Name: atomic mass and mole calculations
Uploaded: 2023-06-23T15:38:52.000Z
Duration: 1 min 25 s
Channel: JoVE

La masse atomique d'un élément, parfois appelée masse atomique, est approximativement égale à la somme du nombre de protons et de neutrons. Celle-ci est exprimée en unités de masse atomique, abrégée en amu, ou Daltons, et est représentée près de son symbole chimique sur le tableau périodique.
Mais, la plupart des éléments ont plusieurs isotopes qui diffèrent par le nombre de neutrons et donc par leurs masses.
Pour de tels éléments, le poids atomique est la masse moyenne de tous les isotopes d’un élément, pondérée en fonction de leur abondance relative ou du pourcentage de chaque isotope présent dans la nature.
Par exemple, le chlore a deux isotopes naturels. La masse de chlore-35 est de 34,97 amu et celle de chlore-37 est de 36,97 amu.
Lorsqu’on multiplie par leurs abondances relatives, 75,77 % pour le chlore 35 et 24,23 % pour le chlore 37, et qu’on additionne, on obtient la masse atomique finale de 35,45 amu.
Étant donné que le chlore 35 est plus abondant dans la nature que le chlore 37, le poids atomique du chlore est proche de 35 amu.

masse atomique

Les protons et les neutrons ont approximativement la même masse, soit environ 1,67 × 10-24 grammes. Les scientifiques définissent arbitrairement cette quantité de masse comme une unité de masse atomique (uma) ou un Dalton. Les électrons ont une masse beaucoup plus petite que les protons, pesant seulement 9,11 × 10-28 grammes, soit environ 1/1800 d'une unité de masse atomique. Par conséquent, ils ne contribuent pas beaucoup à la masse atomique globale d'un élément. Cela signifie que lorsqu'on considère la masse atomique, il est courant d'ignorer la masse des électrons et de calculer la masse de l'atome en fonction du nombre de protons et de neutrons seuls. Cependant, étant donné que la plupart des éléments naturellement présents sont composés d'isotopes, il est important de prendre en compte la masse et l'abondance naturelle des isotopes lors de la détermination de la masse atomique de ces éléments. Dans de tels cas, la masse atomique est calculée en additionnant les masses de tous les isotopes de l'élément, chacun multiplié par son abondance fractionnaire naturelle. Les chimistes utilisent souvent l'unité "mole" pour déterminer le nombre d'atomes d'un composé participant à une réaction chimique. Une mole d'un élément correspond à sa masse atomique en grammes, tandis qu'une mole d'un composé correspond à la somme des masses atomiques de ses composants, appelée masse moléculaire. Un exemple souvent utilisé est le calcul d'une mole de glucose avec la formule chimique C6H12O6. La masse atomique du carbone (C) est de 12,011 grammes et il y a six carbones dans le glucose, ce qui donne une masse atomique totale de 72,066 grammes. En effectuant les mêmes calculs pour l'hydrogène (H) et l'oxygène (O), la masse moléculaire du glucose est de 180,156 grammes. Ce texte est partiellement adapté de <a href="https://openstax.org/books/biology-ap-courses/pages/2-1-atoms-isotopes-ions-and-molecules-the-building-blocks?query=atomic%20mass&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-idp140338797859808">Openstax, Biology for AP</a><a href="https://openstax.org/books/biology-ap-courses/pages/2-1-atoms-isotopes-ions-and-molecules-the-building-blocks?query=atomic%20mass&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-idp140338797859808">® </a><a href="https://openstax.org/books/biology-ap-courses/pages/2-1-atoms-isotopes-ions-and-molecules-the-building-blocks?query=atomic%20mass&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-idp140338797859808">courses. Section 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks</a> et <a href="https://openstax.org/books/anatomy-and-physiology-2e/pages/2-4-inorganic-compounds-essential-to-human-functioning?query=mole&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-id1218107">Openstax, Anatomy and Physiology 2e, Section 2.4 Inorganic compounds Essential to Human Functioning</a>

Masse atomique

Les protons et les neutrons ont à peu près la même masse, environ 1,67 × 10-24 grammes. Les scientifiques définissent arbitrairement cette quantité de masse comme une masse atomique ...

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Atomic Weight

atomic mass and mole calculations

Atomic mass and mole calculations help explain how much matter is in atoms and compounds. Protons and neutrons have about the same mass, about 1.67 × 10 -24 grams. Scientists define that mass as one atomic mass unit, or amu, also called one Dalton.

Electrons are much lighter than protons. They weigh only 9.11 × 10 -28 grams, or about 1/1800 of an atomic mass unit. Because of that small mass, electrons do not add much to an element’s overall atomic mass. When chemists calculate atomic mass, they usually ignore electrons and use only the protons and neutrons.

Many naturally occurring elements are made of isotopes, which are atoms of the same element with different masses. For these elements, atomic mass must include each isotope’s mass and its natural fractional abundance. Chemists find this by adding the masses of all the isotopes after multiplying each one by how common it is.

Chemists also use the mole to count atoms and compounds in reactions. A mole of an element is its atomic weight in grams. A mole of a compound is the sum of the atomic weights of its parts, and that total is called the molecular weight. Glucose, C6H12O6, is a common example. Carbon has an atomic weight of 12.011 grams, and six carbons in glucose give 72.066 grams. Doing the same for hydrogen and oxygen gives glucose a molecular weight of 180.156 grams.

Atomic Mass and Mole Calculations

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Atomic Mass and Mole Calculations

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