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

K12 Standards

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Structure And Properties Of Matter

Quantifying matter

Measurement: Standard Units

W nauce pomiar to przypisanie wartości liczbowej do cechy, takiej jak masa, rozmiar i temperatura materii.

Każdy pomiar dostarcza dwóch rodzajów informacji. Pierwszym z nich jest liczba – rozmiar lub wielkość pomiaru. Drugi to jednostka – standard porównawczy dla pomiaru.

Tak więc, gdy woda wrze w temperaturze 100 °C, liczba 100 reprezentuje wielkość pomiaru, a °C reprezentuje jednostkę. Razem oddają one względną wielkość właściwości danej materii – temperatury wrzenia.

Bez jednostek liczba może być bez znaczenia lub myląca. Na przykład, jeśli wysokość wieży Eiffla wynosi 300, bez podania jednostek, kontekst tej liczby 300 jest niejasny. Oznacza to, że wieża Eiffla może być stosunkowo wysoka lub bardzo niska, ale bez jednostek do porównania trudno to ocenić.

Czasami podawana jest również trzecia forma informacji, błąd. Dane te wskazują na niepewność pomiaru.

W związku z tym, aby dokładnie mierzyć i raportować różne właściwości materii, opracowano powszechne systemy miar. Międzynarodowy Układ Jednostek Miar lub układ SI jest powszechnie używany przez naukowców.

W układzie SI ustalono jednostki miary dla 7 podstawowych właściwości – długości, masy, czasu, temperatury, prądu elektrycznego, ilości substancji i natężenia światła. Są to tak zwane jednostki standardowe lub jednostki podstawowe. Spośród nich jednostki podstawowe dla pierwszych czterech właściwości są najczęściej stosowane w chemii.

Standardową jednostką długości w układzie SI jest metr, reprezentowany przez symbol jednostki "m". Dłuższe odległości są często podawane w kilometrach lub "km", gdzie 1 kilometr jest równy 1000 metrów. I odwrotnie, krótsze odległości można podawać w centymetrach lub "cm", gdzie 1 centymetr jest równy 0,01 metra; lub milimetrów, "mm", gdzie 1 milimetr jest równy 0,001 metra.

Podobnie standardową jednostką masy w układzie SI jest kilogram, reprezentowany przez symbol jednostki "kg". Kilogram jest równy 1000 gramów.

Temperatura to kolejna podstawowa właściwość, z jednostką SI kelwinów i symbolem jednostki "K" zapisanym wielkimi literami. Jest to wyświetlane bez słowa "stopień" lub symbolu stopnia, aby odróżnić go od stopni Celsjusza i stopni Fahrenheita. Dla przykładu, woda wrze w temperaturze 373.15 K, a normalna temperatura ciała człowieka wynosi około 310 K.

Czwartą najczęściej używaną jednostką podstawową jest druga, jednostka czasu SI, o symbolu jednostki "s". Podczas gdy krótsze interwały czasowe mogą być wyrażone w milisekundach, gdzie 1 milisekunda jest równa 0,001 sekundy, dłuższe interwały czasowe mogą być wyrażone jako megasekundy, gdzie 1 megasekunda jest równa 1 milionowi sekund. Alternatywnie używane są również jednostki inne niż SI, takie jak godziny, dni i lata.

pomiar: jednostki standardowe

Każdy pomiar dostarcza trzech rodzajów informacji: rozmiaru lub wielkości pomiaru (liczba), miary pomiaru (jednostka) oraz wskazania niepewności pomiaru. Chociaż liczba i jednostka są dokładnie opisywane podczas zapisywania wielkości, niepewność jest związana z błędami w wynikach pomiarów.

Liczbę w pomiarze można przedstawić na różne sposoby, w tym w formie dziesiętnej czy w formie notacji naukowej, znanej również jako notacja wykładnicza. Na przykład maksymalna masa startowa samolotu Boeing 777-200ER wynosi 298 000 kilogramów, co można również zapisać jako 2,98 × 105 kg.

Jednostki, takie jak litry, funty i centymetry, są standardami porównawczymi pomiarów. Butelka napoju bezalkoholowego o pojemności 2 litrów zawiera objętość napoju dwukrotnie większą niż dopuszczalna objętość 1 litra. Bez jednostek liczba może być pozbawiona znaczenia lub myląca, a nawet może potencjalnie zagrażać życiu. Załóżmy, że lekarz przepisuje fenobarbital w celu opanowania napadów padaczkowych pacjenta i podaje dawkę „100” bez określenia jednostek. Będzie to nie tylko dezorientujące dla lekarza podającego dawkę, ale może również nieść tragiczne konsekwencje: 100 mg podawane trzy razy dziennie może być skuteczne jako lek przeciwdrgawkowy, ale pojedyncza dawka 100 g to ponad 10-krotność dawki śmiertelnej.

<h4>Międzynarodowy układ jednostek (jednostki SI)</h4>

Jednostki miary siedmiu podstawowych właściwości („jednostek podstawowych”): długości, masy, czasu, temperatury, prądu elektrycznego, ilości substancji i natężenia światła zostały ustalone w porozumieniu międzynarodowym. Nazywa się je Międzynarodowym Układem Jednostek lub Jednostkami SI. Jednostki innych właściwości można wyprowadzić z tych siedmiu jednostek podstawowych. Używane w życiu codziennym jednostki miary są często definiowane jako ułamki lub wielokrotności innych jednostek. Mleko jest zwykle pakowane w pojemniki o pojemności 1 galona (4 kwarty), 1 kwarty (0,25 galona) i jednego kufla (0,5 kwarty). To samo podejście jest stosowane w przypadku jednostek SI, ale te ułamki lub wielokrotności są zawsze potęgami liczby 10. Nazwy wielu jednostek SI opierają się na przedrostku i nazwie jednostki podstawowej. Na przykład długość 1000 metrów jest również nazywana kilometrem, ponieważ przedrostek kilo oznacza „tysiąc”, co w notacji naukowej wynosi 103 (1 kilometr = 1000). m = 103 m).

<h4>Standardowe jednostki SI</h4>

Początkowe jednostki systemu metrycznego, który ostatecznie przekształcił się w system SI, powstały we Francji podczas Rewolucji Francuskiej. Pierwotne standardy metra i kilograma zostały tam przyjęte w 1799 roku, a następnie w innych krajach. Poniżej przedstawiono cztery podstawowe jednostki SI powszechnie stosowane w chemii.

<h4>1. Długość</h4>

Standardową jednostką długości w układzie SI jest metr (m). Metr definiuje się jako odległość, jaką światło pokonuje w próżni w ciągu 1/299 792 458 sekundy. Dłuższe odległości są często podawane w kilometrach (1 km = 1000 m = 103 m), podczas gdy krótsze odległości można podawać w centymetrach (1 cm = 0,01 m = 10-2 m) lub milimetrach (1 mm = 0,001 m = 10-3 m).

<h4>2. Masa</h4>

Standardową jednostką masy w układzie SI jest kilogram (kg). Kilogram definiuje się jako masę obiektu wzorcowego - metalowego cylindra wykonanego ze stopu platyny i irydu o wysokości i średnicy 39 mm. Mówi się, że każdy obiekt o tej samej masie co to odniesienie ma masę 1 kilograma. Gram (g) jest dokładnie równy 1/1000 masy kilograma (10-3 kg).

Terminu „waga” często używa się zamiennie z określeniem „masa”. Jednak te dwie ilości są różne. Podczas gdy masa obiektu mierzy ilość zawartej w nim materii, jego waga mierzy siłę grawitacji wywieraną na materię. Na przykład, gdybyśmy mogli zważyć się na Księżycu, który ma słabszą grawitację niż Ziemia, ważylibyśmy mniej niż na Ziemi. Jednakże masa – ilość materii w naszym ciele – pozostanie taka sama.

<h4>3. Temperatura</h4>

Jednostką temperatury w układzie SI jest kelwin (K), chociaż w układzie SI dozwolony jest również stopień Celsjusza (°C), przy czym do pomiarów stopni Celsjusza używane jest zarówno słowo „stopień”, jak i symbol stopnia. Stopnie Celsjusza mają tę samą wielkość co Kelwiny, ale w obu skalach zera są umieszczane w różnych miejscach. Woda z definicji zamarza w temperaturze 273,15 K (0 ° C) i wrze w temperaturze 373,15 K (100 ° C), a normalna temperatura ciała człowieka wynosi około 310 K (37 ° C). Skala Fahrenheita (°F) to kolejna powszechnie stosowana jednostka pomiaru temperatury. W skali Fahrenheita woda zamarza w temperaturze 32°F i wrze w temperaturze 212°F, a normalna temperatura ciała człowieka wynosi 96°F.

Podczas gdy skale Fahrenheita i Celsjusza pozwalają na ujemne temperatury, skala Kelvina, zwana także skalą absolutną, nie. W skali Kelvina 0 K to najniższa temperatura, znana jako zero absolutne.

Skale temperatur można wzajemnie konwertować przy użyciu następujących wzorów przeliczeniowych:

<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. Czas</h4>

Podstawową jednostką czasu w układzie SI jest sekunda (s). Małe i duże przedziały czasowe można wyrazić za pomocą odpowiednich przedrostków. Na przykład:

<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;">

Alternatywnie można zastosować godziny, dni i lata.

Pomiar: Jednostki standardowe

Każdy pomiar dostarcza trzech rodzajów informacji: rozmiar lub wielkość pomiaru (liczba), standard porównania dla pomiaru...

Education

JoVE Core

Chemia

Jeden

Introduction: Matter and Measurement

Wprowadzenie: Materia i pomiary

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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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physical and chemical properties of matter

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the scientific method

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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...

measurement: derived units

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Texas Essential Knowledge and Skills

High School

Specialised Topic in Science

112.51.c.1.

112.51.c.1.E.

CHEM.1

CHEM.1.E

Biology

BIO.1

BIO.1.E

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

The Bohr Model

Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded ...

The Quantum-Mechanical Model of an Atom

Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Quantum Numbers

It is said that the energy of an electron in an atom is quantized; that is, it can be equal only to certain specific values and can jump from one energy ...

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

electron-configuration-of-multielectron-atoms

Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

Thomson's e/m Experiment

In a beam of charged particles created by a heated cathode, the particles move at different speeds. However, many applications need a beam with uniform ...

Atomic structure
• Evolution of atomic models/theory
• 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

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Ionic Radii

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Ionization Energy

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Electron Affinity

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This process can be either ...

Alkali Metals

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Halogens

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Noble Gases


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Periodic Table
• Properties
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Periodic Table

Types of Chemical Bonds

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Lewis Symbols and the Octet Rule

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Ionic Bonding and Electron Transfer

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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 ...

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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