Atoms
All matter is made of An atom is the simplest particle of an element that can exist on its own in a stable environment..
The word “atom” is derived from the ancient Greek word atomos – “something that cannot be divided”.
The modern atomic model
Today, scientists use an atomic model that has a central, positively-charged nucleus that contains:
- positively-charged Subatomic particle with a positive charge and a relative mass of 1.
- neutral neutrons
Negatively-charged electrons orbit the nucleus in shells.
An atom’s nucleus gives it most of its mass.
Protons, neutrons and electrons - Higher tier only
The The relative mass is the number of times heavier a particle is, compared to another., relative charge and location in the atom for each Objects found in atoms, including protons, neutrons and electrons. is given in the table.
| Subatomic particle | Relative mass | Relative charge | Location in atom |
| Proton | 1 | +1 | In nucleus |
| Neutron | 1 | 0 | In nucleus |
| Electron | \(\frac{1}{1840}\) | -1 | In shells around nucleus |
| Subatomic particle | Proton |
|---|---|
| Relative mass | 1 |
| Relative charge | +1 |
| Location in atom | In nucleus |
| Subatomic particle | Neutron |
|---|---|
| Relative mass | 1 |
| Relative charge | 0 |
| Location in atom | In nucleus |
| Subatomic particle | Electron |
|---|---|
| Relative mass | \(\frac{1}{1840}\) |
| Relative charge | -1 |
| Location in atom | In shells around nucleus |
Atomic number
An atom’s The number of protons in the nucleus of an atom. Also called the proton number. is the number of protons in its nucleus.
Atomic number = number of protons
Mass number
An atom’s The number of protons and neutrons found in the nucleus of an atom. is the total number of protons and neutrons.
Mass number = number of protons + number of neutrons
Electrical charge
An An atom is the simplest particle of an element that can exist on its own in a stable environment. has no overall charge, because it has an equal number of protons and electrons (Higher tier only).
When we write the symbol for an atom, we can place its mass number at the top left and its atomic number at the bottom left.
You can use these numbers to calculate the number of protons, neutrons and electrons in an atom.
Number of protons = number of electrons = atomic number
Number of neutrons = mass number – atomic number
Question
The atomic number of a sodium atom is 11 and its mass number is 23. Calculate the number of protons, neutrons and electrons it contains.
Number of protons = 11
Number of electrons = 11
Number of neutrons (mass number - atomic number) = 23 - 11 = 12