Atomic Masses and Composition of Nucleus

In every atom, the positive charge and mass are densely concentrated at the centre of the atom, forming its nucleus.
The overall dimensions of a nucleus are much smaller than those of an atom.
Experiments on the scattering of alpha particles showed that the radius of a nucleus is smaller than the radius of an atom by a factor of about 10⁴.
Therefore, the volume of a nucleus is about 10⁻¹² times the volume of an atom, and an atom is almost empty.
If an atom were enlarged to the size of a classroom, the nucleus would be the size of a pinhead.
Nevertheless, the nucleus contains more than 99.9% of the mass of an atom.

Accurate measurement of atomic masses is carried out with a mass spectrometer.
These measurements revealed the existence of different types of atoms of the same element that have the same chemical properties but different masses.
Such observations led to the identification of isotopes.

In 1932, James Chadwick verified the existence of neutral matter inside the nucleus.
He observed neutral radiation when beryllium nuclei were bombarded with alpha particles and explained this radiation as a stream of neutral particles called neutrons.
Chadwick later received the 1935 Nobel Prize in Physics for this discovery.

A different mass unit used for expressing atomic masses is the atomic mass unit (u), defined as one-twelfth of the mass of a carbon-12 atom, called atomic mass unit.

Mathematically: 1 u = \[\frac{\text{mass of one }^{12}\mathrm{C~atom}}{12}\] = 1.660539 × 10⁻²⁷ kg

Atomic species of the same element differing in mass are called isotopes.

The nucleus of the lightest atom of hydrogen, which has a relative abundance of 99.985%, is called the proton.

A new type of neutral particle whose mass is very nearly the same as that of a proton is called neutron.

A proton or a neutron is called a nucleon.

All nuclides with the same mass number are called isobars.

Nuclides with the same neutron number but different atomic number are called isotones.

m_p ​= 1.00727 u = 1.67262 × 10⁻²⁷ kg

m_n​ = 1.00866 u = 1.6749 × 10⁻²⁷ kg

A = Z + N

Where:

• Z = atomic number = number of protons.
• N = neutron number = number of neutrons.
• A = mass number = total number of protons and neutrons.

• Kilograms are not convenient for atomic masses because atomic masses are very small.
• So, atomic masses are measured in atomic mass units.
• Chlorine has an atomic mass of 35.46 u, which is not a whole number.
• This happens because chlorine is a mixture of isotopes.
• Chlorine has two isotopes with masses 34.98 u and 36.98 u.
• Their abundances are 75.4% and 24.6%, respectively.
• Hydrogen has three isotopes with masses 1.0078 u, 2.0141 u and 3.0160 u.
• The other two isotopes of hydrogen are deuterium and tritium.
• Tritium is unstable and is produced artificially.
• The positive charge of the nucleus is due to protons.
• The number of protons in the nucleus is equal to the atomic number Z.
• Deuterium and tritium must contain neutral matter in addition to one proton.
• This neutral matter was identified as a neutron.
• A free neutron is unstable, but a neutron is stable inside the nucleus.
• Nuclides are represented as \[\frac {A}{Z}\]​X.
• Isotopes have the same number of protons but a different number of neutrons.
• Isotopes have identical chemical behaviour because their electronic structure is the same.