Particles in the atom
Section: 1 Atomic Structure | Syllabus: Cambridge AS Level Chemistry (9701)
Structure of the Atom
An atom consists of a very small, dense nucleus at its centre, surrounded by a large region of mostly empty space in which electrons are found. The nucleus is made up of protons and neutrons, collectively called nucleons.
Figure 1.1: Structure of the Atom
(Labelled diagram showing the tiny nucleus containing protons and neutrons at the centre, surrounded by electron shells in the vast empty space)
Key PointAtoms are mostly empty space. The nucleus occupies only about 1/10,000 of the diameter of the atom, yet contains almost all of its mass. This was demonstrated by Rutherford's gold foil experiment, in which most alpha particles passed straight through the foil, while a very small number were deflected at large angles - indicating a tiny, dense, positively charged nucleus.
Figure 1.2: Rutherford's Gold Foil Experiment
(Diagram showing alpha particle source, thin gold foil, and detectors. Most particles pass straight through; a few are deflected at large angles; very rarely one bounces straight back. Conclusion: atom is mostly empty space with a tiny dense positive nucleus)
Subatomic Particles
The three fundamental subatomic particles and their properties are summarised below:
| Particle | Location | Relative Mass | Relative Charge |
|---|---|---|---|
| Proton | Nucleus | 1 | +1 |
| Neutron | Nucleus | 1 | 0 |
| Electron | Shells around nucleus | 1/1840 (≈ 0) | −1 |
The relative masses are approximate. Protons and neutrons each have a relative mass of 1; the electron's mass is so small it is considered negligible (approximately 1/1840 that of a proton).
Atomic Number and Mass Number
Atomic (Proton) Number, Z The number of protons in the nucleus of an atom. This defines the element - all atoms of the same element have the same atomic number. Mass (Nucleon) Number, A The total number of protons and neutrons (nucleons) in the nucleus of an atom.From these two numbers, all particle counts can be found:
- Number of protons = Atomic number (Z)
- Number of neutrons = Mass number (A) − Atomic number (Z)
- Number of electrons = Atomic number (Z) for a neutral atom
- For ions: adjust the electron count by the charge. A 2+ ion has 2 fewer electrons; a 2− ion has 2 extra electrons.
Z = 17 → protons = 17
A = 35 → neutrons = 35 − 17 = 18
Charge = −1 → electrons = 17 + 1 = 18
Distribution of Mass and Charge
- Mass is concentrated almost entirely in the nucleus (protons + neutrons). Electrons contribute negligible mass.
- Positive charge is concentrated in the nucleus (from protons).
- Negative charge is spread through the electron shells surrounding the nucleus.
- A neutral atom has equal numbers of protons and electrons, so overall charge = 0.
Behaviour of Particles in an Electric Field
When a beam of subatomic particles moving at the same velocity passes between charged plates (an electric field), each particle is deflected according to its charge and mass:
Figure 1.3: Deflection of Particles in an Electric Field
(Diagram showing two parallel plates - positive plate at top, negative at bottom. Three particle paths enter from the left: proton curves gently toward the negative plate; neutron travels straight through undeflected; electron curves sharply toward the positive plate. Label the relative degree of deflection.)
| Particle | Charge | Direction of deflection | Amount of deflection |
|---|---|---|---|
| Proton | +1 | Towards negative plate | Small (high mass) |
| Electron | −1 | Towards positive plate | Large (very low mass) |
| Neutron | 0 | Not deflected - travels straight | None |
Electrons are deflected much more than protons because they have the same magnitude of charge but approximately 1/1840 of the mass - so the same force causes a much greater acceleration. Protons deflect in the opposite direction to electrons.
Atomic Radius and Ionic Radius Trends
Atomic radius is defined as half the distance between the nuclei of two identical adjacent atoms.
Across a Period (left → right)
- Atomic radius decreases across a period.
- Nuclear charge (number of protons) increases, pulling electrons closer to the nucleus.
- The number of electron shells stays the same, and shielding is roughly constant across the period.
- The increased effective nuclear charge therefore draws the outer electrons inward.
Figure 1.4: Atomic Radius Trend Across Period 3
(Bar chart or scaled circles showing atomic radius decreasing from Na → Mg → Al → Si → P → S → Cl → Ar across Period 3. Label radii values in pm.)
Down a Group (top → bottom)
- Atomic radius increases down a group.
- Each successive element has an extra electron shell, placing the outer electrons further from the nucleus.
- Increased shielding from inner shells also reduces the effective nuclear charge felt by the outer electrons.
Figure 1.5: Atomic Radius Trend Down Group 1
(Scaled circles for Li, Na, K, Rb, Cs showing radius increasing down the group. Each circle should be noticeably larger than the one above.)
Ionic Radius Compared to Atomic Radius
- Cations (positive ions) are smaller than the parent atom - removing electrons reduces electron–electron repulsion and the remaining electrons are pulled closer to the nucleus.
- Anions (negative ions) are larger than the parent atom - adding electrons increases repulsion, expanding the electron cloud.
When comparing ionic radii across a period, remember that isoelectronic ions (same number of electrons) become smaller as nuclear charge increases. For example: O²⁻ > F⁻ > Na⁺ > Mg²⁺ - all have 10 electrons, but more protons pull them in tighter.
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