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
