Atomic Structure & Bonding

IGCSE Edexcel Chemistry
1.14–1.51 Atomic structure, the periodic table, and chemical bonding
Key Concepts: Atoms contain protons, neutrons and electrons. The periodic table organises elements by atomic number. Bonding — ionic (electron transfer) or covalent (electron sharing) — determines the properties of substances.

Section A — Atomic Structure

1. State the relative mass and relative charge of a proton, neutron and electron. [6]
ParticleRelative massRelative charge
Proton
Neutron
Electron
2. Define atomic number, mass number, and isotope. [3]
3. Chlorine has two isotopes: Cl-35 (75%) and Cl-37 (25%). Calculate the relative atomic mass of chlorine. Show your working. [2]
4. Write the electronic configuration for each element. [4]

a) Sodium (Na, atomic number 11)

b) Chlorine (Cl, atomic number 17)

c) Calcium (Ca, atomic number 20)

d) Argon (Ar, atomic number 18)

Section B — The Periodic Table

5. Describe how the periodic table is organised and define the terms period and group. [3]
6. For each electronic configuration, identify the element's group and period. [6]
Electronic configurationGroupPeriod
2, 8, 1
2, 8, 7
2, 8, 8, 2
7. Explain why elements in the same group have similar chemical properties. [2]
8. Explain why noble gases are unreactive. [2]

Section C — Ionic Bonding

9. State the charges of ions formed by elements in each group. [6]
GroupCharge of ion formed
Group 1
Group 2
Group 3
Group 5
Group 6
Group 7
10. Write the correct ionic formula for each compound. [5]
CompoundFormula
Magnesium chloride
Aluminium oxide
Sodium sulfide
Calcium nitrate
Ammonium sulfate
11. Draw dot-and-cross diagrams showing the formation of ions in sodium chloride (NaCl). [3]
12. Explain why ionic compounds have high melting points and conduct electricity when molten or in solution. [4]

Section D — Covalent Bonding and Structures

13. Draw dot-and-cross diagrams for the following covalent molecules. [6]

a) H₂

b) H₂O

c) NH₃

d) CO₂

14. Explain why simple molecular substances (e.g. water, iodine) have low melting and boiling points. [2]
15. Compare the structure and properties of diamond and graphite. Refer to bonding, hardness, and electrical conductivity. [6]

Total marks: 60

Mark Scheme

1. Proton: mass 1, charge +1; Neutron: mass 1, charge 0; Electron: mass ~0 (negligible), charge −1 [6]
2. Atomic number = number of protons in the nucleus [1]; mass number = total number of protons + neutrons [1]; isotopes = atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons) [1] [3]
3. (35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5 [2]
4. Na: 2,8,1; Cl: 2,8,7; Ca: 2,8,8,2; Ar: 2,8,8 [4]
5. Arranged in order of increasing atomic number [1]; periods are horizontal rows [1]; groups are vertical columns [1] [3]
6. 2,8,1 → Group 1, Period 3; 2,8,7 → Group 7, Period 3; 2,8,8,2 → Group 2, Period 4 [6 — 2 per row]
7. Elements in the same group have the same number of outer-shell (valence) electrons [1]; the outer electrons determine chemical reactivity and how the element reacts/bonds [1] [2]
8. Noble gases have full outer electron shells [1]; they do not need to gain, lose, or share electrons to achieve stability, so they are very unreactive [1] [2]
9. Group 1: +1; Group 2: +2; Group 3: +3; Group 5: −3; Group 6: −2; Group 7: −1 [6]
10. MgCl₂; Al₂O₃; Na₂S; Ca(NO₃)₂; (NH₄)₂SO₄ [5]
11. Na loses one electron to form Na⁺; Cl gains one electron to form Cl⁻; show outer shells only with electron transfer arrow [3]
12. Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions [1]; a lot of energy is needed to break these forces → high melting point [1]; in the solid state ions are fixed; when molten or dissolved, ions are free to move [1]; free mobile ions carry electrical charge → conduct electricity [1] [4]
13. H₂: one shared pair between two H; H₂O: O shares one pair with each H, two lone pairs on O; NH₃: N shares one pair with each H, one lone pair on N; CO₂: C shares two pairs (double bond) with each O [6 — accept correct dot-and-cross, 1.5 per molecule]
14. Simple molecular substances have weak intermolecular forces between molecules [1]; little energy is needed to overcome these forces → low melting and boiling points [1] [2]
15. Diamond: giant covalent structure; each carbon bonded to four others (tetrahedral) [1]; very hard [1]; does not conduct electricity (no free electrons/ions) [1]; Graphite: layers of hexagonal rings; each carbon bonded to three others [1]; layers can slide over each other → soft/lubricant [1]; delocalised electrons between layers → conducts electricity [1] [6]