The nuclear model of the atom

Key points

5.1.1 The atom

• An atom has a nucleus at its centre. The nucleus is positively charged.
• Electrons orbit the nucleus. Electrons are negatively charged.
• An atom has the same number of electrons as protons, so it has no overall charge. We say it is neutral.
• An ion is an atom that has lost or gained electrons, so it has an overall charge.
• An atom that loses electrons becomes a positive ion.
• An atom that gains electrons becomes a negative ion.
• In the alpha-particle scattering experiment, α-particles (alpha particles) are fired at a very thin sheet of metal, usually gold foil. A detector records where they go. Extended
• Most α-particles pass straight through the foil. So most of the atom is empty space, and the nucleus is very small. Extended
• A few α-particles are deflected. α-particles are positive. They are repelled by the nucleus, and like charges repel. So the nucleus is positively charged. Extended
• Very few α-particles bounce back. Only something with a large mass can turn back a fast α-particle. The electrons are far too light to do this. So the nucleus contains most of the mass of the atom. Extended
• The very small number of large deflections also shows that the nucleus is tiny. An α-particle must pass very close to the nucleus to be deflected through a large angle. Extended

5.1.2 The nucleus

• The nucleus is made of protons and neutrons. Protons and neutrons together are called nucleons.
• Relative charges: proton +1, neutron 0, electron −1.
• Proton number Z (atomic number) is the number of protons in the nucleus.
• Nucleon number A (mass number) is the total number of protons and neutrons in the nucleus.
• Number of neutrons = A − Z.
• Nuclide notation shows a nucleus as ^A_ZX. X is the chemical symbol. A is written at the top and Z at the bottom. Example: ¹⁴₆C has 6 protons and 14 − 6 = 8 neutrons.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. So they have the same Z but a different A.
• An element can have more than one isotope. Example: ¹²₆C and ¹⁴₆C are both isotopes of carbon.
• The relative charge on a nucleus is +Z, because each proton has a charge of +1 and neutrons have no charge. Extended
• The relative mass of a nucleus is about equal to its nucleon number A, because a proton and a neutron each have a relative mass of about 1. Extended
• Nuclear fission is the splitting of a large, heavy nucleus into two smaller nuclei. Extended
• In fission, a uranium-235 nucleus absorbs a neutron and becomes unstable. It splits into two smaller nuclei (called daughter nuclei) and releases neutrons and energy. Extended
• One fission equation: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + energy. Other pairs of daughter nuclei are possible. Extended
• Nuclear fusion is the joining of two light nuclei to form one heavier nucleus. Extended
• One fusion equation: ²₁H + ³₁H → ⁴₂He + ¹₀n + energy. Extended
• Extra detail: fusion needs a very high temperature. The nuclei must move fast enough to overcome the repulsion between their positive charges.
• In both fission and fusion, the total mass after the reaction is slightly less than the total mass before. This mass has been converted into the energy released. You do not need any values. Extended
• In every nuclide equation, the nucleon numbers (top) add up to the same total on both sides. The proton numbers (bottom) also add up to the same total on both sides. Extended
• Stable stars, such as the Sun, are powered by the fusion of hydrogen into helium (see 6.2). Extended

Common mistakes

• Students write that the nucleus is made of protons, neutrons and electrons. / The mark scheme wants protons and neutrons only. The electrons orbit outside the nucleus.
• Students write that a positive ion has gained protons. / The mark scheme wants: a positive ion has lost electrons. The nucleus does not change when an ion forms.
• Students write that isotopes have a different number of protons. / The mark scheme wants: the same number of protons but a different number of neutrons.
• Students write that “the atom is empty space” for the α-particles that bounce back. / The mark scheme wants each observation linked to its own conclusion: passing straight through → mostly empty space; deflected → positive nucleus; bounced back → small nucleus with most of the mass. Extended
• Students write that fission is the joining of nuclei and fusion is the splitting. / The mark scheme wants: fission = splitting a heavy nucleus; fusion = joining light nuclei. Extended
• Students write that mass is created in fission or fusion, or that the mass stays the same. / The mark scheme wants: the total mass after the reaction is slightly less, and this mass is converted into energy. Extended

Exam tips

• State means give a short, clear answer with no explanation. “State the relative charge of a neutron” needs only “0”.
• Define means give the exact meaning. For proton number, write “the number of protons in the nucleus”. For nucleon number, write “the number of protons and neutrons in the nucleus”.
• Describe the alpha-particle scattering results as three separate observations: most pass straight through, a few are deflected, very few bounce back. Extended
• Explain what the scattering shows by joining each observation to its conclusion with “so” or “because”. A 3-mark answer usually needs all three links. Extended
• A typical 1-mark answer: “An isotope has the same number of protons but a different number of neutrons.”
• A typical 2-mark answer to “Describe nuclear fusion”: “Two light nuclei join to make a heavier nucleus (1). Energy is released and the total mass decreases (1).” Extended
• In a nuclide equation question, always check both rows: the top numbers (A) and the bottom numbers (Z) must each balance. Extended
• Write nuclide notation with A at the top and Z at the bottom, both on the left of the symbol.