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The mass number of an element is equal to the number of protons plus the average number of neutrons. The atomic mass or atomic weight is another name for the mass number.

Every element has a different atomic number and mass than the others. The elements are grouped in a certain arrangement based on their atomic numbers.

Atoms are the basic building blocks of all substances. The nucleus, which is made up of protons and neutrons, is the atom's core. Neutrons do not have an electric charge, but protons do. Electrons, which are smaller and lighter negatively charged particles, surround the nucleus.

Lithium (Li) is an atom that consists of 3 protons and 4 neutrons.

Because the mass number is equal to the sum of protons and neutrons, the mass number of lithium may be computed using the formula:

$\text{Mass number}=\text{Number of protons}+\text{Number of neutrons}$

It is given that

Number of protons = 3;

Number of neutrons = 4.

Now, substitute the values into the formula:

$$\begin{gathered} \text{Mass number =}3\text{+}4 \\ =7 \end{gathered}$$

Thus, the mass number of lithium is 7.