The Nucleus (Cambridge (CIE) IGCSE Physics): Revision Note

Composition of the nucleus

• A nucleus is composed of:

  • positively charged protons

  • neutrally charged neutrons

• Hence a nucleus has an overall positive charge

Structure of the atom

Describing the nucleus

Proton number, Z

• The number of protons in an atom is called its proton number (it can also be called the atomic number)

  • Elements in the periodic table are ordered by their atomic number

  • Therefore, the number of protons determines which element an atom is

• The atomic number of a particular element is always the same

• For example:

  • Hydrogen has an atomic number of 1 — it always has just one proton

  • Sodium has an atomic number of 11 — it has 11 protons

  • Uranium has an atomic number of 92 — it has 92 protons

• The atomic number is also equal to the number of electrons in an atom

  • This is because atoms have the same number of electrons and protons in order to have no overall charge

Nucleon number, A

• The total number of particles in the nucleus of an atom is called its nucleon number (or mass number)

• The mass number is the number of protons and neutrons in the atom

Calculating the number of neutrons

• The number of neutrons can be found by subtracting the atomic number from the mass number

$\mathrm{number}\mathrm{of}\mathrm{neutrons}=\mathrm{nucleon}\mathrm{number}-\mathrm{proton}\mathrm{number}$

• For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12

Nuclide notation

• Atomic symbols are written in a specific notation called AZX or nuclide notation

  • The top number A represents the nucleon number

  • The lower number Z represents the proton number

• A nuclide is a group of atoms containing the same number of protons and neutrons

  • For example, five atoms of oxygen are all the same nuclide but are five separate atoms

• An example of nuclide notation is:

Isotopes

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

• Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons

  • This means that each element can have more than one isotope

• Isotopes tend to be more unstable due to their imbalance of protons and neutrons

  • This means they are more likely to decay (opens in a new tab)

• In the diagram below are three isotopes of hydrogen:

• Isotopes occur naturally, but some are more rare than others

• For example, about two in every 10 000 hydrogen atoms is deuterium

  • Tritium is even more rare (about 1 in every billion billion hydrogen atoms)