Structure of Atom

Dalton's concept of the indivisibility of atom was completely discredited by a series of experimental evidence obtained by scientists. As a result of extensive studies, it was concluded that atoms are made up of three subatomic particles: electrons, protons, and neutrons.

Sub- atomic particles of atom

The mass of sub-atomic particles is very small. The mass of these particles cannot be expressed in gram. Therefore, their masses are expressed in a different unit called atomic mass unit. This atomic mass unit is written as a. m. u. in short.

Proton and electron both have an electric charge. This charge is measured in a unit called Coulomb in short.

1. Electron

Characteristics of an electron:An electron is a negatively charged particle found in the atom of all the elements. The electron are located outside the nucleus in an atom. The only hydrogen atom contains one electron, all other contain more than one electron.

Mass: The mass of an electron is about 1/1836 the mass of hydrogen atom. The mass of hydrogen atom is 1 a. m. u, hence the relatives mass of an electron = 1/ 1836 a. m. u. The atomic mass is a unit of mass used to express relative atomic masses. It is equal to 9.109 x 10 ^-31 kg.

Charges: The electron is found to carry 1.602 x 10^-19 coulomb of negative charge. This quantity of charge has been shown to be the smallest negative charge carried by any particle. Thus, the charge carried by an electron is taken to be a unit negative charge (-1).

2. Proton

Characteristics of a protonA proton is a positively charged particle found in the atoms of all the elements. They are located in the nucleus of an atom. Only hydrogen atoms contain one proton in its nucleus, an atom of all other elements contains more than one proton.

a. A proton is a positively charged particle present in the nucleus of atoms of all elements.

b. Mass: The mass of a proton is practically equal to the mass of the hydrogen atom i. e. 1836 times that of an electron. The relative mass of an element is equal to 1.005757 a. m. u. which is taken to be equal to 1 a. m. u. The absolute mass of a proton is 1.672 x 10 ^-27kg.

c. Charge: The electron on a proton is equal in magnitude and opposite in sign to that of an electron. The charge carried by a proton is equal to 1.602 x 10^-19 coulomb which is taken to be one unit of positive charge (+). Thus, a proton is said to carry a unit positive charge.

3. Neutron

Characteristics of neutronsThe neutron is a neutral particle found in the nucleus of an atom. Atoms of all elements contain neutrons except hydrogen atom which does not contain any neutron. Thus, the fundamental particle not present in a hydrogen atom is neutron. A hydrogen atom contains only one proton and one electron.

a. Charge: A neutron carries no charge, i. e. it is a neutral particle.

b. Mass: The mass of the neutron is nearly equal to that of the proton. Thus, the relative mass of a neutron = 1 a. m. u. and its absolute mass = 1.6 x 10^-27kg.