Atomic Structure – In the previous classes you have studied that the smallest part of an element is called an atom. You also know that the atom consists of two parts and its central part is called a nucleus. The nucleus consists of protons and neutrons. The proton is a positively charged particle. So the nucleus carries a positive charge.

The electrons revolve round the nucleus in nearly circular orbits. Since an atom is a neutral particle, so the number of electrons is it is equal to the number of protons. The mass of the proton & neutron is nearly the same i.e., 1.67×10-27 kg. Since the protons and neutrons exist inside the nucleus so these are called nucleons.

atomic structure

Atomic Structure (10th –Physics-Chapter-18) Lesson 1

A nucleon is nearly 1836 times heavier than an electron. So the mass of an atom is nearly equal to the total sum of the masses of all the protons and neutrons present is the nucleus of that atom. The total number of protons and neutrons in the nucleus is called the Atomic Mass Number and is denoted by the letter Z. since the number of protons in an atom of different elements is different so the number of protons in the nucleus indicate the charge on that nucleus.

The number of protons in a nucleus is called the charge number or Atomic number and is denoted by the letter A. similarly the number of neutrons in the nucleus is denoted by the letter N.

If atomic number of an atom is A and its Atomic Mass Number is Z, then this atom is represented by the symbol A Z x which is called a nuclide. For example there is only one proton in the nucleus of hydrogen atom so its atomic number is 1 and its atomic mass number is also 1. Hence it is denoted by 1/1 H.


The experiments have shown that in the atoms of some elements, the number of protons is the same, due to which the chemical properties are the same, but the total number of nucleons in their nuclei is different. Such elements are called isotopes.

Isotopes are those elements which have the same atomic numbers but their atomic mass numbers are different.