Atoms, Elements and Compounds: Unit Review

(a) element; (b) solution (acetic acid dissolved in water); (c) compound; (d) mechanical mixture.

D = 107.5 ÷ 12.0 ≈ 8.96 g/cm³ → copper.

Chemical: burning makes a new substance (magnesium oxide).

A new colour (brown), a new smell, and it can't be reversed: any two.

Octagon: the contents are dangerous (here, flammable). Triangle: the container is dangerous (it could burst or explode).

Lemon juice, milk, pure water, ammonia cleaner.

3 steps: 10 × 10 × 10 = 1000 times.

They react to give off toxic chlorine gas.

Democritus's was an idea with no evidence; Dalton's theory was supported by experimental evidence (laws of conservation of mass and definite proportions).

9.0 g (mass is conserved).

Atoms contain tiny negative particles (electrons), found in all atoms.

(a) the atom is mostly empty space; (b) there is a concentrated positive charge; (c) the nucleus is tiny and dense.

It explained why electrons don't spiral into the nucleus and why elements give off line spectra: electrons occupy fixed energy levels. Evidence: line spectra.

The neutron: it explains the extra mass of atoms that protons alone can't account for.

Nucleus, relative mass 1, charge 0.

18 p, 22 n, 18 e⁻.

Same element (both 17 protons) but different masses (18 vs 20 neutrons): they are isotopes.

Calcium-40.

A: 7 protons, arrangement 2, 5 → nitrogen, 5 valence electrons. B: 13 protons, arrangement 2, 8, 3 → aluminium, 3 valence electrons.

2, 8, 8, 1.

Cl with 7 dots: three pairs and one single dot.

Period 3, group 16: sulfur.

Similar properties, because they have the same number of valence electrons.

Atomic number (protons) identifies each element; ordering by it puts every element with the right family, fixing cases like argon and potassium.

Metal.

Along the staircase line; for example boron, silicon, germanium, arsenic.

It conducts electricity, which most nonmetals don't.

Potassium: its valence electron is in a higher energy level, farther from the nucleus, so it is lost more easily.

Fluorine: smaller atoms attract an incoming electron more strongly; halogen reactivity decreases down the group.

Property: extremely unreactive (full valence shell). Use: helium balloons, neon signs, argon in welding (any one).

Sodium hydroxide and hydrogen gas.

Ca²⁺ and N³⁻ → Ca₃N₂, calcium nitride.

13 − 3 = 10 electrons.

Potassium sulfide; magnesium bromide.

When melted, the ions can move and carry charge; in the solid they are locked in the lattice.

Dinitrogen pentoxide, carbon tetrachloride; SF₆, N₂O.

C in the centre with 4 single bonds to H atoms; no lone pairs.

It is molecular: the formula gives the actual atoms in one molecule and is not reduced.

Molecular: SO₂, NH₃. Ionic: CaCl₂, Li₂O.

They are opposite reactions: photosynthesis uses CO₂ and H₂O and makes glucose and O₂; respiration does the reverse. At equal rates, the amounts of CO₂ and O₂ stay about steady.

Ice is less dense and floats, insulating the water, and the densest water (about 4 °C) stays liquid at the bottom.

Warm water holds less dissolved oxygen.

Extract, process/refine, manufacture/package, transport/sell, use, end of life.

Social: jobs, or harm to community health. Environmental: habitat loss or water pollution. Economic: income and exports, or cleanup costs (any reasonable set).

Keep it longer / repair it; use recycled metals; recycle it through e-waste programs (any two).

Positive: less mining, less waste. Negative: uses chemicals and energy; damaged batteries can catch fire.

It is a noble gas with a full valence shell, so it doesn't react and keeps oxygen away from the hot metal.

Any three: cost, raw materials, regulations, public demand, environmental impact, skilled workers, consultation with affected communities.