Table of Contents
The Atom
The atom is the building block of the universe – the smallest particle of a chemical element that can exist! So small that it cannot be broken down any further by chemical reactions.
Now, let’s explore the structure of the atom…
An atom is mostly empty space! At its centre is a small and dense positively charged nucleus. At some significant distance, this nucleus is surrounded by a cloud of negatively charged subatomic particles called electrons.
P.S. The diagram is definitely not to scale!
Subatomic Particles
An atom can be split into smaller constituents, called subatomic particles: protons, neutrons and electrons. It is the number of electrons that determines an atom’s chemical properties. The table below details the relative mass and charge of different subatomic particles.
| Relative Mass | Relative Charge | |
|---|---|---|
|
Proton |
1 amu |
+1 |
|
Neutron |
1 amu |
0 |
|
Electron |
1/1840 amu |
-1 |
Atomic Mass Unit (amu) is the unit used for relative mass in the above table.
The atomic mass unit is equal to one twelfth of the mass of a single atom of a carbon-12.
Each atom of carbon-12 contains 6 protons, 6 neutrons and 6 electrons, and has a relative mass of 12 amu.
Atomic Number
Atomic Number
Mass Number
Mass Number
The Nucleus
Situated at the centre of an atom, the nucleus comprises approximately 99.9% of its mass. Yet, it only makes up 0.01% of the atom’s diameter, hence it is very little in relation to the atom overall!
Protons and neutrons make up this atomic nucleus. While neutrons are neutral subatomic particles, protons are positively charged subatomic particles. Something called the strong force is what holds the protons and neutrons together inside the nucleus. This strong force is so great that it can even overcome the repulsion between the positively charged protons, to hold the nucleus together! Without the strong force, the positively charged protons would repel one another like two north poles on a magnet, tearing the nucleus apart.
Isotopes
Atoms with the same number of protons but varying number of neutrons are known as isotopes.
There are stable isotopes that do not emit radiation and unstable isotopes, known as radioisotopes, that emit radiation.
Hydrogen Isotopes
For example, hydrogen (H) naturally exists as three different isotopes. Hydrogen-1 (1H) and hydrogen-2 (2H, also known as deuterium) are both stable and do not emit radiation. Hydrogen-3 (3H, also known as tritium) is unstable; it is radioactive and emits beta radiation.
Hydrogen, 1H
1H
has an atomic number of 1:
1 Proton
0 Neutrons
1 Electron
Deuterium, 2H
2H
commonly known as Deuterium (D),
has an atomic number of 2:
1 Proton
1 Neutron
1 Electron
Tritium, 3H
3H
commonly known as Tritium (T),
has an atomic number of 3:
1 Proton
2 Neutrons
1 Electron
Ellie Field
