Revision notes for Edexcel GCSE Physics Atomic structure and ionising radiation. Open the guide for explanations and worked examples. Written against the Edexcel GCSE Physics (1PH0) specification, so the content matches what's examinable rather than general Physics background.
Atomic structure and ionising radiation
What you'll learn
- What atoms are made from, and where most of their mass is found.
- How to use atomic number, mass number and isotope notation.
- How atoms become positive ions by losing electrons.
- What alpha, beta and gamma radiation are, how they are detected, and how they compare.
Atoms: tiny structures with a dense nucleus
An atom is the smallest part of an element that still behaves like that element. Every atom has a tiny central nucleus, with electrons around it.
Definition
Structure of an atom
An atom has a positively charged nucleus made of protons and neutrons, surrounded by negatively charged electrons. The nucleus is much smaller than the atom, but contains almost all of the atom’s mass.
The diagram below shows the GCSE model: a small nucleus in the centre, electrons in shells around it, and isotope notation on the left.
Size of atoms and nuclei
An order of magnitude is the nearest power of ten. For GCSE, you should recall that atoms and small molecules are typically about 10−10 m10^{-10}\,\text{m}10−10m across. A molecule is a group of atoms joined together.
A nucleus is much smaller, around 10−14 m10^{-14}\,\text{m}10−14m in radius. That means most of an atom is empty space.
Example
Comparing atom and nucleus size
- Use a typical atom radius of 10−10 m10^{-10}\,\text{m}10−10m and a typical nucleus radius of 10−14 m10^{-14}\,\text{m}10−14m.
- Compare them by dividing: 10−1010−14=104\frac{10^{-10}}{10^{-14}}=10^410−1410−10=104.
- The atom’s radius is about 10,000 times larger than the nucleus radius, so the nucleus is tiny compared with the whole atom.
The particles inside atoms
A subatomic particle is a particle smaller than an atom. The main ones here are protons, neutrons, electrons and positrons.
| Particle | Where it is found | Relative electric charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells around nucleus | -1 | about 1/1836 |
| Positron | Emitted in beta plus radiation | +1 | about 1/1836 |
A positron is the antimatter partner of an electron: it has the same very small relative mass as an electron, but positive charge.
Key Idea
Most of the mass is nuclear
Protons and neutrons have much more mass than electrons, so almost all the mass of an atom is in the nucleus.
In a neutral atom, the number of protons equals the number of electrons. Their charges cancel out.
Common Mistake
Forgetting neutral atoms have balanced charge
If an atom has 11 protons, it must have 11 electrons only if it is neutral. Ions do not have equal numbers of protons and electrons.
Atomic number, mass number and isotopes
The atomic number is the number of protons in the nucleus. It is also called the proton number.
The mass number is the total number of protons and neutrons in the nucleus. It is also called the nucleon number, because a nucleon means a proton or neutron.
Definition
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
For example, carbon always has 6 protons. Carbon-12 and carbon-13 are both carbon because they both have 6 protons, but carbon-13 has one extra neutron.
The symbol format is:
ZAX{}^{A}_{Z}\text{X}ZAXwhere AAA is the mass number, ZZZ is the atomic number, and X is the chemical symbol.
To find neutrons:
neutrons=A−Z\text{neutrons}=A-Zneutrons=A−ZExample
Using isotope notation
For 1737Cl{}^{37}_{17}\text{Cl}1737Cl:
- The lower number is the atomic number, so chlorine has 17 protons.
- The upper number is the mass number, so protons plus neutrons is 37. The number of neutrons is 37−17=2037-17=2037−17=20.
- If the atom is neutral, electrons equal protons, so it has 17 electrons.
Tip
Top and bottom numbers
In isotope notation, the top number is the total number of protons and neutrons. The bottom number is the protons.
Electron shells and energy changes
Electrons are found at different set distances from the nucleus. These are often called shells or energy levels.
Electrons cannot sit just anywhere. They can move between shells if energy is transferred. Electromagnetic radiation is energy transferred as waves, such as visible light, ultraviolet, X-rays or gamma rays.
- If an electron moves to a shell farther from the nucleus, it has absorbed electromagnetic radiation.
- If an electron moves to a shell closer to the nucleus, it has emitted electromagnetic radiation.
Example
Deciding whether radiation is absorbed or emitted
- An electron moving to a shell farther from the nucleus has gained energy.
- That energy must have come from electromagnetic radiation, so the atom has absorbed radiation.
- If the electron later moves back closer to the nucleus, it loses energy, so electromagnetic radiation is emitted.
Ions: atoms with charge
An ion is an atom or group of atoms with an overall electric charge. In this topic, focus on positive ions.
Atoms form positive ions by losing outer electrons. The nucleus does not change during ordinary ion formation: protons and neutrons stay the same.
Example
Finding the charge on a positive ion
A magnesium atom has 12 protons and 12 electrons. It loses two outer electrons.
- Before losing electrons, magnesium is neutral because it has 12 protons and 12 electrons.
- Losing two electrons leaves 12−2=1012-2=1012−2=10 electrons, but the nucleus still has 12 protons.
- The overall charge is +12+(−10)=+2+12+(-10)=+2+12+(−10)=+2, so the ion is Mg2+\text{Mg}^{2+}Mg2+.
Radioactivity: unstable nuclei giving out radiation
A nucleus is unstable if it may change by emitting radiation. This process is called radioactive decay.
Radioactive decay is random. You cannot predict exactly which nucleus will decay next, or exactly when a particular nucleus will decay. However, with a large sample, you can predict patterns statistically.
Key Idea
Random does not mean impossible to measure
Radioactive decay is random for individual nuclei, but a large sample gives a measurable count rate.
Unstable nuclei can emit:
- Alpha radiation: an alpha particle, which is the same as a helium nucleus with 2 protons and 2 neutrons.
- Beta minus radiation, β−\beta^{-}β−: a high-speed electron emitted from the nucleus.
- Beta plus radiation, β+\beta^{+}β+: a positron emitted from the nucleus.
- Gamma radiation, γ\gammaγ: electromagnetic radiation from the nucleus.
- Neutron radiation: neutrons emitted from unstable nuclei.
Definition
Ionising radiation
Ionising radiation is radiation that can remove electrons from atoms, forming ions.
Alpha, beta minus, beta plus and gamma radiation are ionising radiations. For this specification point, the main comparison is between alpha, beta and gamma.
Common Mistake
Beta is not an orbiting electron
A beta minus particle is an electron emitted from the nucleus, not one of the electrons already orbiting in shells.
Background radiation
Background radiation is the low-level ionising radiation that is always around us, even when no radioactive source has been placed nearby.
It comes from natural sources on Earth and from space:
- Rocks and soil, including radon gas from some rocks.
- Building materials made from rocks.
- Food and living things, because tiny amounts of radioactive substances are naturally present.
- Cosmic rays from space, especially at higher altitudes.
Detecting radioactivity
A photographic film darkens when exposed to ionising radiation. This can be used in film badges worn by people who work near radiation.
A Geiger-Müller tube, or GM tube, detects ionising radiation by producing electrical pulses. A counter records these as clicks or counts. The count rate is the number of counts per second or per minute.
Example
Correcting a Geiger-Müller reading
A GM tube measures 90 background counts in 3 minutes. With a source present, it measures 420 counts in 3 minutes.
- Find the background count rate: 90÷3=30 counts/min90 \div 3=30\,\text{counts/min}90÷3=30counts/min.
- Find the source plus background count rate: 420÷3=140 counts/min420 \div 3=140\,\text{counts/min}420÷3=140counts/min.
- Subtract background: 140−30=110 counts/min140-30=110\,\text{counts/min}140−30=110counts/min, so the corrected count rate from the source is 110 counts/min.
Tip
Longer counts are more reliable
Because decay is random, a longer counting time usually gives a more reliable average count rate.
Penetration and ionisation
Penetrating power means how far radiation can travel through materials. Ionising power means how easily it forms ions.
The diagram below compares alpha, beta and gamma radiation using common absorbers.
Alpha, beta and gamma compared
- Alpha is strongly ionising but weakly penetrating. It is stopped by paper or skin.
- Beta is moderately ionising and moderately penetrating. It is stopped by thin aluminium.
- Gamma is weakly ionising but highly penetrating. It is only reduced by thick lead or concrete.
Example
Identifying radiation from absorbers
A radioactive source gives a high count rate. Paper makes little difference, but thin aluminium reduces the count rate close to background.
- Paper does not stop the radiation, so it is unlikely to be mainly alpha.
- Thin aluminium stops it, which matches beta radiation.
- Gamma would mostly pass through thin aluminium, so the radiation is most likely beta.
Common Mistake
Mixing up penetration and ionisation
Alpha is the most ionising but the least penetrating. Gamma is the least ionising but the most penetrating.
Exam technique
In the exam
- For isotope symbols, read the bottom number as protons, then subtract from the top number to find neutrons.
- For ion questions, remember that atoms form positive ions by losing electrons; the nucleus does not lose protons.
- For radiation questions, separate ionising power from penetrating power, and subtract background radiation from count rates when needed.
Self review
Check yourself
- In 614C{}^{14}_{6}\text{C}614C, how many protons, neutrons and electrons are in a neutral atom?
- What happens to an atom’s overall charge if it loses two electrons?
- Which radiation is stopped by paper, and which is only reduced by thick lead or concrete?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?