Atomic Masses and Composition of Nucleus | Physics Class 12 - NEET PDF Download

Nucleus

The entire positive charge and nearly the entire mass of atom is concentrated in a very small space called the nucleus of an atom.

Atomic Nucleus

The nucleus consists of protons and neutrons. They are called nucleons.

Terms Related to Nucleus:

(i) Atomic Number: The number of protons in the nucleus of an atom of the element is called atomic number (Z) of the element.

(ii) Mass Number: The total number of protons and neutrons present inside the nucleus of an atom of the element is called mass number (A) of the element.

(iii) Nuclear Size: The radius of the nucleus R ∝ A^1/3

⇒ R = R_o A^1/3

where, R_o = 1.1 * 10^-15 m is an empirical constant.

(iv) Nuclear Density: Nuclear density is independent of mass number and therefore same for all nuclei.

ρ = mass of nucleus / volume of nucleus ⇒ ρ = 3m / 4π R³_o

where, m = average mass of a nucleon.

(v) Atomic Mass Unit: It is defined as 1 / 12th the mass of carbon nucleus.

It is abbreviated as amu and often denoted by u. Thus

1 amu = 1.992678 * 10^-26 / 12 kg

= 1.6 * 10^-27 kg = 931 Me V

Isotopes:

The atoms of an element having same atomic number but different mass numbers are called isotopes.

e.g., ₁H¹, ₁H², ₁H³ are isotopes of hydrogen.

Fig: Isotopes of hydrogen

Isobars:

The atoms of different elements having same mass numbers but different atomic numbers, are called isobars.

e.g., ₁H³, ₂He³ and ₁₀Na²², ₁₀Ne²² are isobars.

Isotones:

The atoms of different elements having different atomic numbers and different mass numbers but having same number of neutrons, are called isotones.

e.g., ₁H³, ₂He⁴ and ₆C¹⁴, ₈O¹⁶ are isobars.

Isomers:

Atoms having the same mass number and the same atomic number but different radioactive properties are called isomers.

Nuclear Force:

The force acting inside the nucleus or acting between nucleons is called nuclear force.

Nuclear forces are the strongest forces in nature.

• It is a very short range attractive force.
• It is non-central. non-conservative force.
• It is neither gravitational nor electrostatic force.
• It is independent of charge.
• It is 100 times that of electrostatic force and 10³⁸ times that of gravitational force.

According to the Yukawa, the nuclear force acts between the nucleon due to continuous exchange of meson particles.

Mass Defect:

The difference between the sum of masses of all nucleons (M) mass of the nucleus (m) is called mass defect.

Mass Defect (Δm) = M – m = [Zm_p + (A – Z)m_n – m_n]

Nuclear Binding Energy:

The minimum energy required to separate the nucleons up to an infinite distance from the nucleus, is called nuclear binding energy.

Nuclear binding energy per nucleon = Nuclear binding energy / Total number of nucleons

Binding energy, E_b = [Zm_p + (A – Z) m_n – m_N]c²

Packing Fraction (P):

p = (Exact nuclear mass) – (Mass number) / Mass number

= M – A / M

The larger the value of packing friction. greater is the stability of the nucleus.

[The nuclei containing even number of protons and even number of neutrons are most stable.

The nuclei containing odd number of protons and odd number of neutrons are most unstable.]