Atomic Structure and Periodic Table: Models and Trends

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.

The correct answer is A, Fluorine. Electronegativity is the tendency of an atom to attract a shared pair of electrons towards itself. It increases from left to right across a period, as nuclear charge grows, and decreases down a group, as each new shell puts the outer electrons further from the nucleus. Fluorine stands at the top of group 17 and is the smallest halogen, so it holds a shared pair most tightly and is the most electronegative element of all, with a value of about 4.0 on the Pauling scale. Options B, C and D are wrong because bromine, iodine and chlorine all lie below fluorine in the same group, and electronegativity falls as you go down, in the order fluorine, then chlorine, then bromine, then iodine. Remember also that fluorine is the most electronegative element in the whole table, not merely among the halogens, and that caesium is among the least electronegative.

The correct answer is B, atomic number. Henry Moseley showed in 1913, from the frequencies of the X-rays that elements emit, that the atomic number, the number of protons in the nucleus, is the more fundamental property of an element. The periodic law was therefore restated: the properties of elements are a periodic function of their atomic numbers. Option A is wrong because atomic mass was the basis of Mendeleev's law of 1869, which could not place isotopes and needed pairs such as cobalt and nickel to be put out of mass order. Option C is wrong because density is a physical property that shows no simple periodic pattern of this kind. Option D is wrong because valency follows from the electron arrangement and so is itself a consequence of the atomic number, not the basis of the law.

The correct answer is B, Thorium. John Newlands arranged the fifty-six elements known in 1866 in order of increasing atomic mass, beginning with hydrogen, the lightest, and ending with thorium, the fifty-sixth. He noticed that every eighth element had properties like the first, and named the pattern the Law of Octaves after the musical scale. The law failed once more elements were found, because there was no room left in the arrangement for them. Option A is wrong because radium was discovered only in 1898 by Marie and Pierre Curie, long after Newlands' table. Option C is wrong because iron lies in the middle of the arrangement, not at its end. Option D is wrong because rhodium too sits among the earlier elements and was not the last in his list.

The correct answer is C, 7 periods and 18 groups. The modern periodic table has seven horizontal rows, called periods, and eighteen vertical columns, called groups. The first period holds only two elements, the second and third hold eight each, the fourth and fifth hold eighteen each and the sixth holds thirty-two, while the lanthanides and actinides are shown in two separate rows below. Option A is wrong because it reverses the two counts. Option B is wrong because eighteen is the number of groups, not of periods. Option D is wrong because eight groups belonged to Mendeleev's older table, which had eight vertical columns; the eighteen-group form came with the long form of the modern table, in which elements of a group share the same number of outermost electrons. A useful check is that seven is always the smaller of the two numbers, because the table is wider than it is tall.

The correct answer is A, atomic number but different mass number. Isotopes are atoms of the same element, so they have the same number of protons and therefore the same atomic number, but they carry different numbers of neutrons, which makes their mass numbers differ. Hydrogen has three isotopes, protium, deuterium and tritium, and chlorine occurs naturally as chlorine-35 and chlorine-37. Option B is wrong because atoms with the same mass number but different atomic numbers are isobars, such as argon-40 and calcium-40. Option C is wrong because atoms with the same number of neutrons but different numbers of protons are called isotones. Option D is wrong because atoms with the same number of protons and the same number of neutrons are simply identical atoms of one isotope, not isotopes of each other. Because isotopes differ only in neutrons, they share the same chemical behaviour and differ in physical properties such as density and radioactivity.

The correct answer is B, J. J. Thomson. After discovering the electron in 1897, J. J. Thomson pictured the atom as a sphere of positive charge with negatively charged electrons stuck in it, a picture often compared to plums in a pudding or seeds in a watermelon. It explained why an atom is electrically neutral, but it had no nucleus and could not survive Rutherford's scattering experiment. Option A is wrong because John Dalton's 1808 theory treated the atom as indivisible and offered no internal structure at all. Option C is wrong because Rutherford replaced this model with the nuclear one after the gold foil experiment. Option D is wrong because Niels Bohr then added fixed permitted shells to Rutherford's nucleus in order to explain why the atom is stable.

The correct answer is C, Scandium, gallium and germanium. Mendeleev arranged the elements by increasing atomic mass in 1869 and left blank spaces where his pattern demanded an element that was not yet known. He described in advance the mass, density and compounds of three of them, calling them eka-boron, eka-aluminium and eka-silicon, and when scandium, gallium and germanium were discovered they matched his descriptions closely, which won the periodic law wide acceptance. Option A is wrong because sodium, potassium and calcium were all well known long before 1869. Option B is wrong because cobalt, nickel and copper were known too, and cobalt and nickel were in fact the pair he had to place out of strict mass order. Option D is wrong because radium, radon and rhenium were found much later and were not among his three predictions.