Answer – One lithium atom contains 4 neutrons.

Explanation:

To find out the number of neutrons an atom has, one must know its atomic number (Z) as well as its mass number (M). 

The atomic number, signified by Z, of an element is the number of protons it contains within its nucleus. (Knowing this value also gives us the number of electrons in one atom of the element, as in the ground state, the number of protons in an atom is equal to the number of electrons).

The mass number of an element is represented by the letter M. This is the total number of nucleons in the atom. Therefore, it is the number of protons plus the number of neutrons in an atom. It is more or less equal to the element’s atomic mass, which is the average mass of an atom, and is expressed in amu (atomic mass units) or daltons (D). [While the atomic mass of an element may run into decimal points, mass numbers are always whole numbers as they represent the number of nucleons, and there cannot be halves or quarters or other such fractional bits of these subatomic particles].

In the case of lithium (Li), according to the periodic table, the atomic number is 3, while the atomic mass is 6.941u.

Therefore, here, Z = 3 and M = 7 (as mass numbers have to be whole numbers, when atomic mass is in decimals, it is rounded off).

The number of neutrons = M – Z

Thus, the number of neutrons in lithium is 7 – 3 = 4.

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