5.1.2 The Nucleus

Composition of the Nucleus

The nucleus is the dense, central core of an atom. It contains almost all of the atom's mass. The nucleus is composed of two types of subatomic particles: protons and neutrons.

Protons

Protons are positively charged particles located within the nucleus. Each proton has a charge of +1e (where 'e' is the elementary charge, approximately 1.602 x 10^-19 Coulombs).

• Symbol: p or
• Charge: +1e
• Relative mass: approximately 1 atomic mass unit (amu)
• The number of protons in an atom determines the element. This number is known as the atomic number (Z).

Neutrons

Neutrons are electrically neutral particles found in the nucleus. They have no charge.

• Symbol: n
• Charge: 0
• Relative mass: approximately 1 atomic mass unit (amu)
• Neutrons contribute to the mass of the atom and help to stabilize the nucleus.

Table Summarizing Nucleon Properties

Particle	Symbol	Charge	Relative Mass (amu)
Proton	p	+1	1
Neutron	n	0	1

Atomic Mass Unit (amu)

The atomic mass unit (amu) is a unit of mass used to describe the mass of atoms and molecules. It is defined as 1/12 the mass of a carbon-12 atom.

$$1 \, \text{amu} \approx 1.66 \times 10^{-27} \, \text{kg}$$

Number of Nucleons

The total number of nucleons in the nucleus is the sum of the number of protons and the number of neutrons. This number is often denoted by 'A' and is called the mass number.

$$A = \text{Number of protons} + \text{Number of neutrons}$$

$$A = Z + N$$

Where:

• A = Mass number
• Z = Atomic number (number of protons)
• N = Number of neutrons

Isotopes

Atoms of the same element (same number of protons) may have different numbers of neutrons. These variations are called isotopes.

For example, Carbon-12 ($^{12}C$) and Carbon-14 ($^{14}C$) are isotopes of carbon. They both have 6 protons but differ in the number of neutrons (6 and 8 respectively).