Atomic Nuclei (OCR Gateway GCSE Physics: Combined Science)

• Atoms are extremely small with a radius of about 1 x 10^-10 metres
  • Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus

• The central nucleus contains protons and neutrons only which are packed close together in a small region of space
  • Because protons are positive and neutrons are neutral, the charge of the nucleus is positive
  • The number of protons in the nucleus determines the element
  • Therefore the nucleus of each element contains a characteristic positive charge
    • e.g. a helium nucleus, with 2 protons, has a charge of +2
    • e.g. a uranium nucleus, with 92 protons, has a charge of +92

Diagram showing the structure of a Lithium atom. If drawn to scale then the electrons would be around 100 metres away from the nucleus!

• The radius of the nucleus is about 10 000 times smaller than that of the atom, so it is an extremely small region of space compared to the overall size of the atom
• This means that rather than being evenly spread out throughout the atom, virtually all of the atom's mass is concentrated inside the nucleus

• Electrons in an atom orbit around the nucleus at particular distances, known as energy levels
• A certain number of electrons can occupy each energy level
  • For example, only two electrons can orbit in the first energy level
  • Only eight electrons can fit in the second energy level, and eight in the third as well

• The higher the energy level, the further the distance of the electron from the nucleus

In this diagram the first two energy levels are full. Electrons further from the nucleus have more energy

• Like moving up a ladder, electrons in higher energy levels have greater potential energy because they have more distance between them and the nucleus
• Electrons can move between energy levels by absorbing or emitting electromagnetic radiation

• Energy is required for an electron to move from a lower to a higher energy level
  • This transition is called an excitation
• Energy is released if the electron moves from a higher to a lower energy level
  • This transition is called a de–excitation

Ionisation

• If an atom gains or loses an electron from the outer energy level this is known as ionisation
  • The charged particle is now known as an ion
  • An ion is an atom or particle with a non-zero charge
• Positive ions are formed when electrons in the outer energy level are knocked out from an atom
  
  • This is because an electron is negatively charged
• Positive ionisation can happen in a number of ways:
  • When objects are rubbed together, electrons can be removed by friction
  • When electrons absorb electromagnetic radiation they can gain enough energy to leave the atom
  • From chemical reactions

When radiation passes close to atoms it can knock electrons out, leaving the atom with an overall positive charge

• Negative ions are formed when the outer shell of an atom gains an electron
• Negative ionisation most commonly occurs during chemical reactions