The AtomCambridge IGCSE Physics: Revision notes
Section 1
What is the basic structure of an atom?
An atom consists of two main parts:
- Nucleus: a very small, dense region at the centre of the atom
- Electrons: negatively charged particles that orbit the nucleus in shells (or energy levels)
The nucleus itself contains protons (positively charged) and neutrons (no charge). The positive charge of the protons in the nucleus attracts the negative electrons, keeping them in orbit.
Key structural features:
- The nucleus is extremely small compared to the overall size of the atom — most of the atom is empty space
- The nucleus contains almost all the mass of the atom, even though it occupies only a tiny fraction of the volume
- Electrons have negligible mass compared to protons and neutrons
- The number of protons equals the number of electrons in a neutral atom, making the overall charge zero
Think of the atom like a solar system: the nucleus is like the Sun (very small but containing most of the mass), and electrons are like planets orbiting at a distance. Most of the space between them is empty.
Examiners expect you to emphasize that the atom is mostly empty space. When describing the nucleus, always mention both its small size AND its high density/mass concentration.
Section 2
How do atoms form ions by gaining or losing electrons?
Ions are formed when atoms gain or lose electrons (not protons or neutrons). This creates an imbalance between the number of protons and electrons.
Positive ions (cations):
- Formed when an atom loses electrons
- Protons now outnumber electrons
- The atom becomes positively charged
- Example: Sodium atom loses 1 electron to form Na⁺ ion
Negative ions (anions):
- Formed when an atom gains electrons
- Electrons now outnumber protons
- The atom becomes negatively charged
- Example: Chlorine atom gains 1 electron to form Cl⁻ ion
| Type of Ion | How it forms | Charge | Example |
|---|---|---|---|
| Positive ion (cation) | Loses electrons | Positive | Na⁺, Mg²⁺, Al³⁺ |
| Negative ion (anion) | Gains electrons | Negative | Cl⁻, O²⁻, N³⁻ |
Important: The number of protons in the nucleus does NOT change when an ion forms — only the number of electrons changes.
Students often think the nucleus changes when an ion forms. Remember: only electrons move, never the protons or neutrons. The nucleus remains unchanged.
A magnesium atom has 12 protons and 12 electrons (neutral). When it loses 2 electrons to form Mg²⁺, it still has 12 protons but now only 10 electrons, so the charge is +2.
Section 3
What does Rutherford's alpha particle scattering experiment tell us?
Rutherford's experiment involved firing alpha particles (positively charged helium nuclei) at a very thin sheet of gold foil and observing how they were deflected.
Key observations and conclusions:
-
Most alpha particles passed straight through with little or no deflection
- Conclusion: atoms are mostly empty space
- Very few particles encountered a nucleus that would cause them to deflect
-
Some alpha particles were deflected at large angles (some even bounced backwards)
- Conclusion: there must be a very small, dense, positively charged region (the nucleus) at the centre
- When alpha particles came close to this nucleus, the strong repulsive force caused large deflections
-
The nucleus must contain most of the atom's mass
- Conclusion: despite occupying only a tiny volume, the nucleus is very dense
- Alpha particles could only be heavily deflected if they hit something with significant mass and charge concentrated in a small space
This experiment provided experimental evidence for the nuclear model of the atom, replacing the earlier plum pudding model.
When answering questions about Rutherford's experiment, link each observation directly to a conclusion: explain WHAT was observed and WHY it shows something about atomic structure. For example, 'Most particles passed straight through, showing atoms are mostly empty space.'
Imagine firing bullets at a thin metal foil. Most bullets pass straight through (mostly empty space), but some bounce off a tiny, hard lump hidden inside (the nucleus).
Section 4
How does the charge in an atom create the nuclear model?
The nuclear model of the atom explains how opposite charges interact:
- Nucleus: contains protons, which are positively charged
- Electrons: are negatively charged and orbit the nucleus
- Electrostatic attraction: the positive nucleus attracts the negative electrons, keeping them in orbit around the nucleus
This attraction is the force that holds the atom together.
Evidence from alpha particle scattering:
- Alpha particles (positively charged) are repelled when they approach the positively charged nucleus
- The closer an alpha particle gets to the nucleus, the stronger the repulsive force
- Only alpha particles that come very close to the nucleus experience significant deflection
- This confirms the nucleus is positively charged and located at the centre
The very small size of the nucleus means that the electric field is extremely strong in that region, causing dramatic deflections of alpha particles that pass near it.
Examiners want to see that you understand charges cause forces. State clearly: 'The positive nucleus attracts negative electrons' or 'Alpha particles are repelled by the positive nucleus' — always link charge to the resulting force.
Section 5
What is the evidence that the nucleus contains most of the atom's mass?
Rutherford's alpha particle scattering experiment provided direct evidence that the nucleus contains nearly all of an atom's mass:
Why this conclusion was drawn:
-
Alpha particles were significantly deflected by the nucleus
- A large deflection only occurs if the alpha particle encounters something with substantial mass
- If the atom's mass were spread evenly throughout (as the old plum pudding model suggested), alpha particles would not be deflected so dramatically
-
The deflecting force was very strong in a tiny region
- The nucleus is extremely small (diameter about 10,000 times smaller than the atom)
- Yet it can deflect an alpha particle through large angles
- This is only possible if the nucleus is very dense — meaning lots of mass packed into a tiny volume
-
Electrons have negligible mass
- Electrons are about 2000 times lighter than protons
- The electrons orbiting at a distance from the nucleus contribute almost nothing to the atom's total mass
- Therefore, nearly all the atom's mass must be in the nucleus
Quantitatively: a nucleus with diameter ~10⁻¹⁵ m contains >99.9% of the atom's mass, while the atom itself has diameter ~10⁻¹⁰ m.
If an alpha particle (mass ~4 u) is strongly deflected by the nucleus, the nucleus must have significant mass to exert such a strong repulsive force. If the atom's mass were spread thinly throughout the entire atom, the deflection would be minimal.
Must Know
-
Atomic structure: atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons orbiting in shells. Most of the atom is empty space.
-
The nucleus: contains protons (positive) and neutrons (neutral); contains almost all the atom's mass in a tiny volume; is positively charged.
-
Electron shells: electrons orbit at relatively large distances from the nucleus; attracted by the positive nucleus; have negligible mass.
-
Ion formation: positive ions form when atoms lose electrons (cations); negative ions form when atoms gain electrons (anions); the nucleus is unchanged.
-
Rutherford's alpha scattering experiment: most alpha particles pass straight through (empty space); some are deflected at large angles (small, dense, positive nucleus exists); provides evidence for the nuclear model.
-
Forces: electrostatic attraction between the positive nucleus and negative electrons holds atoms together; electrostatic repulsion between alpha particles and the positive nucleus causes deflection.
That's the notes covered.
Carry on to the next subtopic.