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

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.1

Atomic Mass

La masa de un solo átomo es muy pequeña, por lo que no resulta práctico medirla en gramos o kilogramos. La masa de los átomos y las moléculas se mide en una unidad llamada Dalton, abreviado Da"o unidad de masa atómica, abreviado amu"o a veces solo u"La masa de un átomo en unidades de masa atómica es aproximadamente igual a la suma del número de protones y neutrones. Sin embargo, la mayoría de los elementos tienen varios isótopos naturales.Cada uno de estos isótopos tiene un número distinto de neutrones y, por lo tanto, una masa diferente. Para cada elemento, la masa promedio se calcula sumando las masas de los isótopos del elemento, cada una multiplicada por su abundancia fraccional de origen natural en la Tierra. La masa y la abundancia relativa de distintos isótopos de un elemento se pueden determinar por medio de la espectrometría de masas.De esta forma, los isótopos se separan en función de su masa y el espectro de masas de una muestra indica la abundancia relativa de los isótopos. Estos valores se usan para calcular la masa atómica promedio que aparece en la tabla periódica. El espectro de masas de una muestra representativa de boro indica que 19, 9%de la misma es boro-10 y tiene una masa de 10, 0129 unidades de masa atómica.El resto es boro-11 y tiene una masa de 11, 0093 unidades de masa atómica. Las masas del boro-10 y el boro-11 se multiplican individualmente por sus abundancias fraccionales, es decir, 19, 9 entre 100 para el boro-10 y el remanente, 80, 1 entre 100, para el boro-11. Estos valores luego se suman para hallar la masa atómica promedio del boro.

masa atómica

Los átomos — y los protones, neutrones y electrones que los componen — son extremadamente pequeños. Por ejemplo, un átomo de carbono pesa menos de 2 &times 10−23 g. Al describir las propiedades de objetos diminutos como los átomos, utilizamos unidades de medida apropiadamente pequeñas, como la unidad de masa atómica (uma). La uma fue originalmente definida en base al hidrógeno, el elemento más ligero, más tarde en términos de oxígeno. Desde 1961, se ha definido con respecto al isótopo más abundante del carbono, cuyos átomos tienen masas asignadas de exactamente 12 uma. Así, una uma es exactamente 1/12 de la masa de un átomo de carbono-12: 1 uma = 1,6605 × 10−24 g. El Dalton (Da) y la unidad de masa atómica unificada (u) son unidades alternativas equivalentes a la uma.  
 
 Debido a que cada protón y cada neutrón contribuyen aproximadamente con una uma a la masa de un átomo y cada electrón contribuye con mucho menos, la masa atómica de un solo átomo es aproximadamente igual a su número de masa (una suma de números enteros de protones y neutrones en el átomo). Por ejemplo, el número de masa de un solo átomo de nitrógeno es 14 (7 protones + 7 neutrones). Sin embargo, las masas medias de átomos de la mayoría de los elementos no son números enteros porque la mayoría de los elementos existen naturalmente como mezclas de dos o más isótopos. Los isótopos son átomos del mismo elemento con el mismo número de protones pero un número diferente de neutrones. La masa de un elemento que se muestra en una tabla periódica o se enumera en una tabla de masas atómicas es una masa media ponderada de todos los isótopos presentes en una muestra natural de ese elemento. La masa media es igual a la suma de la masa de cada isótopo individual multiplicada por su abundancia fraccional.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/807/807_Equations-01.png" style="height: 40px;" />
 
 Por ejemplo, el elemento cloro (número atómico 17) está compuesto por dos isótopos: 35Cl y cloro 37Cl. Alrededor del 75,78% de todos los átomos de cloro son 35Cl con una masa de 34,969 uma, y el 24,22% restante son 37Cl con una masa de 36,966 uma. Se calcula que la masa atómica promedio del cloro es:
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/807/807_Equations-02.png" style="height: 40px;" />
 
 Es importante entender que ningún átomo de cloro pesa exactamente 35,45 uma; este valor es la masa media de todos los átomos de cloro y los átomos de cloro individuales pesan aproximadamente 35 uma o 37 uma. Además, debido a que el cloro natural contiene más átomos de 35Cl que átomos de 37Cl, la masa media ponderada del cloro está más cerca de 35 uma que de 37 uma.
 
 La ocurrencia y abundancia natural de isótopos puede determinarse experimentalmente utilizando un instrumento llamado espectrómetro de masas. La espectrometría de masas (EM) se utiliza ampliamente en química, medicina forense, ciencias ambientales y muchos otros campos para analizar y ayudar a identificar las sustancias en una muestra de material. En un espectrómetro de masas típico, la muestra se vaporiza y se expone a un haz de electrones de alta energía que hace que los átomos o moléculas de la muestra se carguen eléctricamente, normalmente al perder uno o más electrones. Estos cationes pasan entonces a través de un campo magnético variable que desvía la trayectoria de cada catión hasta el punto que depende tanto de su masa como de su carga. Finalmente, se detectan los iones y se realiza un gráfico del número relativo de iones generados frente a sus relaciones masa-carga, un espectro de masas. La altura de cada característica vertical o pico en un espectro de masas es proporcional a la fracción de cationes con la relación masa-carga especificada. Desde su primer uso durante el desarrollo de la teoría atómica moderna, la EM ha evolucionado hasta convertirse en una poderosa herramienta para el análisis químico en una amplia gama de aplicaciones.
 
 Texto adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism">Openstax Química 2e, Sección 2.3: Estructura atómica y simbolismo.</a>

Masa Atómica

Los átomos, y los protones, neutrones y electrones que los componen, son extremadamente pequeños. Por ejemplo, un átomo de carbono pesa menos de 2 ...

Education

Educación

JoVE Core

Química

Atoms and Elements

Átomos y elementos

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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

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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...

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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

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ohios-learning-standards-and-model-curriculum

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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

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CHEM.11

CHEM.11.E

Alabama Course of Study Science

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9-12.PS.3.4.d

Next Generation Science Standards

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HS-ESS1-3

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SC.912.P.8

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Isotopes and Atomic Mass

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Average Atomic Mass and Isotopes

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