величина измерения (число), стандарт сравнения для измерения (единица измерения) и показатель неопределенности измерения. В то время как число и единица отображаются явным образом при записи количества, неопределенность является показателем ошибок в результатах измерения. 
 
 Число в измерении может быть представлено различными способами, включая десятичную форму и экспоненциальную форму, с оговоренным числом знаков после запятой. Например, максимальный взлётный вес авиалайнера Boeing 777-200ER составляет 298,000 килограммов, что также можно записать как 2.98× 105 кг.  
 
 Единицы измерения, такие как литры, фунты и сантиметры, являются эталонами для сравнения. 2-литровая бутылка безалкогольного напитка содержит в два раза больше напитка, чем 1 л. Без единиц, число может быть бессмысленным, запутанным или, возможно, опасным для жизни. Предположим, что врач предписывает фенобарбитал, чтобы контролировать приступы пациента’ и указывает дозировку “100” без указания единиц. Это не только сбивает с толку врача, администрирующего дозу, но и может привести к печальным последствиям: 100 мг, даемые три раза в день, могут быть эффективны, как противосудорожный препарат, но одна доза в 100 г более чем в 10 раз превышает смертельную дозу. 
 
 <h4>Международная система единиц (единицы СИ) </h4>
 
 Единицы измерения для семи основных свойств (“базовые единицы”): длина, масса, время, температура, электрический ток, количество вещества и интенсивность света были закреплены международным соглашением. Они называются Международной системой единиц или единицами СИ. Единицы измерения для других свойств могут быть получены из этих семи базовых единиц. Привычные нам единицы измерений часто определяются как фракции или кратные другим единицам. Молоко обычно упаковывается в контейнеры емкостью 1 галлон (4 кварты), 1 кварта (0.25 галлонов) и одна пинта (0.5 кварты). Такой же подход используется с единицами СИ, но эти доли или числа всегда являются степенями 10. Дробные или множественные единицы СИ называются с помощью префикса и имени базовой единицы. Например, длина 1000 метров также называется километром, потому что префикс кило означает “тысячу,” что в научной записи составляет 103 (1 километр = 1000 м = 103 м). 
 
 <h4>Стандартные единицы СИ </h4>
 
 Первоначальные единицы метрической системы, которые в конечном итоге превратились в систему СИ, были установлены во Франции во время Французской революции. Первоначальные стандарты для метра и килограмма были приняты там в 1799 году и, в конечном итоге, былиприняты и другими странами. Ниже приведены четыре базовых единицы СИ, которые обычно используются в химии. 
 
 <h4>1. Длина </h4>
 
 Стандартная единица измерения длины в системе СИ - это метр (м). Измерительный прибор определяется как расстояние, которое свет проходит в вакууме за 1/299,792,458 секунды. Большие расстояния часто даются в километрах (1 км = 1000 м = 103 м), тогда как более короткие расстояния можно сообщить в сантиметрах (1 см = 0.01 м = 10−2 м) или миллиметрах (1 мм = 0.001 м = 10−3 м). 
 
 <h4>2. Масса </h4>
 
 Стандартная единица измерения массы в системе СИ — килограмм (кг). Килограмм определяется массой стандартного объекта — металлического цилиндра, изготовенного из платиноиридиевого сплава высотой и диаметром 39 мм. Любой объект с той же массой, что и этот стандарт, имеет массу 1 килограмм. Грамм (g) точно равен 1/1000 массы килограмма (10−3 кг). 
 
 Термин “вес” часто используется, как взаимозаменяемый с термином “масса.” Однако, эти два количества различаются. В то время как масса объекта измеряет количество вещества внутри него, его вес измеряет силу притяжения, приложенную к этому веществу. Например, если бы мы могли взвешивать себя на Луне, которая обладает более слабой гравитацией, чем Земля, мы бы весили меньше, чем мы весим на Земле. Однако масса – количество вещества в нашем теле – останется прежней. 
 
 <h4>3. Температура </h4>
 
 Единица СИ для измерения температуры – кельвин (K), хотя градусы Цельсия (°C) также разрешены в системе СИ, при этом для измерений в градусах Цельсия используется слово “градус” и символ градуса. Градусы Цельсия являются такой же величиной, как и градусы Кельвина, но две шкалы размещает нули в разных местах. Вода замерзает при температуре 273.15 К (0 °C) и кипит при температуре 373.15 К (100 °C) по определению, а нормальная температура тела человека составляет приблизительно 310 К (37 °C). Шкала Фаренгейта (°F) является еще одной часто используемой единицей измерения температуры. На шкале Фаренгейта вода замерзает при 32 °F и кипит при 212 °F, а нормальная температура тела человека составляет 96 °F.  
 
 В то время как градусы Фаренгейта и Цельсия допускают отрицательные температуры, шкала Кельвина, также называемая абсолютной шкалой, не позволяет этого. По шкале Кельвина, 0 K является самой низкой температурой, которая называется абсолютным нулем.  
 
 Температурные шкалы взаимозаменяемы по следующим формулам преобразования: 
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1010/1010_Equations-01.png" style="height: 40px;" />
 
 <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1010/1010_Equations-02.png" style="height: 40px;" />
 
 <h4>4. Время </h4>
 
 Базовая единица времени в СИ — секунда (с). Малые и большие временные интервалы можно выразить с помощью соответствующих префиксов. Например:  
 
 <img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1010/1010_Equations-03.png" style="height: 40px;" />
 
 <img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1010/1010_Equations-04.png" style="height: 40px;" />
 
 В качестве альтернативы можно использовать часы, дни и годы. 
 
 Этот текст адаптирован из <a href="https://openstax.org/books/chemistry-2e/pages/1-4-measurements">Openstax, Химия 2e, раздел 1.4: Измерения.</a>

Каждое измерение предоставляет три вида информации: размер или величина измерения (число), стандарт сравнения для измерения ...

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Introduction: Matter and Measurement

Введение: Вещество и измерения

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

measurement: si units and conversions

Measurement gives three kinds of information: a number, a unit, and an estimate of uncertainty. The number shows size or magnitude. The unit gives a standard for comparison. The uncertainty reflects possible error in the measurement.

Numbers can be written in more than one way. A value may appear in decimal form or in scientific notation, which is also called exponential notation. For example, the maximum take-off weight of a Boeing 777-200ER airliner is 298,000 kilograms, or 2.98 × 10^5 kg.

Units are essential because they make a number meaningful. Common units such as liters, pounds, and centimeters help people compare measurements. A 2-liter bottle of soft drink contains twice the volume of a 1-liter bottle. Without units, a number can be confusing or even dangerous. A doctor who writes a dose as “100” without units could create a serious mistake, because 100 mg taken three times a day can be effective, while 100 g is more than 10 times the lethal amount.

The International System of Units, or SI units, is based on seven fundamental properties called base units. These properties are length, mass, time, temperature, electric current, amount of substance, and luminous intensity. Other units are derived from these base units. Everyday units often use fractions or multiples of a base unit, and SI units do the same with powers of 10. Prefixes show these multiples. For example, kilo means one thousand, so 1 kilometer equals 1000 meters, or 10^3 m.

The metric system began in France during the French Revolution. The original standards for the meter and kilogram were adopted in 1799 and later used by other countries. In chemistry, the four SI base units used most often are length, mass, temperature, and time.

The SI unit of length is the meter, or m. A meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second. Longer distances are often given in kilometers, while shorter distances are often given in centimeters or millimeters. These units relate directly to the meter: 1 km = 1000 m = 10^3 m, 1 cm = 0.01 m = 10^-2 m, and 1 mm = 0.001 m = 10^-3 m.

The SI unit of mass is the kilogram, or kg. It is defined by a reference object, a metal cylinder made from platinum-iridium alloy with a height and diameter of 39 mm. Any object with the same mass as that reference has a mass of 1 kilogram. The gram, or g, is exactly 1/1000 of a kilogram, which is 10^-3 kg. Weight is often used like mass, but they are not the same. Mass is the amount of matter in an object, while weight is the gravitational force on that matter. That is why a person would weigh less on the moon, where gravity is weaker, but the mass would stay the same.

The SI unit of temperature is the kelvin, or K, although degree Celsius, or °C, is also allowed in SI. Celsius and kelvin have the same size unit, but their zero points are different. Water freezes at 273.15 K, which is 0 °C, and boils at 373.15 K, which is 100 °C. Normal human body temperature is about 310 K, or 37 °C. Fahrenheit, or °F, is another common scale. On that scale, water freezes at 32 °F and boils at 212 °F, and normal body temperature is 96 °F. Celsius and Fahrenheit can show negative temperatures, but Kelvin cannot. The Kelvin scale starts at 0 K, called absolute zero. Temperature scales can be converted using the given formulas.

The SI base unit of time is the second, or s. Small and large time intervals can be written with prefixes, and hours, days, and years are also used. Together, these units help scientists and students describe measurements clearly and compare results with confidence.

Measurement: SI Units and Conversions

scientific laws and theories

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Distinguishing Physical and Chemical Changes

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measurement: standard units

Measurement gives three kinds of information: a number, a unit, and an estimate of uncertainty. The number shows size or magnitude. The unit gives a standard for comparison. The uncertainty reflects possible error in the measurement. Numbers can be written in more than one way. A value may appear in decimal form or in scientific notation, which is also called exponential notation. For example, the maximum take-off weight of a Boeing 777-200ER airliner is 298,000 kilograms, or 2.98 × 10^5 kg. <br...

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SI Units, Density, and Property Types

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Measurement Error: Reading Results Correctly

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Measurement Uncertainty and Significant Figures

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Learn how measurements use numbers, units, and uncertainty, plus SI base units, metric prefixes, and temperature and time conversions.

Measurement: SI Units and Conversions | JoVE K12

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The Pauli Exclusion Principle

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• Electrons
• Electron configurations

Content elaboration: Physical Science included properties and locations of protons, neutrons and electrons, atomic number, mass number, cations and anions, isotopes and the strong nuclear force which holds the nucleus together. In this course, the historical development of the atomic model and the positions of electrons are explored in greater detail. Atomic models are constructed to explain experimental evidence and make predictions. The changes in the atomic model over time exemplify how scientific knowledge changes as new evidence emerges and how technological advancements like electricity extend the boundaries of scientific knowledge. Thompson's study of electrical discharges in cathode-ray tubes led to the discovery of the electron and the development of the plum pudding model of the atom. Rutherford's experiment, in which he bombarded gold foil with a-particles, led to the discovery that most of the atom consists of empty space with a relatively small, positively charged nucleus. Bohr used data from atomic spectra to propose a planetary model of the atom in which electrons orbit the nucleus, like planets around the sun. Later, Schrödinger used the idea that electrons travel in waves to develop a model in which electrons travel randomly in regions of space called orbitals (quantum mechanical model). Based on the quantum mechanical model, it is not possible to predict exactly where electrons are located but there is a region of space surrounding the nucleus in which there is a high probability of finding an electron (electron cloud or orbital). Data from atomic spectra (emission and absorption) gives evidence that electrons can only exist at certain discrete energy levels and not at energies between these levels. Atoms are usually in the ground state where the electrons occupy orbitals with the lowest available energy. However, the atom can become excited when the electrons absorb a photon with the precise amount of energy (indicated by the frequency of the photon) to move to an orbital with higher energy. Any photon without this precise amount of energy will be ignored by the electron. The atom exists in the excited state for a very short amount of time. When an electron drops back down to the lower energy level, it emits a photon that has energy equal to the energy difference between the levels. The amount of energy is indicated by the frequency of the light that is given off and can be measured. Each element has a unique emission and absorption spectrum due to its unique electron configuration and specific electron energy jumps that are possible for that element. Being aware of the quantum mechanical model as the currently accepted model for the atom is important for science literacy as it explains and predicts subatomic interactions, but details should be reserved for more advanced study. Electron energy levels consist of sublevels (s, p, d and f), each with a characteristic number and shape of orbitals. Orbital diagrams and electron configuration can be constructed to show the location of the electrons in an atom using established rules. Valence electrons are responsible for most of the chemical properties of elements. In this course, electron configuration (extended and noble gas notation) and orbital diagrams can be shown for any element in the first three periods. Although the quantum mechanical model of the atom explains the most experimental evidence, other models can still be helpful. Thinking of atoms as indivisible spheres is useful in explaining many physical properties of substances, such as the state (solid, liquid or gas) of a substance at room temperature. Bohr's planetary model is useful to explain and predict periodic trends in the properties of elements.

Atomic structure

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Periodic Table
• Properties
• Trends

Content elaboration: In the Physical Science course, the concept that elements are placed in order of increasing atomic number in the periodic table such that elements with similar properties are placed in the same column is introduced. How the periodic table is divided into groups, families, periods, metals, nonmetals and metalloids is also included and will be revisited here. In this course, with more information about the electron configuration of elements, similarities in the configuration of the valence electrons for a particular group can be observed. The electron configuration of an atom can be determined from the position on the periodic table. The repeating pattern in the electron configuration for elements on the periodic table explains many of the trends in the properties observed. Atomic theory is used to describe and explain trends in properties across periods or down columns including atomic radii, ionic radii, first ionization energies, electronegativities and whether the element is a solid or gas at room temperature. Additional ionization energies, electron affinities and periodic properties of the transition elements, and the lanthanide and actinide series are reserved for more advanced study.

Periodic Table

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

VSEPR Theory and the Basic Shapes

Overview of VSEPR Theory

Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure, including approximate ...

vsepr-theory-and-the-effect-of-lone-pairs

VSEPR Theory and the Effect of Lone Pairs

Effect of Lone Pairs of Electrons&nbsp;on Molecule Geometry

It is important to note that electron-pair geometry around a central atom is not the same ...

Predicting Molecular Geometry

VSEPR Theory for Determination of Electron Pair Geometries

The following procedure uses VSEPR theory to determine the electron pair geometries and the ...

Molecular Shape and Polarity

Dipole Moment of a Molecule

Polar covalent bonds connect two atoms with differing electronegativities, leaving one atom with a partial positive charge ...

Valence Bond Theory

Overview of Valence Bond Theory

Valence bond theory describes a covalent bond as the overlap of half-filled atomic orbitals (each containing a single ...

Hybridization of Atomic Orbitals I

The mathematical expression known as the wave function, &#x3C8;, contains information about each orbital and the wavelike properties of electrons in an ...

Hybridization of Atomic Orbitals II

sp3d and sp3d&nbsp;2 Hybridization

To describe the five bonding orbitals in a trigonal bipyramidal arrangement, we must use five of the valence shell ...

Molecular Orbital Theory I

Overview of Molecular Orbital Theory

Molecular orbital theory describes the distribution of electrons in molecules in the same way as the distribution of ...

Molecular Orbital Theory II

Molecular Orbital Energy Diagrams

The relative energy levels of atomic and molecular orbitals are typically shown in a molecular orbital diagram. For a ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Chemical bonding
• lonic
• Polar/covalent

Content elaboration: Content in the Physical Science course included recognizing that atoms with unpaired electrons tend to form ionic and covalent bonds with other atoms, forming molecules, ionic lattices or network covalent structures. In this course, electron configuration, electronegativity values and energy considerations will be applied to bonding and the properties of materials with different types of bonding. Atoms of many elements are more stable when they are bonded to other atoms. In such cases, as atoms bond, energy is released to the surroundings, resulting in a system with lower energy. An atom's electron configuration, particularly the valence electrons, determines how an atom interacts with other atoms. Molecules, ionic lattices and network covalent structures have different, yet predictable, properties that depend on the identity of the elements and the types of bonds formed. Differences in electronegativity values can be used to predict where a bond fits on the continuum between ionic and covalent bonds. The polarity of a bond depends on the electronegativity difference and the distance between the atoms (bond length). Polar covalent bonds are introduced as an intermediary between ionic and pure covalent bonds. The concept of metallic bonding is also introduced to explain many of the properties of metals (e.g., conductivity). Since most compounds contain multiple bonds, a substance may contain more than one type of bond. Carbon atoms can bond together and with other atoms, especially hydrogen, oxygen, nitrogen and sulfur, to form chains, rings and branching networks that are present in a variety of important compounds, including synthetic polymers, fossil fuels and the large molecules essential to life.

Chemical bonding

Chemical Formulas

A chemical formula presents information about the proportions of atoms constituting a particular chemical compound or molecule, mainly using symbols of ...

ionic-compounds-formulas-and-nomenclature

Ionic Compounds: Formulas and Nomenclature

An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal) to ...

molecular-compounds-formulas-and-nomenclature

Molecular Compounds: Formulas and Nomenclature

Molecular compounds or covalent compounds result when atoms share electrons to form covalent bonds. Since there is no electron transfer, molecular ...

Organic Compounds

All living things are formed mostly of carbon compounds called organic compounds. The category of organic compounds includes both natural and synthetic ...

Classification of Elements and Compounds

Pure substances consist of only one type of matter. A pure substance can be an element or a compound. An element consists of only one type of atom, while ...

Molecular Models

Physical models representing molecular architectures of chemical compounds play essential roles in understanding chemistry. The use of molecular models ...

Representing compounds
• Formula writing
• Nomenclature
• Models and shapes (Lewis structures, ball and stick, molecular geometries)

Content elaboration: Using the periodic table, formulas of ionic compounds containing specific elements can be predicted. This can include ionic compounds made up of elements from groups 1, 2, 17, hydrogen, oxygen and polyatomic ions (given the formula and charge of the polyatomic ion). Given the formula, a compound can be named using conventional systems that include Greek prefixes and Roman numerals where appropriate. Given the name of an ionic or covalent substance, formulas can be written. Many different models can be used to represent compounds including chemical formulas, Lewis structures, and ball and stick models. These models can be used to visualize atoms and molecules and to predict the properties of substances. Each type of representation provides unique information about the compound. Different representations are better suited for particular substances. Lewis structures can be drawn to represent covalent compounds using a simple set of rules and can be combined with valence shell electron pair repulsion (VSEPR) theory to predict the three-dimensional electron pair and molecular geometry of compounds. Lewis structures and molecular geometries will only be constructed for the following combination of elements: hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and the halogens.

Representing compounds

Every measurement provides three kinds of information: the size or magnitude of the measurement (a number), a standard of comparison for the measurement ...

Measurement: Derived Units

The International System of Units or SI system, by international agreement, has fixed measurement units for seven fundamental properties: length, mass, ...

Uncertainty in Measurement: Accuracy and Precision

Scientists typically make repeated measurements of a quantity to ensure the quality of their findings and to evaluate both the precision and the accuracy ...

Uncertainty in Measurement: Reading Instruments

Counting is the type of measurement that is free from uncertainty, provided the number of objects being counted does not change during the process. Such ...

Uncertainty in Measurement: Significant Figures

All the digits in a measurement, including the uncertain last digit, are called significant figures or significant digits. Note that zero may be a ...

Dimensional Analysis

Dimensional analysis, also known as the factor label method, is a versatile approach for mathematical operations. The main principle behind this approach ...

Quantifying matter

Content elaboration: In earlier grades, properties of materials were quantified with measurements that were always associated with some error. In this course, scientific protocols for quantifying the properties of matter accurately and precisely are studied. Using the International System of Units (SI), significant digits or figures, scientific notation, error analysis and dimensional analysis are vital to scientific communication. There are three domains of magnitude in size and time: the macroscopic (human) domain, the cosmic domain and the submicroscopic (atomic and subatomic) domain. Measurements in the cosmic domain and submicroscopic domains require complex instruments and/or procedures. Matter can be quantified in a way that macroscopic properties such as mass can reflect the number of particles present. Elemental samples are a mixture of several isotopes with different masses. The atomic mass of an element is calculated given the mass and relative abundance of each isotope of the element as it exists in nature. Because the mass of an atom is very small, the mole is used to translate between the atomic and macroscopic levels. A mole is equal to the number of atoms in exactly 12 grams of the isotope carbon-12. The mass of one mole of a substance is equal to its molar mass in grams. The molar mass for a substance can be used in conjunction with Avogadro's number and the density of a substance to convert between mass, moles, volume and number of particles of a sample.

Measurement: SI Units and Conversions

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