Atomic Mass of Elements - Overview, Examples, Comparison, FAQs

Why Do Elements Have Different Atomic Masses?

Elements are described by their atomic mass. An atom's mass is the combined mass of all its protons. The unit is known as the unified atomic mass unit and is represented by 'u'.

As a measure of the average atomic mass of a mixture of isotopes in a given sample of an element, the standard atomic weight is used. Each element has a different set of protons and neutrons in its nucleus, hence different atomic masses are observed. Below is the modern periodic table of elements, depicting atomic number and atomic mass of the elements.

Apart from atomic mass, trends for various other physical quantities such as ionisation enthalpy, electronegativity, etc., can be analysed from the modern periodic table.

Atomic Mass of First 30 Elements

The following table contains a list of the atomic mass of the first 30 elements ordered by atomic number and atomic mass.

The above table shows the atomic mass of all elements and the atomic number of elements from 1 to 30. Such details can help in knowing the various physical quantities of elements.

To calculate a molecule's molecular mass, add the mass of each of its constituent atoms. There are several methods of finding the atomic mass of an element, but the simplest is to look it up on the periodic table.

Atomic Mass vs Atomic Number

Learn how elements differ from one another in terms of their atomic number and atomic mass now.

Also read :

Some Solved Examples

Question 1: What is the mass of one Helium atom in $u$ if its mass in $g$ is $6.646629 \times 10^{-24} g ?$
1) 4
2) 3.998
3) (correct) 4.0026
4) 4.568

Solution:

As we learnt in

Relation between amu and Gram -
${ }^{12} \mathrm{C}$ is assigned a mass of exactly 12 atomic mass unit (amu).
$1 \mathrm{amu}=1.66056 \times 10^{-24} \mathrm{~g}$
mass in $u=\frac{6.646629 \times 10^{-24}}{1.66056 \times 10^{-24}}=4.0026 u$
Hence, the answer is (4.0026u).

Question 2: The weight of a molecule of the compound $C_{60} H_{22}$ is W :

$W=X \times 10^{-21}$
What will be the $X$ ?
(Response should be up to only two decimal digits like $X=78.78$ or 0.09 ).
1) $1.09$

2) (correct) $1.23$

3) $5.025$

4) $16.02$

Solution:

Relation between amu and Gram -
${ }^{12} \mathrm{C}$ is assigned a mass of exactly 12 atomic mass units (amu).

$1 \mathrm{amu}=1.66056 \times 10^{-24} \mathrm{~g}$
Weight of molecule $=$ molecular weight of $\mathrm{C}_{60} \mathrm{H}_{22} /$ Avogadro's number

$\begin{aligned}
& =>742 / 6.023 \times 10^{23} \\
& =>1.23 \times 10^{-21} \mathrm{~g}
\end{aligned}$
Hence, the answer is option (2).

Question 3: Amu is a measure of

1) mass of one mole of an atom

2) (correct) mass of one atom of that element

3) mass of one electron

4) None of these

Solution:

As we learned from

amu is used to measure the mass of individual atoms.

Hence, the answer is option (2).

Question 4: The mass of one atom of Oxygen will be
1) $2.65 \times 10^{-24} \mathrm{~g}$

2) $2.65 \times 10^{-22} \mathrm{~g}$

3) (correct) $2.65 \times 10^{-23} \mathrm{~g}$

4) $5.3 \times 10^{-23} \mathrm{~g}$

Solution:

As we learned from

Mass of oxygen $=16 \mathrm{~g} / \mathrm{mol}$. One mole of Carbon is $6.022 \times 10^{23}$ atoms of Carbon (Avogadro number), mass of one atom $=$ mass of one mole of atom $/ 6.022 \times 10^{23}$

$=16 / 6.022 \times 10^{23}=2.65 \times 10^{-23} \mathrm{~g}$

Hence, the answer is option (3).