Atoms, isotopes and ions (AQA GCSE Physics Combined Science): Revision Notes
Atoms, isotopes and ions
Particles can exist as neutral atoms or as charged particles called ions. Understanding the difference between these is essential for GCSE physics.
What are atoms and isotopes?
All atoms of the same element contain exactly the same number of protons in their nucleus. However, they can have different numbers of neutrons. When atoms of the same element have different numbers of neutrons, we call them isotopes.
For example, all carbon atoms have 6 protons, but some carbon isotopes have:
- 6 neutrons (carbon-12)
- 7 neutrons (carbon-13)
- 8 neutrons (carbon-14)
The key thing to remember is that changing the number of neutrons doesn't change what element it is. Carbon will always have 6 protons, no matter how many neutrons it has.
Important masses to remember:
- Protons have a mass of 1
- Neutrons have a mass of 1
- Electrons have a mass of 0 (they're much lighter)
How positive ions form
In a normal atom, the number of electrons equals the number of protons. This means there's no overall charge - the atom is neutral.
However, atoms can lose electrons and become positive ions. This happens because there are now more positive protons than negative electrons.
Electrons can leave an atom in two main ways:
1. Electromagnetic radiation When an atom absorbs enough electromagnetic energy (like UV light, X-rays, or gamma rays), electrons can escape from the atom's pull.
2. Particle collision When high-energy particles like alpha or beta particles hit an atom, they can knock electrons out of their shells.
How negative ions form
Atoms can also gain extra electrons. When this happens, there are more negative electrons than positive protons, so the atom becomes a negative ion with an overall negative charge.
Worked Example: Understanding Atomic Charges
Let's look at some charges:
(a) What's the charge of a normal atom? Answer: (neutral) - because electrons equal protons
(b) What's the charge of a sodium atom that loses one electron?
Answer: - because there's now one more proton than electrons
(c) What's the charge when an electron moves to a lower energy level? Answer: (neutral) - no electrons are lost or gained, just moved
Key Points to Remember:
- Isotopes are atoms of the same element with different numbers of neutrons
- Positive ions form when atoms lose electrons
- Negative ions form when atoms gain electrons
- Normal atoms are neutral because they have equal protons and electrons
- Electromagnetic radiation and particle collisions can knock electrons out of atoms