The nuclear model of the atom
Structure of the Atom and Subatomic Particles
An atom consists of a central nucleus surrounded by electrons in orbit. The nucleus contains most of the atom's mass but occupies a tiny fraction of its volume. This structure is supported by Rutherford's alpha-particle scattering experiments, which showed that atoms are mostly empty space with a dense, positively charged center.
The nucleus is composed of two types of particles: protons and neutrons. Protons carry a positive charge, while neutrons have no charge (they are neutral). Electrons orbit the nucleus and carry a negative charge. In a neutral atom, the number of electrons equals the number of protons.
| Particle | Charge (relative) | Location |
|---|---|---|
| Proton | +1 | ~1 |
| Neutron | 0 | ~1 |
| Electron | -1 | ~1/1836 (negligible) |
Why this matters: The relative charge of a nucleus is determined solely by its proton number (Z). Since each proton has a charge of +1, the total relative charge of the nucleus is +Z. Neutrons contribute to mass but not charge. Electrons are outside the nucleus and do not affect the nuclear charge.
Proton Number (Atomic Number) and Nucleon Number (Mass Number)
Proton number (Z): The number of protons in the nucleus. This defines the element's identity. For example, all carbon atoms have Z = 6. It also equals the number of electrons in a neutral atom.
Nucleon number (A): The total number of protons and neutrons in the nucleus. Since protons and neutrons each have a relative mass of approximately 1, A represents the approximate relative mass of the atom.
Calculating Neutrons: The number of neutrons (N) is found by subtracting the proton number from the nucleon number: N = A - Z
Nuclide Notation
Atoms are represented using nuclide notation: ^{A}_{Z}X where X is the chemical symbol, A is the nucleon number (top), and Z is the proton number (bottom).
Example: Carbon-14 is written as ^{14}_{6}C. This tells us: - Proton number (Z) = 6 (so it has 6 protons). - Nucleon number (A) = 14. - Number of neutrons = 14 - 6 = 8. - In a neutral atom, there are also 6 electrons.
Isotopes
Isotopes are atoms of the same element (same proton number Z) that have different numbers of neutrons (different nucleon number A).
Because they have the same number of protons and electrons, isotopes have identical chemical properties but different physical masses. For example, Carbon-12 (^{12}<em>{6}C) and Carbon-14 (^{14}</em>{6}C) are isotopes.
Alpha Particle Scattering Evidence
Rutherford's experiment fired alpha particles (positively charged helium nuclei) at a thin sheet of gold foil. The observations provided key evidence for the nuclear model:
| Observation | Evidence For |
|---|---|
| Most alpha particles passed straight through. | The atom is mostly empty space. |
| Some alpha particles were deflected at small angles. | There is a concentrated positive charge in the center (nucleus) that repels them. |
| A very few alpha particles bounced back (deflected > 90°). | The nucleus is very small, dense, and contains most of the atom's mass. |
Why this supports the nuclear model: If the positive charge were spread out (as in the 'plum pudding' model), alpha particles would not experience strong enough repulsion to bounce back. The large deflections prove a dense, positively charged nucleus exists.
Ions and Nuclear Reactions
Ions: Atoms become ions by gaining or losing electrons, not protons. - Positive ion (cation): Formed when an atom loses one or more electrons. The nucleus remains unchanged. - Negative ion (anion): Formed when an atom gains one or more electrons.
Nuclear Fission: A heavy, unstable nucleus splits into two smaller nuclei, releasing energy and extra neutrons. This process is used in nuclear power stations.
Nuclear Fusion: Two light nuclei join together to form a heavier nucleus, releasing large amounts of energy. This powers the Sun and stars.
Relationships: Charge and Mass
Relative Charge of Nucleus: The relative charge of a nucleus is exactly equal to its proton number (Z). Since each proton has a charge of +1, the total nuclear charge is +Z. Neutrons have no charge and do not contribute.
Relative Mass of Nucleus: The relative mass of a nucleus is approximately equal to its nucleon number (A). This is because protons and neutrons each have a relative mass of ~1, while electrons have negligible mass.
⚠︎ Confusing Ions with Nuclear Changes
Mistake: Thinking that forming a positive ion involves losing protons from the nucleus. Correction: Ions are formed by gaining or losing electrons from the electron shells. The nucleus (protons and neutrons) remains unchanged during chemical ionization.
Mistake: Calculating the number of protons by subtracting the atomic number from the mass number. Correction: The proton number is given directly by the atomic number (Z). To find neutrons, you subtract Z from A (N = A - Z).
Describing Alpha Scattering Evidence
When asked to describe how alpha scattering supports the nuclear model, you must link specific observations to conclusions. Do not just list facts.
Correct phrasing: 'Most alpha particles passed through because the atom is mostly empty space.' This is accepted because it directly links the observation (passing through) to the structural implication (empty space).
Incorrect phrasing: 'There is a nucleus.' This is insufficient. You must explain why the evidence points to a nucleus (e.g., 'Some were deflected because they came close to a dense, positive center').
Past Paper Style Questions
Q:
State the relative charge of a proton and a neutron.
A:
Proton: +1; Neutron: 0 (or neutral).
Q:
An atom has 11 protons, 12 neutrons, and 10 electrons. What is its nuclide notation and charge?
A:
Nuclide: ^{23}_{11}Na; Charge: +1 (since it has one fewer electron than proton).
Q:
Explain what is meant by the term 'isotope'.
A:
Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (different nucleon number).
Q:
Describe one difference between nuclear fission and nuclear fusion.
A:
Fission is the splitting of a heavy nucleus into lighter nuclei; Fusion is the joining of light nuclei to form a heavier nucleus.
Q:
How does the structure of a neutral atom differ from that of a positive ion?
A:
A positive ion has fewer electrons than protons, whereas a neutral atom has an equal number of protons and electrons. The nucleus is identical in both.