2.1.1 Constituents of the atom

1. Protons and neutrons are found in the nucleus at the centre of an atom and are collectively known as nucleons.
2. Electrons orbit the nucleus in regions called shells or energy levels. Each of these particles has unique properties, which can be measured in SI units (Standard International units) and relative units. A summary of these properties is presented below:

Particle	Charge (Coulombs)	Relative Charge	Mass (kg)	Relative Mass	Specific Charge (C/kg)
Proton	$+1.6 \times 10^{-19} C$	+1	$1.67 \times 10^{-27} kg$	1	$9.58 \times 10^7 C/kg$
Neutron	0	0	$1.67 \times 10^{-27} kg$	1	0
Electron	$-1.6 \times 10^{-19} C$	-1	$9.11 \times 10^{-31} kg$	0.0005	$1.76 \times 10^{11} C/kg$

Specific Charge

Specific charge is a measure of the charge-to-mass ratio of a particle. It is calculated by dividing the particle's charge by its mass.

$$\text{Specific charge} = \frac{\text{Charge}}{\text{Mass}}$$

Atomic Notation and Definitions

• Proton Number ($Z$): The number of protons in an atom. This defines the element's identity (e.g., Hydrogen has ( $Z = 1$ ), Carbon has ( $Z = 6$).
• Nucleon Number (A): The total number of protons and neutrons in the nucleus. It represents the mass number and gives an approximation of the atom's mass.
• Nuclide Notation: A specific way of representing atoms using the form $^A_Z X$, where:
  • $A$ = Nucleon number,
  • $Z$ = Proton number,
  • $X$ = Element symbol.

Isotopes and Carbon Dating

Isotopes are variants of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron count leads to different mass numbers for isotopes of the same element.