Atomic Structure & Bonding
Protons, electrons and the three kinds of bond, the section every later topic assumes, and the one where a dot-and-cross diagram is worth as much as an essay.
Inside the atom, and the two numbers that describe it
An atom is a nucleus of protons (charge +1, mass 1) and neutrons (charge 0, mass 1) surrounded by electrons (charge −1, mass negligible) in shells. Proton number Z is the number of protons and defines the element. Nucleon number A is protons plus neutrons. Neutrons = A − Z. A neutral atom has as many electrons as protons.
Electrons fill shells 2, 8, 8. Write the configuration as a comma list: sodium (Z = 11) is 2,8,1; chlorine (17) is 2,8,7; argon (18) is 2,8,8. The outer-shell count is the chemistry. It is the group number, it decides the charge of the ion, and it decides how many bonds the atom makes.
Isotopes are atoms of the same element with different numbers of neutrons, same Z, different A. Same electrons, so same chemical reactions; different mass. Relative atomic mass Ar is the weighted average: chlorine at 75% Cl-35 and 25% Cl-37 gives (75 × 35 + 25 × 37) ÷ 100 = 35.5.
Ionic bonding: transfer, then attraction
Between a metal and a non-metal. The metal loses its outer electrons to reach a full shell and becomes a positive ion; the non-metal gains them and becomes negative. Sodium 2,8,1 → Na⁺ (2,8). Chlorine 2,8,7 → Cl⁻ (2,8,8). The bond is the electrostatic attraction between oppositely charged ions, in every direction, throughout a giant lattice.
The charges fix the formula. Mg²⁺ needs two Cl⁻: MgCl₂. Al³⁺ and O²⁻ must balance six each way: Al₂O₃. Brackets for compound ions with a subscript: Ca(NO₃)₂, (NH₄)₂SO₄.
Properties follow from the structure, and the examiner wants the link stated, not just the property. High melting point: a lot of energy is needed to overcome strong forces between many ions. Conducts when molten or dissolved, not as a solid: the ions are only free to move then. Usually soluble in water.
Covalent bonding: sharing, and two very different structures
Between non-metals. Each shared pair is one bond; each atom counts the shared electrons as its own. H₂ has one shared pair, O₂ a double bond (two pairs), N₂ a triple. Water: O shares one pair with each H and keeps two lone pairs. Methane: four single bonds. Dot-and-cross diagrams show outer shells only. Draw them for H₂, Cl₂, H₂O, CH₄, NH₃, HCl, CO₂ until you do not have to think.
Simple molecular substances (water, CO₂, iodine, methane) have strong covalent bonds inside the molecule but only weak forces between molecules. So they have low melting and boiling points and do not conduct: there are no ions and no free electrons. When iodine sublimes, the I₂ molecules stay intact; it is the forces between them that break. That sentence, correctly written, is a mark every year.
Giant covalent structures (diamond, graphite, silicon dioxide) are a single network of covalent bonds. Very high melting points; hard (diamond); do not conduct (except graphite). Diamond: each carbon bonded to four others tetrahedrally. Graphite: each carbon to three in flat hexagonal layers, the fourth electron delocalised, so graphite conducts, and the layers slide because the forces between them are weak. That is why it is a lubricant and a pencil.
Metallic bonding, and predicting properties
A metal is a lattice of positive ions in a sea of delocalised electrons. The attraction between the two is the bond. Delocalised electrons carry charge, so metals conduct; layers of ions can slide without breaking the structure, so metals are malleable and ductile. Alloys are harder because atoms of a different size disrupt the layers.
Given an unfamiliar substance, the paper expects you to reason from properties to structure and back. High m.p. conducts only when molten → ionic. Low m.p. never conducts → simple molecular. Very high m.p. never conducts → giant covalent. Conducts as a solid → metal (or graphite).
Worked: "Substance X melts at 801 °C, dissolves in water, and the solution conducts." Ionic. "Substance Y melts at −95 °C and does not conduct in any state." Simple molecular. Give the reason with the label: ions free to move when dissolved; weak forces between molecules, no charged particles free to move.
Where marks go
- "Covalent bonds are weak." They are strong. It is the forces between molecules that are weak.
- "Ionic compounds conduct because electrons move." Ions move. Electrons move in metals and graphite.
- Formula from names without balancing charge. Magnesium chloride is MgCl₂, not MgCl.
- Isotopes described as different elements. Same element, same proton number.
- Dot-and-cross with inner shells drawn. Outer shell only, and every shared pair shown as a dot and a cross.
- "Sodium wants to lose an electron." Atoms do not want. Say: loses one electron to achieve a full outer shell.
What to remember
- Z = protons; A = protons + neutrons; configuration 2,8,8; outer shell = group number.
- Ionic: metal + non-metal, transfer, lattice, high m.p. conducts molten/aqueous.
- Covalent: non-metals, shared pairs. Simple molecular: weak forces between, low m.p. no conduction. Giant covalent: network, very high m.p.
- Graphite conducts and slides; diamond does neither.
- Metallic: ions in a sea of electrons, conducts, malleable.
- Always link property → structure in the answer.
Watch one worked through
Here is one done slowly, with the reason for every move. Then one for you.
Describe the bonding in sodium chloride and explain why it conducts electricity when molten but not when solid.
- Sodium loses one electron, chlorine gains it.Na (2,8,1) → Na⁺ (2,8) Cl (2,8,7) → Cl⁻ (2,8,8)Both reach a full outer shell. Showing the arrangements before and after is what demonstrates you understand it.
- The opposite charges attract, forming an ionic bond.Na⁺ ⟷ Cl⁻ ionic bondAn ionic bond is electrostatic attraction between ions, not a shared pair. That distinction is the question being tested.
- In the solid the ions are locked in a lattice.solid: ions fixed in place → no mobile charges → no conductionConduction needs charged particles that can move. The charges exist in the solid but cannot go anywhere.
- Melting frees the ions.molten: ions free to move → conductsThe same reasoning explains why a solution conducts too. Always answer with mobile ions, not just with "it is ionic".
Answer: Ionic bonding between Na+ and Cl-. Solid ions are fixed in a lattice so cannot carry charge; molten ions are free to move, so it conducts.
What charge does a sodium ion carry? Write it as 1+ or 1-.
Show the answer
1+
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