Name: average atomic mass and isotopes
Uploaded: 2020-09-03T13:38:25.000Z
Duration: 1 min 52 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.1

Atomic Mass

De massa van een enkel atoom is erg klein, dus het is niet praktisch om de massa in gram of kilogram te meten. De massa van atomen en moleculen wordt gemeten in eenheden die Dalton worden genoemd, afgekort Da, of atomaire massa-eenheden, afgekort amu of soms alleen u. De massa van een atoom in amu is ongeveer gelijk aan de som van het aantal protonen en het aantal neutronen;de meeste elementen hebben echter verschillende natuurlijk voorkomende isotopen.Elk van deze isotopen heeft een ander aantal neutronen en dus een andere massa. Voor elk element wordt zijn gemiddelde massa berekend door de massa's van de isotopen van het element op te tellen, elk vermenigvuldigd met zijn natuurlijke fractionele overvloed op aarde. De massa en relatieve abundantie van verschillende isotopen van een element kan worden bepaald met behulp van massaspectrometrie.De isotopen worden gescheiden door massa en het massaspectrum van een monster toont de relatieve overvloed van de isotopen. Deze waarden worden gebruikt om de gemiddelde atomaire massa te berekenen die op het periodiek systeem verschijnt. Het massaspectrum van een representatief boriummonster laat zien dat 19, 9%van dit monster borium-10 is met een massa van 10, 0129 amu.De rest is borium-11 met een massa van 11.0093 amu. De massa van borium-10 en borium-11 worden elk vermenigvuldigd met hun fractionele abundantie, 19, 9 per 100 voor borium-10 en 80, 1 per 100, de rest, voor borium-11. Deze waarden worden vervolgens opgeteld om de gemiddelde atoommassa van borium te vinden.

atoommassa

Atomen — en de protonen, neutronen en elektronen waaruit ze bestaan — zijn extreem klein. Een koolstofatoom weegt bijvoorbeeld minder dan 2 × 10−23 g. Bij het beschrijven van de eigenschappen van kleine objecten zoals atomen gebruiken we op passende wijze kleine maateenheden, zoals de atomaire massa-eenheid (amu). De amu werd oorspronkelijk gedefinieerd op basis van waterstof, het lichtste element, en later in termen van zuurstof. Sinds 1961 is de definitie gebaseerd op de meest voorkomende isotoop van koolstof, waarvan de atomen een massa van precies 12 amu krijgen toegewezen. Eén amu is dus precies 1/12 van de massa van één koolstof-12-atoom: 1 amu = 1,6605 × 10−24 g. De Dalton (Da) en de verenigde atomaire massa-eenheid (u) zijn alternatieve eenheden die gelijkwaardig zijn aan de amu.  Omdat elk proton en elk neutron ongeveer één amu bijdragen aan de massa van een atoom, en elk elektron veel minder bijdraagt, is de atoommassa van een enkel atoom ongeveer gelijk aan zijn massagetal (een geheel getal van protonen en neutronen in het atoom). Het massagetal van een enkel stikstofatoom is bijvoorbeeld 14 (7 protonen + 7 neutronen). Echter, de gemiddelde atoommassa van een element (zoals weergegeven in het periodiek systeem) is geen heel getal. Omdat de meeste elementen van nature voorkomen als mengsels van twee of meer isotopen. Isotopen zijn atomen van hetzelfde element met hetzelfde protongetal maar een verschillend aantal neutronen. De massa van een element weergegeven in een periodiek systeem of vermeld in een tabel met atoommassa's is een gewogen gemiddelde massa van alle isotopen die aanwezig zijn in een natuurlijk voorkomend monster van dat element. De gemiddelde massa is gelijk aan de som van de massa van elke individuele isotoop, vermenigvuldigd met zijn fractionele overvloed. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/807/807_Equations-01.png" style="height: 40px;"> Het element chloor (atoomnummer 17) bestaat bijvoorbeeld uit twee isotopen: 35Cl en chloor 37Cl. Ongeveer 75,78% van alle chlooratomen is 35Cl met een massa van 34,969 amu, en de resterende 24,22% is 37Cl met een massa van 36,966 amu. De gemiddelde atoommassa voor chloor wordt als volgt berekend: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/807/807_Equations-02.png" style="height: 40px;"> Het is belangrijk om te begrijpen dat geen enkel chlooratoom precies 35,45 amu weegt; Deze waarde is de gemiddelde massa van alle chlooratomen, en individuele chlooratomen wegen ongeveer 35 amu of 37 amu. Omdat natuurlijk voorkomend chloor meer 35Cl-atomen dan 37Cl-atomen bevat, ligt de gewogen gemiddelde massa van chloor dichter bij 35 amu dan bij 37 amu. Het voorkomen en de natuurlijke overvloed van isotopen kunnen experimenteel worden bepaald met behulp van een instrument dat massaspectrometer wordt genoemd. Massaspectrometrie (MS) wordt veel gebruikt in de chemie, het forensisch onderzoek, de geneeskunde, de milieuwetenschappen en vele andere gebieden om de stoffen in een materiaalmonster te analyseren en te helpen identificeren. In een typische massaspectrometer wordt het monster verdampt en blootgesteld aan een hoogenergetische elektronenbundel die ervoor zorgt dat de atomen of moleculen van het monster elektrisch geladen raken, meestal door het verlies van een of meer elektronen. Deze kationen gaan vervolgens door een variabel magnetisch veld dat het pad van elk kation afbuigt in de mate dat dit afhangt van zowel de massa als de lading. Ten slotte worden de ionen gedetecteerd en wordt er een grafiek gemaakt van het relatieve aantal gegenereerde ionen versus hun massa-ladingsverhouding, een massaspectrum. De hoogte van elk verticaal kenmerk of elke piek in een massaspectrum is evenredig met de fractie kationen met de gespecificeerde massa-ladingsverhouding. Sinds het eerste gebruik ervan tijdens de ontwikkeling van de moderne atoomtheorie, is MS geëvolueerd tot een krachtig hulpmiddel voor chemische analyse in een breed scala aan toepassingen. Text adapted from <a href="https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism">Openstax Chemistry 2e, Section 2.3: Atomic Structure and Symbolism.</a>

Atoommassa

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Education

JoVE Core

Scheikunde

Eén

Atoms and Elements

Atomen en elementen

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

average atomic mass and isotopes

Average atomic mass explains why many elements on the periodic table have decimal values. Atoms are extremely small, so scientists use the atomic mass unit, or amu, to measure them. A carbon atom weighs less than 2 × 10−23 g, and the amu is defined from carbon-12. One amu is exactly 1/12 the mass of one carbon-12 atom, which is 1.6605 × 10−24 g. The dalton (Da) and the unified atomic mass unit (u) mean the same thing as the amu.

For a single atom, atomic mass is usually close to its mass number. The mass number is the total of protons and neutrons in the atom. Each proton and neutron contributes about 1 amu, while each electron adds far less. For example, a nitrogen atom has a mass number of 14 because it has 7 protons and 7 neutrons.

Most elements do not have a whole-number atomic mass because they exist as mixtures of isotopes. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The atomic mass listed for an element in the periodic table is a weighted average of the masses of all naturally occurring isotopes. This average is found by multiplying each isotope’s mass by its fractional abundance and then adding the results.

Chlorine is a good example. It has two isotopes, chlorine-35 and chlorine-37. About 75.78% of chlorine atoms are chlorine-35 with a mass of 34.969 amu, and 24.22% are chlorine-37 with a mass of 36.966 amu. The average atomic mass of chlorine is 35.45 amu. No single chlorine atom has exactly that mass. Individual chlorine atoms are closer to 35 amu or 37 amu, depending on the isotope.

Scientists can measure isotopes and their natural abundance with a mass spectrometer. Mass spectrometry, or MS, is used in chemistry, forensics, medicine, environmental science, and other fields to identify substances in a sample. In a mass spectrometer, the sample is vaporized and hit with a high-energy electron beam. This makes the atoms or molecules become charged, usually by losing one or more electrons. The cations then move through a magnetic field, which bends their paths based on mass and charge. The detector records them and creates a mass spectrum, a graph of ion count versus mass-to-charge ratio. Each peak shows how many ions have a given mass-to-charge ratio. Since its early use in modern atomic theory, MS has become a powerful tool for chemical analysis.

Average Atomic Mass and Isotopes

the atomic theory of matter

Dalton’s Laws and Early Atomic Ideas

Dalton’s atomic theory explains matter with atoms, elements, and compounds. It also shows how chemical changes follow simple laws. This idea gave chemistry a microscopic model for what can be seen in the lab. The earliest recorded ideas about matter came from ancient Greek philosophers. Leucippus and Democritus said all matter was made of tiny, finite particles called atomos, meaning “indivisible.” Aristotle and others later argued that matter was made from four elements: fire, earth, air, and...

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Atomic structure is built from three subatomic particles: protons, neutrons, and electrons. Atoms make up all matter, and these particles explain an atom’s mass and charge. The electron was first identified at the end of the 19th century by J.J. Thomson. He used a cathode ray tube, a sealed glass tube with almost all the air removed and two metal electrodes. When high voltage was applied, a visible beam called a cathode ray formed. The beam bent toward positive charge and away from negative...

elements: chemical symbols and isotopes

Reading Element Symbols and Isotope Notation

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atomic mass

Average atomic mass explains why many elements on the periodic table have decimal values. Atoms are extremely small, so scientists use the atomic mass unit, or amu, to measure them. A carbon atom weighs less than 2 × 10−23 g, and the amu is defined from carbon-12. One amu is exactly 1/12 the mass of one carbon-12 atom, which is 1.6605 × 10−24 g. The dalton (Da) and the unified atomic mass unit (u) mean the same thing as the amu. For a single atom, atomic mass is usually close to its mass...

molar mass

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The mole links mass and the number of particles in a substance. It gives chemists a standard way to count atoms and molecules in samples that are far too small to see. It also helps show why substances with the same kinds of atoms can still have very different properties, like water, H2O, and hydrogen peroxide, H2O2. The mole is an amount unit, like a pair or a dozen. One mole contains 6.02214076 × 10^23 discrete entities, such as atoms or molecules. This number is usually rounded to 6.022 ×...

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Learn how isotopes, fractional abundance, and mass spectrometry explain average atomic mass values on the periodic table for high school chemistry.

Average Atomic Mass and Isotopes | JoVE K12

chemistry

JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

PS.Chm.4.3

PS.Chm.4.4

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Physical Science

Study Of Matter

Atoms

Structure And Properties Of Matter

Quantifying matter

Physical Geology

Minerals

Atoms and elements

Virginia  Standards of Learning

HIgh School

CH.2

CH.2.a

CH.4

CH.4.a

PS.4

PS.4.a

Texas Essential Knowledge and Skills

High School

CHEM.8

CHEM.8.A

CHEM.11

CHEM.11.E

Alabama Course of Study Science

Matter and Its Interactions

9-12.C.1.1

9-12.PS.3.4.d

Next Generation Science Standards

Earth and Space Science

Earth’s Place in the Universe

HS-ESS1-3

Florida Department of Education

SC.912.P.8

SC.912.P.8.4

Chemistry and Physics

IPC.7

IPC.7.A

Georgia Standards of Excellence

SC1.

Isotopes and Atomic Mass

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Analyze the structure of atoms and isotopes.

Average Atomic Mass and Isotopes

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