Question

The atomic number of Lithium is 3, its mass number is 7.
a.) How many protons and neutrons are present in a lithium atom ?
b.) Draw a structure of lithium atoms.

Answer 1

Hint: The atomic number of an atom is equal to the number of protons or number of electrons in a neutral atom.
Atomic number = Number of protons = Number of electrons
The mass number is the sum of the number of protons and neutrons in an atom.
Mass number = number of protons + number of neutrons

Complete step by step answer:
We have studied that the atomic number of an atom is equal to the number of protons or number of electrons in a neutral atom.
Atomic number = Number of protons = Number of electrons
Further, the mass number of an atom is equal to the sum of the total number of nucleons.
So, what are these nucleons? The nucleons are the particles present in the nucleus of an atom. So, mass number is the sum of the number of protons and neutrons in an atom.
Mass number = number of protons + number of neutrons
We have been given,
Atomic number of Lithium = 3
So, the number of protons = atomic number = 3
Mass number = 7
Mass number = number of protons + number of neutrons
number of protons + number of neutrons = 7
3 + number of neutrons = 7
number of neutrons = 7 - 3
number of neutrons = 4
So, a.) number of protons = 3
Number of neutrons = 4
b.) The structure of Lithium can be drawn as -

Note: The lithium ion has 3 atomic numbers. The number of electrons is three in a neutral lithium atom. The first shell 1s can accommodate only two electrons. The third electron will be in second shell 2s. The nucleons will remain bound by internuclear forces.