Atomic Structure and the Periodic Table.
Notes
Subatomic particles
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | very small (about 1/2000) |
- Atomic number = number of protons. Mass number = protons + neutrons.
- Atoms are neutral: protons = electrons.
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Relative atomic mass (Ar)
Ar = Σ(isotope mass × abundance) ÷ 100.
Chlorine: 75% ³⁵Cl and 25% ³⁷Cl gives Ar = (35 × 75 + 37 × 25) ÷ 100 = 35.5.
History of the atom
Dalton (solid spheres) → Thomson (plum pudding, with electrons found) → Rutherford (nuclear model: alpha scattering showed a small, dense, positive nucleus) → Bohr (electrons in shells) → Chadwick (neutrons).
Electron configuration
Shells hold 2, 8, 8 electrons. Calcium (20 electrons) is 2,8,8,2. The group number equals the number of outer-shell electrons, and the period number equals the number of shells.
Group 1: alkali metals
Soft metals with low melting points. Reactivity increases down the group: the outer electron is further from the nucleus, with more shielding, so it is lost more easily. React with water to form a metal hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂.
Group 7: halogens
Diatomic molecules. Reactivity decreases down the group: the atom gains an electron less easily because the outer shell is further from the nucleus. A more reactive halogen displaces a less reactive one from its salt:
Cl₂ + 2KBr → 2KCl + Br₂. Ionic: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
Group 0: noble gases
Full outer shells, so unreactive and monatomic. Boiling points increase down the group.
Transition metals
Higher melting points and densities, form coloured compounds and act as catalysts.
Common mistakes
- Saying "atoms want a full outer shell". Explain with electron energy and attraction instead.
- Mixing up atomic number and mass number.
- Forgetting that reactivity trends in Group 1 and Group 7 go in opposite directions.
Exam tips
- For trend explanations, use the three-part chain: distance from nucleus, shielding, ease of losing or gaining an electron.
Worksheet
Total: 25 marks.
- Chlorine has two main isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). Calculate the relative atomic mass of chlorine. (2)
- Boron has two isotopes, ¹⁰B and ¹¹B. Its relative atomic mass is 10.8. Calculate the percentage abundance of ¹¹B. (3)
- State the number of protons, neutrons and electrons in an atom of ²⁷Al (atomic number 13). (2)
- Write the electron configuration of calcium (atomic number 20). (1)
- Explain why potassium is more reactive than lithium. (3)
- Write a balanced equation for the reaction of sodium with water, and describe two observations. (4)
- Chlorine gas is bubbled through potassium bromide solution.
(a) Describe what you would see. (1)
(b) Write the balanced symbol equation. (2)
(c) Write the ionic equation. (2)
(d) Explain why chlorine displaces bromine but not the reverse. (2) - In Rutherford's alpha scattering experiment, most alpha particles passed straight through gold foil, a few were deflected through large angles, and very few bounced back. Explain what each observation showed about the atom. (3)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- (35 × 75 + 37 × 25) ÷ 100 (M1) = 35.5. (A1)
- Let x = % of ¹¹B. Then (11x + 10(100 − x)) ÷ 100 = 10.8 (M1). 11x + 1000 − 10x = 1080, so x = 80 (M1). 80% ¹¹B (and 20% ¹⁰B). (A1)
- Protons 13, electrons 13, neutrons 27 − 13 = 14. (B1 for protons and electrons, B1 for neutrons)
- 2,8,8,2. (B1)
- Both have one electron in the outer shell (B1). In potassium the outer electron is further from the nucleus and there is more shielding from inner shells (B1), so the attraction to the nucleus is weaker and the electron is lost more easily. (B1)
- 2Na + 2H₂O → 2NaOH + H₂ (B1 formulae, B1 balanced). Observations: fizzing/effervescence (B1) and the sodium melts into a ball and moves on the surface (B1). (Also acceptable: dissolves, indicator turns purple.)
- (a) The colourless solution turns orange (bromine formed). (B1)
(b) Cl₂ + 2KBr → 2KCl + Br₂ (B1 formulae, B1 balanced)
(c) Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ (B1 for correct species, B1 for balance and charges)
(d) Chlorine is more reactive than bromine (B1). Its outer shell is closer to the nucleus, so it attracts an extra electron more strongly (B1). - Most pass straight through: the atom is mostly empty space (B1). A few large deflections: there is a small, dense, positively charged nucleus (B1) that repels the positive alpha particles. A few bounced back: the nucleus holds most of the mass, since the alpha particle is repelled head-on by a dense centre (B1).