Nucleus Structure - Practice Questions & MCQ

An atom is the basic unit of matter. 

The atom consists of a central core called ‘nucleus’ and the electrons revolve around it in nearly circular orbits as shown in the below figure.

The nucleus of an atom consists of neutrons and protons, collectively referred to as nucleons. The neutron carries no electrical charge and has a mass slightly larger than that of a proton.

Constituents of the nucleus (Nucleons)-

(a) Protons:-

Mass of proton, $\mathrm{m}_{\mathrm{p}}=1.6726 \times 10^{-27} \mathrm{~kg}$
Charge of proton $=1.602 \times 10^{-19} \mathrm{C}$
(b) Neutron:-

Mass of neutron, $\mathrm{m}_{\mathrm{n}}=1.6749 \times 10^{-27} \mathrm{~kg}$

The proton is the main part of an atom and carries a positive charge. The number of protons and neutrons is usually the same except in the case of the hydrogen atom which contains a single proton that exists on its own.

The number of protons in a nucleus (called the atomic number or proton number) is represented by the symbol Z.  

The number of neutrons  (neutron number)  is represented by N.

The total number of neutrons and protons in a nucleus is called its mass number and it is represented by N.

And we have $A=Z+N$.

Different types of nuclei-
- Isotopes- The atoms of an element having the same atomic number but a different mass numbers are
called isotopes. All isotopes have the same chemical properties.
The isotopes of some elements are the following

$
{ }_1 H^1{ }_{, 1} H^2{ }_{, 1} H^3 \quad \text { and }{ }_8 O^{16}{ }_{, 8} O^{17}{ }_{, 8} O^{18}
$

• Isobars: The nuclei which have the same mass number (A) but a different atomic number (Z) are called isobars. Isobars occupy different positions in the periodic table so all isobars have different chemical properties.

Some of the examples of isobars are ${ }_1 H^3$ and ${ }_2 \mathrm{He}^3, \quad{ }_6 \mathrm{C}^{14}$ and ${ }_7 N^{14}{ }_{88} O^{17}$ and ${ }_9 F^{17}$
- Isotones: The nuclei having an equal number of neutrons are called isotones. For them both the atomic number $(Z)$ and mass number $(A)$ are different, but the value of $(N=A-Z)$ is the same.

Some examples are ${ }_4 \mathrm{Be}^9$ and ${ }_5 \mathrm{~B}^{10}{ }_{, 6} \mathrm{C}^{13}$ and ${ }_7 N^{14}{ }_{, 8} \mathrm{O}^{18}$ and ${ }_9 \mathrm{~F}^{19}$
Size of the nucleus-
- Nuclear radius - The radius r of the nucleus depends upon the atomic mass A of the element as

$
R=R_0 \cdot A^{1 / 3} \text { where } \mathrm{R}_0=\text { Constant }=1.2 \mathrm{fm} \text { and } \mathrm{A}=\text { Mass number of the }
$

nucleus
- Nuclear volume: The volume of the nucleus is given by

$
V=\frac{4}{3} \pi r^3=\frac{4}{3} R_0^3 A
$

• Nuclear density: Mass per unit volume of a nucleus is called nuclear density. And it is given as

$\begin{aligned} & \rho=\frac{M}{V} \\ & \rho=\frac{(A) \cdot m_p}{\frac{4 \pi}{3} R^3} \\ & \rho=\frac{A \cdot m_p}{\frac{4 \pi}{3} \cdot R_0^3 \cdot A}=\frac{3 m_p}{4 \pi R_0^3}\end{aligned}$

Density is constant for all the nucleus. It is independent of size and mass numbers.