Flashcards · 9202 · Topic 2

Structure, bonding and the properties of matter flashcards

31 cards on Topic 2, subtopics 3.2.1 to 3.2.4. Pick a subtopic or study them all.

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3.2.1 Chemical bonds: ionic, covalent and metallic

Chemical bonding: Transferring or sharing electrons in the highest occupied energy levels (outer shells) so that atoms reach a noble gas electron arrangement.
Ion: An atom that has gained or lost electrons, so it carries an overall charge.
Ionic bonding: The strong electrostatic pull between oppositely charged ions, which works in every direction through a giant lattice.
Ion charges for Groups 1, 2, 3, 5, 6 and 7: Group 1: 1+. Group 2: 2+. Group 3: 3+. Group 5: 3−. Group 6: 2−. Group 7: 1−.
Charge and electron arrangement of the phosphide ion (P, atomic number 15): 3−; [2,8,8]³⁻ — the atom (2,8,5) gains three electrons.
Structure of sodium chloride: A regular cubic lattice: each Na⁺ is surrounded by six Cl⁻ and each Cl⁻ by six Na⁺, alternating in every direction.
Covalent bond: A pair of electrons shared by two atoms; it is a strong bond.
Shared pairs and non-bonding pairs in HCl, H₂O and NH₃: HCl: 1 shared, 3 non-bonding (on Cl). H₂O: 2 shared, 2 non-bonding (on O). NH₃: 3 shared, 1 non-bonding (on N).
Giant covalent structures (macromolecules) named in 9202: Diamond and silicon dioxide.
Structure and bonding in a metal: A giant structure of positive ions in a regular pattern, with delocalised outer electrons. Strong electrostatic attractions between the ions and the delocalised electrons hold it together.
Molecule vs giant structure: A molecule has a fixed, small number of atoms. A giant structure has no fixed size; its formula is only a ratio.

3.2.2 How bonding and structure are related to the properties of substances

Explain why ionic compounds have high melting points: A giant ionic lattice has many strong electrostatic attractions between oppositely charged ions, acting in all directions. Overcoming them takes a large amount of energy.
When does an ionic compound conduct electricity, and why?: When molten or dissolved in water: the ions are free to move and carry charge. In the solid they are fixed in the lattice.
Melting and boiling points of simple molecular substances: Relatively low: they are gases, liquids or solids with relatively low melting and boiling points.
What is overcome when a simple molecular substance melts or boils?: Only the weak intermolecular forces between molecules. The covalent bonds are not broken.
Why do simple molecular substances not conduct electricity?: The molecules have no overall electric charge, so there are no charged particles to carry a current.
Why do giant covalent substances have very high melting points?: Every atom is held by strong covalent bonds, so a very large number of them must be broken. There are no separate molecules, so no intermolecular forces.
Why do metals conduct heat and electricity?: Their delocalised electrons can move through the whole structure, carrying charge and energy.
Substance Y melts at −210 °C and boils at −196 °C. What is its state at 25 °C?: Gas — 25 °C is above its boiling point.

3.2.3 Structure and bonding of carbon

Why can a carbon atom form four covalent bonds?: Its electron arrangement is 2,4, so it has four outer electrons to share.
Why is diamond hard?: Every carbon atom is locked in place by four strong covalent bonds pointing in all directions, so scratching it means breaking many strong bonds.
Why is graphite slippery?: There are no covalent bonds between the layers, only weak forces, so the layers slide.
Why does graphite conduct electricity?: One electron from each carbon atom is delocalised and moves through the structure.
Why does graphite still melt at a very high temperature?: Melting breaks the strong covalent bonds inside the layers.
Fullerenes: structure and the four uses named in 9202: Forms of carbon based on hexagonal rings, with different numbers of carbon atoms. Uses: drug delivery into the body, lubricants, catalysts, and nanotubes for reinforcing materials (e.g. tennis rackets).
Graphite vs metal conduction: Both use delocalised electrons. Graphite is not a metal; it is a giant covalent structure.

3.2.4 Nanoparticles

Nanoscale range: 1–100 nm, not "anything small". Of the order of a few hundred atoms.
Nanoparticle vs bulk: Same substance, different size, different properties, because of the far higher surface area to volume ratio.
Surface area to volume ratio of a cube with sides of 2 nm: Surface area = 6 × 2² = 24 nm². Volume = 2³ = 8 nm³. Ratio = 24 ÷ 8 = 3 nm⁻¹ (or 6 ÷ L).
Is a particle 6.5 × 10⁻⁸ m across a nanoparticle?: 6.5 × 10⁻⁸ ÷ 1 × 10⁻⁹ = 65 nm, which is within 1–100 nm, so yes.
Possible applications of nanoparticles named in 9202: New computers, new catalysts, new coatings, highly selective sensors, stronger and lighter construction materials, and new cosmetics such as suntan creams and deodorants.