The structure of an atom — AQA GCSE Physics
Test yourself on The structure of an atom with AQA GCSE practice questions.
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The structure of an atom explained
An atom has a central nucleus containing protons, each with a relative charge of +1, and neutrons, which are uncharged.
Read the full explanation
Both have a relative mass of 1, so the nucleus carries the positive charge and nearly all the mass. Around it, at various distances, move the electrons, each with a relative charge of -1 and a mass roughly one two-thousandth of a proton's. A neutral atom holds exactly as many electrons as protons, so the charges cancel. A typical atomic radius is about 1 × 10⁻¹⁰ m. Charging an object never moves protons: only the loosely held outer electrons transfer, which is why an examiner awards nothing for an answer that speaks of transferring positive charge.
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 has a radius of roughly 1 × 10⁻¹⁰ m, while its nucleus is smaller than 1 × 10⁻¹⁴ m, so the nucleus occupies less than one ten-thousandth of the atomic radius. Scaled up, if an atom were a sports stadium the nucleus would be a pea at the centre, and everything else is empty space through which the electrons move. Despite that tiny size, the nucleus holds nearly all the mass, because protons and neutrons are each about two thousand times heavier than an electron. Alpha particle scattering supplies the evidence for both claims: most alpha particles passed straight through the foil, but a few rebounded from something small, massive and positively charged.
The electrons are arranged at different distances from the nucleus (different energy levels).
Electrons are not scattered at random around the nucleus. They occupy particular distances from it, drawn as shells and described as energy levels. The level closest to the nucleus is the lowest in energy and fills first, taking up to two electrons; the next levels each take up to eight for the first twenty elements. So sodium, with eleven electrons, is written 2,8,1. An electron further from the nucleus is in a higher energy level, because energy has to be supplied to move it out against the attraction of the positive nucleus. The outermost electrons are held least tightly, which is why they are the ones lost when an atom becomes a positive ion.
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 can move between energy levels, but only by gaining or losing exactly the right amount of energy. If an atom absorbs electromagnetic radiation carrying the energy gap between two levels, an electron moves out to the higher level, further from the nucleus. The arrangement is unstable, and when the electron drops back to a lower level, closer in, the atom emits electromagnetic radiation carrying that same energy away. Absorbing moves electrons out, emitting brings them in. If the radiation absorbed is energetic enough, such as ultraviolet or an X-ray, the electron is removed from the atom completely, and the atom becomes a positive ion. That process is what ionising radiation is named for.
Your focus
- Label a diagram of an atom with the nucleus, protons, neutrons and electrons, and give the charge of each particle.
- Explain why an atom has no overall charge, using the numbers and charges of protons and electrons.
- Explain why a cloth becomes positively charged when it is rubbed on a plastic rod, referring only to the movement of electrons.
Show all 12 objectives
- State the approximate radius of an atom and of a nucleus, and calculate the ratio between them.
- Explain why almost all the mass of an atom lies in its nucleus, referring to the masses of protons, neutrons and electrons.
- Interpret the alpha scattering results as evidence that the atom is mostly empty space with a tiny massive centre.
- Draw the electron arrangement of an atom with up to twenty electrons, filling the lowest energy level first.
- Explain why an electron further from the nucleus has more energy than one closer to it.
- Identify the electrons that are lost most easily when an atom forms a positive ion, and justify the choice.
- Describe what happens to an electron when an atom absorbs electromagnetic radiation of the right energy.
- Explain why an atom emits electromagnetic radiation shortly after one of its electrons has been raised to a higher energy level.
- Explain how an atom becomes a positive ion when it absorbs sufficiently energetic radiation.
The structure of an atom exam tips
Marking Points
- one mark for a positively charged nucleus containing protons and neutrons
- one mark for negatively charged electrons surrounding the nucleus
- one mark for relative charges of +1 for a proton, 0 for a neutron and -1 for an electron
- one mark for an atomic radius of about 1 × 10⁻¹⁰ m
- one mark for the radius of the nucleus being less than 1/10 000 of the radius of the atom
- one mark for quoting about 1 × 10⁻¹⁰ m for the atom and about 1 × 10⁻¹⁴ m or smaller for the nucleus
- one mark for stating that the mass of the atom is concentrated in the nucleus
- one mark for stating that most of the atom is empty space
- one mark for electrons occupying particular distances from the nucleus, called energy levels or shells
- one mark for stating that an electron further from the nucleus is at a higher energy level
- one mark for stating that the lowest energy level fills first
- one mark for a correct electron arrangement written or drawn, such as 2,8,1 for sodium
- one mark for absorption of electromagnetic radiation moving an electron to a level further from the nucleus
- one mark for emission of electromagnetic radiation when an electron moves to a level closer to the nucleus
- one mark for identifying the level further from the nucleus as the higher energy level
- one mark for stating that absorbing enough energy removes the electron altogether, leaving a positive ion
Examiner Tips
- 💡Learn the relative charges and masses of all three particles as a table; one-mark recall questions come up every year.
- 💡In any electrostatics answer, transfer electrons and nothing else.
- 💡Say surrounding or orbiting the nucleus rather than around the atom, which is ambiguous.
- 💡Learn the two orders of magnitude, 10⁻¹⁰ m and 10⁻¹⁴ m; comparison questions often just want these.
- 💡Answer mass questions with the phrase concentrated in the nucleus, which is the wording the mark scheme uses.
- 💡If the question asks about the atom being mostly empty space, link it to alpha particles passing straight through.
- 💡Draw each shell as a neat circle and space the electrons around it; crowded dots lose marks on diagrams.
- 💡Count the electrons against the atomic number before you fill the levels.
- 💡Use the phrase energy level as well as shell, because the specification wording uses both.
- 💡Pair the words in your answer: absorb with further out and higher, emit with closer in and lower.
- 💡Name the radiation as electromagnetic; simply saying light or energy often misses the mark.
- 💡Link this to ionisation if the question is about ultraviolet, X-rays or gamma rays.
Common Mistakes
- placing the electrons inside the nucleus
- giving the neutron a negative charge because of its name
- explaining charging by the transfer of protons or positive charge, which scores zero
- writing about positive electrons, which also scores zero
- quoting 1/10 000 as a ratio of masses rather than of radii
- giving the nucleus a radius of 10⁻¹⁰ m, which is the size of the whole atom
- saying the electrons account for most of the mass because there are several of them
- claiming the nucleus is massive because it is large
- drawing electrons scattered randomly in the space around the nucleus
- placing more than two electrons in the first energy level
- saying the outermost electrons are at the lowest energy level
- treating the shells as orbits that can take any radius at all
- saying radiation is emitted when an electron jumps outwards
- claiming the electron is destroyed or created during the change
- confusing the emission of electromagnetic radiation with the emission of an electron from the atom
- describing the nucleus, rather than an electron, as the part that absorbs the radiation