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
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 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
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 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
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