The structure of an atom — AQA GCSE Combined Science
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The structure of an atom explained
Atoms are the smallest particles of an element that retain its chemical identity.
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The radius of a typical atom is about 1 × 10⁻¹⁰ m, which is 0.0000000001 m, or one ten-billionth of a metre. This is roughly a million times smaller than the width of a human hair. Because atoms are so small, a single grain of sand contains an enormous number of them. The value is an order-of-magnitude figure: atoms vary slightly in size, but 1 × 10⁻¹⁰ m is a useful benchmark. When comparing atomic size with the nucleus, the radius of the nucleus is less than 1/10 000 of the radius of the atom (about 1 × 10⁻¹⁴ m), so most of an atom is empty space. Students should be able to quote the radius, interpret it in standard form and use it to judge scale.
The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.
An atom has a tiny central nucleus carrying positive charge because it contains protons, which are positively charged, and neutrons, which have no charge. The nucleus therefore has a positive overall charge equal to the number of protons. Surrounding the nucleus are negatively charged electrons, arranged in energy levels or shells. In a neutral atom the number of electrons equals the number of protons, so the positive and negative charges balance and the atom has no overall charge. Most of the atom's mass is in the nucleus, while most of its volume is the space occupied by electrons. For example, a carbon atom has six protons and six neutrons in its nucleus, with six electrons around it.
The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.
An atom is mostly empty space. Its tiny central nucleus has a radius under one ten-thousandth of the whole atom's radius, so if an atom were drawn 1 m across, the nucleus would be smaller than 0.1 mm. Yet nearly all the mass sits in that nucleus, because protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. For example, a carbon atom has a radius of about 7 × 10⁻¹¹ m, so its nucleus is under 7 × 10⁻¹⁵ m across. This explains why alpha particles mostly pass straight through gold foil but a few bounce back: the atom is empty, but its dense nucleus repels them.
The electrons are arranged at different distances from the nucleus (different energy levels). The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) or by the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level).
Electrons occupy energy levels at set distances from the nucleus, not a continuous spread. The lowest available level is filled first, and each level holds a limited number of electrons. An electron can move to a higher level by absorbing electromagnetic radiation of the right energy; it then falls back to a lower level by emitting electromagnetic radiation. For example, atoms can also be excited by heating in a flame; as the electrons drop back to lower levels, they emit visible light of characteristic colours, which is why flame tests work. The energy gap between levels fixes the frequency of radiation absorbed or emitted, linking electron arrangement to spectra.
Your focus
- Recall that the radius of an atom is about 1 × 10⁻¹⁰ m.
- Interpret and convert the atomic radius between standard form and decimal form.
- Recall that the radius of a nucleus is less than 1/10 000 of the radius of an atom (about 1 × 10⁻¹⁴ m).
Show all 12 objectives
- Describe the basic structure of an atom, including the nucleus and surrounding electrons.
- State the relative charge of protons, neutrons and electrons.
- Explain why a neutral atom has no overall charge.
- Describe the nucleus as having a radius less than 1/10 000 of the atomic radius.
- Explain that most of an atom's mass is concentrated in the nucleus because protons and neutrons have much greater mass than electrons.
- Apply the scale of atomic and nuclear radii to interpret simple models or scattering evidence.
- Describe the arrangement of electrons in energy levels at different distances from the nucleus.
- Explain how absorption and emission of electromagnetic radiation change an electron's energy level and distance from the nucleus.
- Interpret simple observations, such as flame colours or line spectra, using electron transitions between energy levels.
The structure of an atom exam tips
Marking Points
- States that the radius of an atom is about 1 × 10⁻¹⁰ m.
- Recognises that this is a typical or approximate value, not an exact size for every atom.
- Converts 1 × 10⁻¹⁰ m into ordinary decimal form as 0.0000000001 m.
- Explains that atoms are mostly empty space because the radius of the nucleus is less than 1/10 000 of the radius of the atom.
- States that the radius of a nucleus is about 1 × 10⁻¹⁴ m.
- States that the nucleus is at the centre of the atom and is positively charged.
- Identifies that the nucleus contains protons, which are positive, and neutrons, which are neutral.
- States that electrons are negatively charged and surround the nucleus.
- Explains that in a neutral atom the number of electrons equals the number of protons, so the charges balance.
- Recognises that most of the atom's mass is in the nucleus while most of its volume is the region occupied by electrons.
- State that the nucleus radius is less than 1/10 000 of the atomic radius, so the atom is mostly empty space.
- Explain that protons and neutrons in the nucleus carry almost all the atom's mass, while electrons contribute negligible mass.
- Use the relative masses of protons, neutrons and electrons to justify why mass is concentrated in the nucleus.
- Apply the scale idea to a concrete example, such as comparing an atom 1 m across with a nucleus under 0.1 mm across.
- Link the small, dense, positive nucleus to the observation that most alpha particles pass through foil while a few are deflected.
- Describe electrons as arranged in energy levels at different distances from the nucleus, with lower levels closer to the nucleus.
- Explain that absorption of electromagnetic radiation moves an electron further from the nucleus to a higher energy level.
- Explain that emission of electromagnetic radiation occurs when an electron moves closer to the nucleus to a lower energy level.
- Relate the size of the energy gap to the energy of the radiation absorbed or emitted.
- Apply the model to a context such as flame colours or line spectra produced by excited atoms.
Examiner Tips
- 💡Learn the value 1 × 10⁻¹⁰ m with its negative exponent and be ready to write it in standard form.
- 💡If asked to compare sizes, convert both values to the same form, such as standard form, before dividing.
- 💡Remember that the nuclear radius is less than 1/10 000 of the atomic radius, which is why the atom is mostly empty space.
- 💡Sketch a simple labelled diagram showing the nucleus with protons and neutrons and electrons in shells around it.
- 💡When explaining why an atom is neutral, state clearly that the number of protons equals the number of electrons.
- 💡Use the words proton, neutron and electron accurately, and give the charge of each particle.
- 💡Quote the comparison as 'less than 1/10 000' rather than a vague phrase such as 'very small'.
- 💡When asked why most alpha particles pass through, refer to the atom being mostly empty space and the nucleus being tiny.
- 💡Use relative masses of 1 for protons and neutrons and about 1/1836 for electrons to support mass statements.
- 💡Use the phrases 'higher energy level' and 'further from the nucleus' together when describing absorption.
- 💡Use the phrases 'lower energy level' and 'closer to the nucleus' together when describing emission.
- 💡If a question gives a spectrum or flame colour, link the observed radiation to an electron dropping between named levels.
Common Mistakes
- Writing the radius as 1 × 10¹⁰ m instead of 1 × 10⁻¹⁰ m; the negative exponent is essential because atoms are extremely small.
- Treating 1 × 10⁻¹⁰ m as the exact radius of every atom; it is a typical value and atoms of different elements vary slightly.
- Confusing the atomic radius with the nuclear radius; the radius of the nucleus is less than 1/10 000 of the radius of the atom.
- Saying the nucleus is negatively charged; it is positive because of the protons, while neutrons are neutral.
- Placing electrons inside the nucleus; electrons occupy energy levels or shells around the nucleus.
- Forgetting that neutrons have no charge and treating them as positive particles like protons.
- Thinking the nucleus occupies a large fraction of the atom: correct this by stressing the nucleus radius is under 1/10 000 of the atomic radius, so the atom is mostly empty space.
- Believing electrons contribute significant mass: correct this by comparing relative masses, where an electron is about 1/1836 of a proton, so nearly all mass is in the nucleus.
- Confusing radius with mass: correct this by separating the two ideas, since the nucleus is tiny in size but contains almost all the mass.
- Saying electrons can sit at any distance from the nucleus: correct this by stating they occupy fixed energy levels at set distances.
- Reversing absorption and emission: correct this by linking absorption to moving further out and a higher level, and emission to moving closer in and a lower level.
- Treating energy levels as evenly spaced or unlimited: correct this by noting each level has a limited capacity and gaps between levels differ.