Subatomic Particles

The atomic number (or proton number) is the number of protons in the nucleus of an atom and has the symbol Z
- The atomic number is also equal to the number of electrons that are present in a neutral atom of an element
- E.g. the atomic number of lithium is 3
  - This means that a neutral lithium atom has 3 protons and, therefore, also has 3 electrons

The chemical symbol of a general element

The mass (nucleon) and atomic (proton) number are given for each element in the Periodic Table

The mass number (or nucleon number) is the total number of protons + neutrons in the nucleus of an atom, and has the symbol A
- Protons and neutrons are also called nucleons, because they are found in the nucleus
The number of neutrons can be calculated by:

Number of neutrons = mass number - atomic number

How to work out protons, neutrons and electrons

An atom is neutral and has no overall charge
Ions on the other hand have either gained or lost electrons causing them to become charged
The number of subatomic particles in atoms and ions can be determined given their:
- Atomic (proton) number
- Mass (nucleon) number
- Charge

Protons

The atomic number of an atom and ion determines which element it is
Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
- E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)
The number of protons equals the atomic (proton) number
The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:

Mass number = number of protons + number of neutrons

Number of protons = mass number - number of neutrons

Worked example

Determine the number of protons of the following ions and atoms:

Mg²⁺ ion
Carbon atom
An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

The atomic number of a magnesium atom is 12
This means that the number of protons in the nucleus of a magnesium atom is 12
Therefore, the number of protons in a Mg²⁺ ion is also 12
- Remember: The number of protons does not change when an ion is formed

Answer 2:

The atomic number of a carbon atom is 6
This means that a carbon atom has 6 protons in its nucleus

Answer 3:

Use the formula to calculate the number of protons
- Number of protons = mass number - number of neutrons
- Number of protons = 63 - 34
- Number of protons = 29

Therefore, element X is copper

Electrons

An atom is neutral and therefore has the same number of protons and electrons
Ions have a different number of electrons to the number of protons, depending on their charge
- A positively charged ion has lost electrons and, therefore, has fewer electrons than protons
- A negatively charged ion has gained electrons and, therefore, has more electrons than protons

Worked example

Determine the number of protons of the following ions and atoms:

Mg²⁺ ion
Carbon atom
An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

The atomic number of a magnesium atom is 12
This means that the number of protons in the nucleus of a magnesium atom is 12
However, the 2+ charge in Mg²⁺ ion indicates that it has lost two electrons
Therefore, an Mg²⁺ ion only has 10 electrons

Answer 2:

The atomic number of a carbon atom is 6
This means that a carbon atom has 6 protons in its nucleus
Since there is no overall charge on a neutral carbon atom, there must be 6 negative electrons to balance the charge of the 6 positive protons

Answer 3:

Use the formula to calculate the number of protons
- Number of protons = mass number - number of neutrons
- Number of protons = 63 - 34
- Number of protons = 29

Since element X is neutral, there must be 29 negative electrons to balance the charge of 29 positive protons

Neutrons

The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:

Number of neutrons = mass number (A) - number of protons (Z)

Worked example

Determine the number of protons of the following ions and atoms:

Mg²⁺ ion
Carbon atom
An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

The atomic number of a magnesium atom is 12 and its mass number is 24
- Number of neutrons = mass number (A) - number of protons (Z)
- Number of neutrons = 24 - 12
- Number of neutrons = 12
The Mg²⁺ ion has 12 neutrons in its nucleus

Answer 2:

The atomic number of a carbon atom is 6 and its mass number is 12
- Number of neutrons = mass number (A) - number of protons (Z)
- Number of neutrons = 12 - 6
- Number of neutrons = 6
The carbon atom has 6 neutrons in its nucleus

Answer 3:

The atomic number of an element X atom is 29 and its mass number is 63
- Number of neutrons = mass number (A) - number of protons (Z)
- Number of neutrons = 63 - 29
- Number of neutrons = 34
The neutral atom of element X has 34 neutrons in its nucleus

Subatomic Particles (SL IB Chemistry)

Revision Note