Atomic Nuclei (WJEC GCSE Physics): Revision Note

Atomic Nuclei

• Atoms are made up of protons, neutrons, and electrons

  • The positively charged nucleus at the centre contains the protons and neutrons

  • The negatively charged electrons are in orbit around the nucleus

• Protons have a positive charge, whilst neutrons have no charge

  • This is why the nucleus has an overall positive charge

• An atom contains the same number of protons and electrons

  • This is why the atom has no charge overall

Structure of the Atom

Protons and neutrons are found in the nucleus of an atom, while electrons are found orbiting around the nucleus

• Atomic symbols are written in a specific notation called AZX notation, as shown below:

• Nucleon number (A) = total number of nucleons (protons and neutrons) in the nucleus

  • This is the top number A (sometimes it is also called the mass number)

• Proton number (Z) = total number of protons in the nucleus

  • This is the bottom number Z (sometimes it is also called the atomic number)

• The atom above represents the element lithium, which has the atomic symbol:

• The atomic symbol for lithium tells us

  • It has an atomic number Z = 3, therefore a lithium atom has 3 protons and 3 electrons

  • It has a mass number A = 7, therefore a lithium atom has 7 nucleons total, so it has 4 neutrons

• When given an atomic symbol, the total number of protons, neutrons and electrons can be determined using the following rules:

  • Protons: the number of protons is equal to the proton number

  • Electrons: atoms are neutral, so the number of electrons is equal to the number of protons

  • Neutrons: the number of neutrons is equal to the nucleon number minus the proton number

Isotopes

• For a particular element, the number of protons is always the same, but the number of neutrons can be different

  • This is because the number of protons determines the element e.g. carbon atoms have 6 protons and iron atoms have 26 protons

• An isotope is

  An atom, or atoms, of the same element that have an equal number of protons but a different number of neutrons

• Each element can have more than one isotope

Isotopes of Hydrogen

• Some isotopes are more unstable than others due to the imbalance of protons and neutrons, which means

  • They may be more likely to decay

  • They may be less likely to occur naturally

• For example, about 2 in every 10 000 atoms of hydrogen are the isotope deuterium

  • The isotope tritium is even rarer (about 1 in every billion billion atoms of hydrogen)