The plum pudding model of the atom, in which electrons are embedded in a sphere of positive charge, was proposed by:

The correct answer is A, Democritus. The Greek philosopher Democritus, working in the fifth century before the common era, argued that matter could be divided only up to a limit, and he called that smallest indivisible particle atomos, from which the word atom comes. His was a philosophical claim, not an experimental one. Option B is wrong because J. J. Thomson discovered the electron in 1897 and proposed the plum pudding model of the atom. Option C is wrong because Ernest Rutherford performed the alpha-particle scattering experiment, discovered the nucleus and identified the proton, but the word was in use long before him. Option D is wrong because John Dalton gave the first scientific atomic theory in 1808, using the existing word rather than coining it. A quick way to keep these apart is that Democritus named the atom, Dalton theorised it, Thomson found what is inside it and Rutherford found its nucleus.

The correct answer is A, J. Chadwick. James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles and showing that the radiation produced consisted of neutral particles of nearly the same mass as the proton. The discovery explained why the mass of an atom is greater than the mass of its protons and electrons together. Option B is wrong because E. Goldstein discovered canal rays, which carried positive charge and led to the identification of the proton. Option C is wrong because Niels Bohr gave the model in which electrons revolve in fixed permitted shells without radiating energy. Option D is wrong because Ernest Rutherford discovered the nucleus through alpha-particle scattering and had only predicted that a neutral particle should exist. Note the years, which papers like to ask together: the electron in 1897, the nucleus in 1911 and the neutron in 1932, the last of the three fundamental particles to be found.

The correct answer is D, almost all the mass and the positive charge lie in a tiny nucleus. Most alpha particles passed straight through the foil, a few were deflected through small angles and about one in twenty thousand bounced back. Rutherford read this as meaning that the atom is mostly empty space, that its positive charge and nearly all its mass are packed into a very small central nucleus, and that electrons move around it. Option A is wrong because that is Thomson's plum pudding model, which the experiment disproved. Option B is wrong because indivisibility was Dalton's postulate, already broken by the discovery of the electron. Option C is wrong because fixed permitted orbits came later, with Bohr, who introduced them to cure the instability in Rutherford's own model.

The correct answer is C, 18. The capacity of a shell is given by the formula 2n squared, where n is the number of the shell counted outward from the nucleus. The M shell is the third, so its capacity is 2 multiplied by 3 squared, which is 18. Option A is wrong because 8 is the capacity of the L shell, the second, and is also the greatest number of electrons an outermost shell may hold, which is a different rule. Option B is wrong because 2 is the capacity of the K shell, the first and nearest to the nucleus. Option D is wrong because 32 is the capacity of the N shell, the fourth. Note that in an atom such as calcium the M shell stops at 8 because it is the outermost shell at that stage, which does not change its full capacity of 18.

The correct answer is D, First group. The alkali metals are lithium, sodium, potassium, rubidium, caesium and francium. Each has a single electron in its outermost shell, loses it readily to form a positive ion, and therefore stands in group 1 of the modern periodic table. They are soft, highly reactive metals whose oxides and hydroxides give strongly basic, or alkaline, solutions, which is where the family name comes from. Option A is wrong because group 2 holds the alkaline earth metals, such as beryllium, magnesium and calcium, with two outermost electrons. Option B is wrong because group 18 holds the noble gases, which are inert because their outermost shells are complete. Option C is wrong because group 3 begins the d-block transition elements, starting with scandium. The safe rule for such questions is to count the outermost electrons: one places the element in group 1, two in group 2, seven in group 17 and a complete shell in group 18.