Table of Contents

What is Atomic Mass ? Atomic Mass Definition

Atomic Mass Definition :-

According to Dalton’s atomic theory, each element has its characteristic atomic mass. Dalton’s theory can easily explain the law of constant proportion, hence, inspired by it the scientists advanced towards measuring the atomic mass and relative atomic mass, using the combination rules. Practically the mass of atom is due to the protons and neutrons present in it. They are also known as nucleon because of their presence in the nucleus. Thus the entire mass of an atom is in its nucleus. The atomic mass of oxygen, which is, 16amu (atomic mass unit) is because of the presence of 8 protons and 8 neutrons in it. Similarly the mass of nitrogen atom is 14 amu (indicating 7 neutron + 7 protons). “The total number of nucleons (number of proton + neutron) present in the nucleus of an atom is known as its mass number.” Mass number is represented by ‘A’. “The number of protons present in an atom is known as the atomic number.” It is denoted by ‘Z’. It can be related to Aas under :

The basic particles of atom are electron, proton and neutron.
The negatively charged particles in the atom are electrons.
The numeric value of the charge on electron and proton is the same but their sign is opposite.
James Chadwick discovered neutrons.
There are 6.022×10²³ particles in one mole. This is known as the Avogadro number.
The NTP volume of 1 mole of a gas is 22.4 litres.
The formula to determine the maximum number of electrons in a shell is 2n² .
When the atomic number is the same but mass number is different they are known as Isotopes.
Isobars are elements having different atomic number and the same mass number.
There are three isotopes of hydrogen, Protium, Deutirium and Tritium.

1. The Plum Pudding Model of atom was given by:
(a) Neil Bohr
(b) Thomson
(c) Ernest Rutherford
(d) Goldstein

Answer⇒ { B}

2. The discoverer of neutron was :
(a) C.V. Raman
(b) Rutherford
(c) J.J. Thomson
(d) James Chadwick

Answer ⇒ { D}

3. The size of atom is :
(a) 10^-6 cm
(b) 10^-15 cm
(c) 10^-2 cm
(d) 10^-8 cm

Answer ⇒ {D }

4. The number of neutrons in the Deutirium Isotope of hydrogen is /are :
(a) one
(b) Two
(c) Three
(d) Not even one

Answer ⇒ {A }