Subatomic Particles (DP IB Chemistry): Revision Note
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
Each element is represented using its chemical symbol, with two key numbers:
Mass number (A) — the total number of protons and neutrons (also called nucleons)
Atomic number (Z) — the number of protons in the nucleus
Protons and neutrons are both nucleons because they are located 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
A neutral atom has no overall charge because it has equal numbers of protons and electrons
Ions form when electrons are gained (forming a negative ion) or lost (forming a positive ion), creating a charged species
You can work out the number of subatomic particles using:
Atomic (proton) number = number of protons
Mass (nucleon) number = total number of protons + neutrons
Charge = difference between number of protons and electrons
Protons
The atomic number (Z) tells you the number of protons in the nucleus
All atoms and ions of the same element have the same number of protons
e.g. Lithium has 3 protons; beryllium has 4 protons
If mass number and number of neutrons are known, calculate protons using:
Mass number = number of protons + number of neutrons
Number of protons = mass number - number of neutrons
Worked Example
Determine the number of protons in 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
A neutral atom has the same number of protons and electrons
Ions have a different number of electrons compared to protons:
A positive ion (cation) has lost electrons, so it has fewer electrons than protons
A negative ion (anion) has gained electrons, so it has more electrons than protons
Worked Example
Determine the number of electrons in 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 neutrons in 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
Unlock more revision notes. It's free!
By signing up you agree to our Terms and Privacy Policy.
Already have an account? Log in
Did this page help you?
