Question

The atomic mass of an element depends upon the _____.

Answer 1

Answer: mass and relative abundance of each isotope of that element

Explanation:-

For example:

If an element X has two isotopes with following masses ad abundance:

Mass of isotope 1 = 267.8 amu

% abundance of isotope 1 = 87.8% = 0.878

Mass of isotope 2 = 269.9 amu

% abundance of isotope 2 = 12.2% = 0.122

Formula used for average atomic mass of an element :

$\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}\times {{\text { fractional abundance}})$

$\text{ Average atomic mass of an element X}=\sum[(267.8\times 0.878)+(269.9\times 0.122)]$

$\text{ Average atomic mass of an element X}=268.06amu$

Answer 2

The atomic mass of an element depends upon the mass and relative abundance of each isotope of that element

Further explanation

Atoms of the same element do not always have the same mass called Isotopes

Atoms in nature have different masses, so the atomic mass is calculated based on the average mass of all atoms in nature. So that the isotopes of some of these atoms, atomic mass is the average mass of all isotopes in nature

$\large {\boxed {\bold{mass ~ average ~ atom ~ X ~ = ~ \frac {mass ~ isotope ~ 1 + mass ~ isotope ~ 2} {total ~ whole ~ atom ~ X}}}$

In determining atomic mass, the standard is the mass of 1 carbon-12 atom whose mass is 1 amu (amu = atomic mass unit)

So the obtained atomic mass is the atomic mass relative to the carbon-12 atom

$\large {\boxed {\bold{ ~ atomic\:mass\:X = \frac {mass ~ average ~ 1 ~ atom ~ X} {\frac {1} {12}. mass ~ 1 ~ atom ~ C-12}}}$

• Example

Isotope Cl-35 75.53%, 34.969 amu

Isotope Cl-37 24.47%, 36.966 amu

$\displaystyle Relative\:atomic\:mass\:Cl=\frac {(75,53.34,969)+(24,47.36,966)} {100}$

Average atomic mass Cl = 35.45 amu

So atomic mass of Cl is:

$\large {Ar ~ Cl ~ = ~ \dfrac {mass ~ average ~ 1 ~ atom ~ Cl} {\frac {1} {12}. mass ~ 1 ~ atom ~ C-12}}$

$\large {Ar ~ Cl ~ = ~ \dfrac {35,45 ~ amu} {\frac {1} {12}. 12 ~ amu}}$

So atomic mass Cl = 35.45

Whereas the Atomic Number (Z) shows the number of protons in an atomic nucleus from an element. If the atom is neutral then the number of protons will equal the number of electrons. So the atomic number can also indicate the number of electrons.

So atomic number = number of protons = number of electrons

Mass Number (A) is the sum of protons and neutrons

Mass Number (A) = Number of protons + Number of Neutrons

So there is a difference between the mass number and atomic mass

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Keywords: mass number, atomic mass, amu, isotope