Coaching Centre
Inside the atom review
A summary of the whole Inside the Atom chapter, key words, and a mixed test covering models of the atom, subatomic particles, isotopes, electron shells, flame tests, radioactivity, half-life and uses of radiation.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 1A Models of the atom | Democritus (atomos); Dalton 1803 (indivisible atoms, fixed ratios); Thomson 1897 discovered the electron with cathode rays, plum pudding model; Rutherford 1911, gold foil: most alpha particles pass through (empty space), a few bounce back (tiny dense positive nucleus); Bohr 1913 shells; Chadwick 1932 neutron; quantum model (electron cloud). Models change with new evidence. |
| 1B Subatomic particles | Proton +1, mass 1, nucleus; neutron 0, mass 1, nucleus; electron −1, mass ≈ 1/1840, shells. Atomic number = protons (decides element); mass number = protons + neutrons. Neutral atom: electrons = protons. Ions: lose electrons → positive, gain → negative. |
| 1C Isotopes | Same protons, different neutrons; same chemistry, different mass. Relative atomic mass = weighted average of isotope masses by abundance. Measured with a mass spectrometer. |
| 1D Electrons | Shells fill 2, 8, 8, 2 for the first 20 elements. Shells = period; outer (valence) electrons = group and control reactions; full outer shell = noble gas. Flame tests: electrons absorb energy, jump up, fall back and emit light of specific colours; line spectra identify elements. |
| 1E Radioactivity | Unstable nuclei decay randomly. Alpha (2p + 2n, +2, stopped by paper, most ionising); beta (electron from n → p, stopped by mm of aluminium); gamma (EM wave, reduced by thick lead). Alpha: A −4, Z −2; beta: Z +1. Background radiation (radon, rocks, cosmic, food, medical). Becquerel, the Curies. Geiger counter, sieverts. |
| 1F Half-life | Time for half the nuclei (or activity) to decay. Amount halves each half-life. Carbon-14 (5730 years) dates once-living things up to about 50 000 years. |
| 1G Uses | Choose by radiation type, half-life, where it goes. Tc-99m imaging, F-18 PET, I-131 thyroid treatment, brachytherapy, Co-60 sterilising, Am-241 smoke alarms, beta thickness gauges, tracers. OPAL reactor at Lucas Heights. Fission power: low CO₂, but waste, accident risk, cost; banned in Australia. Time, distance, shielding. |
Putting it together
Almost everything in this chapter comes back to the nucleus and the electrons around it. The electrons, and especially those in the outer shell, decide how an element behaves in chemical reactions and what colours it gives out when heated. The nucleus decides which element it is (protons), how heavy it is (protons and neutrons), and whether it is stable or radioactive. Chemistry is about electrons moving between atoms; nuclear physics is about the nucleus changing. The same small set of particles explains fireworks, the periodic table, carbon dating, cancer treatment and why the Sun shines.
Key words
activity · alpha particle · atomic mass unit · atomic number · background radiation · becquerel · beta particle · brachytherapy · cathode ray · chain reaction · control rod · electron · electron configuration · energy level · excited state · fission · flame test · fusion · gamma ray · Geiger counter · half-life · ion · ionising radiation · isotope · line spectrum · mass number · mass spectrometer · neutron · nuclear model · nucleus · plum pudding model · proton · radioactive decay · radiocarbon dating · radioisotope · relative atomic mass · shell · sievert · strong nuclear force · tracer · transmutation · valence electron
Test yourself
Rutherford's gold foil experiment led to the discovery of the
The rare bounce-backs showed a tiny dense positive nucleus.
An atom of argon-40 (atomic number 18) contains
40 − 18 = 22 neutrons.
Which pair are isotopes?
Same element, different numbers of neutrons.
Magnesium (Z = 12) has the electron configuration
Fill from the inside: 2, 8, then 2.
Thorium-234 (Z = 90) gives out a beta particle. The new nucleus has
A neutron becomes a proton: Z + 1, A unchanged.
A source's count rate drops from 2000 to 250 counts per minute in 12 hours (background removed). Its half-life is
2000 → 1000 → 500 → 250 is 3 half-lives in 12 hours.
Which radiation would be best for checking for cracks inside a thick steel weld?
Only gamma gets through thick steel.
Name each model of the atom.
What can change?
The particle discovered by Thomson is the . Atoms of the same element with different numbers of neutrons are . Outer-shell electrons are called electrons. A helium nucleus given out by an unstable nucleus is an particle. The time for half of a radioactive sample to decay is its .
Sodium-24 (atomic number 11) is a beta and gamma emitter with a half-life of 15 hours. It is sometimes injected into a water pipe to find leaks underground. (a) State the number of protons, neutrons and electrons in a sodium-24 atom, and its electron configuration. (b) Explain why sodium-24 is suitable for this job. (c) An engineer injects a sample with an activity of 8000 Bq. Calculate its activity after 60 hours.
(a) Protons = 11, neutrons = 24 − 11 = 13, electrons = 11. Electron configuration 2, 8, 1.
(b) Sodium-24 gives out gamma rays, which can pass through the soil above the pipe, so a detector at the surface can pick them up. Where the pipe leaks, water carrying the sodium-24 soaks into the soil and the radioactivity builds up there, so the detector shows a higher reading above the leak. Its half-life of 15 hours is long enough for the tracer to travel through the pipe and for the survey to be done, but short enough that it decays to a very low level within a few days and doesn't leave the water supply radioactive for long. Sodium is also soluble and moves with the water.
(c) Number of half-lives = 60 ÷ 15 = 4. Activity: 8000 → 4000 → 2000 → 1000 → 500 Bq.