Atomic Structure and the Periodic Table
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1.1 Atoms, sub-atomic particles and isotopes
Every atom has a tiny, dense nucleus containing protons and neutrons, surrounded by electrons in shells. Almost all the mass is in the nucleus; almost all the volume is empty space where the electrons are.
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (negligible) | −1 | Shells |
Atomic number, Z: the number of protons in the nucleus. It decides which element the atom is.
Mass number, A: the total number of protons and neutrons in the nucleus.
Neutrons = A − Z. In a neutral atom, electrons = Z.
A nuclide is written with the mass number at the top left and the atomic number at the bottom left, for example 2311Na: 11 protons, 12 neutrons, 11 electrons.
Ions. Only the number of electrons changes. A 2+ ion has lost two electrons; a 3− ion has gained three. So 24Mg2+ has 12 protons, 12 neutrons and 10 electrons.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes have the same chemical properties, because they have the same electron configuration and chemistry is about electrons. They have slightly different physical properties that depend on mass, such as density and rate of diffusion.
How ideas about the atom developed. Dalton pictured solid spheres; Thomson found the electron and proposed the "plum pudding" model; Rutherford's gold-foil experiment showed a small, dense, positive nucleus; Bohr put electrons in fixed energy levels. Each model was replaced when new evidence did not fit it.
Check yourself: How many protons, neutrons and electrons are in 37Cl−?
17 protons, 20 neutrons, 18 electrons
1.2 Relative masses
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1.3 Mass spectrometry
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1.4 Ionisation energies
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1.5 Evidence for shells and sub-shells
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1.6 Shells, sub-shells and orbitals
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1.7 Electronic configurations and blocks
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1.8 Periodicity
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Extra worked questions
Cover the solution, answer on paper, then mark yourself against each step. Each step shows the mark it earns.
All 8 worked solutions are free for members. The first one is open to everyone.
Particles in ions
State the numbers of protons, neutrons and electrons in (a) the ion 56Fe3+ and (b) the ion 32S2−.
Try it first, then show the worked solution
| (a) Protons and neutrons | Fe has atomic number 26, so 26 protons. Neutrons = 56 − 26 = 30. | 1 |
|---|---|---|
| (a) Electrons | A 3+ ion has lost three electrons: 26 − 3 = 23 electrons. | 1 |
| (b) All three | S has atomic number 16: 16 protons, 32 − 16 = 16 neutrons, and 16 + 2 = 18 electrons. | 1 |
Answer: (a) 26 p, 30 n, 23 e (b) 16 p, 16 n, 18 e
Examiner tip: Only the electrons change when an atom becomes an ion. A positive charge means electrons lost; a negative charge means electrons gained.
Relative atomic mass of silicon
A sample of silicon contains 92.23% 28Si, 4.67% 29Si and 3.10% 30Si. Calculate the relative atomic mass of silicon in this sample, to one decimal place.
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Isotope abundances of antimony
A sample of antimony contains only 121Sb and 123Sb. Its relative atomic mass is 121.8. Calculate the percentage abundance of each isotope.
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Configurations of iron and copper
Give the full electronic configuration, in s, p, d notation, of (a) an iron atom, (b) an Fe2+ ion and (c) a copper atom.
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Which group? Successive ionisation energies
The first six ionisation energies of a Period 3 element Z, in kJ mol−1, are 1012, 1907, 2914, 4964, 6274 and 21 267. Deduce the group of Z, explain your answer and identify Z.
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Comparing sodium and potassium
The first ionisation energy of sodium is 496 kJ mol−1 and of potassium is 419 kJ mol−1. The second ionisation energy of sodium is 4562 kJ mol−1. (a) Explain why the first ionisation energy of potassium is lower than that of sodium. (b) Explain why the second ionisation energy of sodium is so much larger than the first.
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The mass spectrum of bromine
The mass spectrum of bromine, Br2, has three peaks for Br2+ at m/z 158, 160 and 162 with relative heights 1 : 2 : 1. (a) Deduce the isotopes of bromine and their ratio. (b) Explain the 1 : 2 : 1 pattern. (c) Calculate Ar(Br).
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Melting points across Period 3
Explain the pattern in melting points across Period 3 from sodium to argon. Refer to structure and bonding.
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Video walkthroughs
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Relative atomic mass and isotope abundance
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First ionisation energy across Period 3
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Emission spectra and quantum shells
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Recall cards
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