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

  • Protons and neutrons are found in the nucleus of an atom

  A) proton

  B) neutron

  C) electron

  
• Be careful with your terminology:

  • Atom = nucleus (proton and neutron) and electrons
  • Nucleus = protons and neutrons at the centre of the atom
• 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

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

  • number of neutrons = nucleon number - proton 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
• You may have noticed that the number of electrons is not part of the mass number

  • This is because electrons have a tiny mass compared to neutrons and protons
  • We say their mass is negligible when compared to the particles in the nucleus
• Atomic symbols are written in a specific notation called ZXA or nuclide notation

  • The top number A represents the nucleon number
  • The lower number Z represents the proton number
• Nuclide notation

  A) nucleon

  B) proton

  C) chemical

  D) element

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

  • For example, 5 atoms of oxygen are all the same nuclide but are 5 separate atoms
• You may recognise this notation from the periodic table in chemistry when mass number and proton number are more commonly used

  • In physics, you are more likely to see nucleon number and proton number. The periodic table is ordered by atomic number.
• 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're more likely to decay
• Isotopes of hydrogen

  • Isotopes occur naturally, but some are more rare than others
  • For example, about 2 in every 10,000 Hydrogen atoms is Deuterium
  • Tritium is even more rare (about 1 in every billion billion hydrogen atoms)