Questions on protons, neutrons, electrons and isotopes

Question 1

Explain how experimental evidence changed the model of the atom from Dalton's solid sphere to Bohr's energy levels.

Accepted Answer

Indicative content

Dalton's atoms were solid, indivisible spheres, which explained fixed combining ratios but not particles coming out of atoms
Thomson found that cathode rays are negative particles far lighter than any atom, so atoms contain electrons
atoms are neutral, so Thomson proposed the plum pudding model: electrons dotted through a sphere of spread-out positive charge
in the gold foil experiment most alpha particles passed straight through, but a few were deflected through more than 90°
Rutherford concluded that the positive charge and nearly all the mass are in a tiny, dense nucleus, with the atom mostly empty space
the line spectrum of hydrogen showed that its electron has only certain energies, so Bohr placed electrons in fixed energy levels

Levels

Level 3 (5–6 marks) every step from Dalton to Bohr is present with the evidence that forced each change: cathode rays and the electron, the plum pudding model, the alpha scattering results and the nucleus, and the hydrogen line spectrum and energy levels; the order is right and each model is linked to its evidence
Level 2 (3–4 marks) three or four models appear in order with evidence for most of the changes; one link is asserted rather than argued, such as a nucleus with no reason drawn from the few alpha particles that bounced back
Level 1 (1–2 marks) two or three models are listed, often with alpha scattering said to have revealed the electron, and no evidence for why each model changed

Question 2

Deduce the numbers of protons, neutrons and electrons in one ³⁹K⁺ ion and in one ¹²⁷I⁻ ion. Potassium is element 19 and iodine is element 53.

Accepted Answer

B1 ³⁹K⁺ has 19 protons, from the atomic number, and 39 − 19 = 20 neutrons B1 ³⁹K⁺ has 18 electrons, one fewer than its protons because the charge is +1 B1 ¹²⁷I⁻ has 53 protons and 127 − 53 = 74 neutrons B1 ¹²⁷I⁻ has 54 electrons, one more than its protons because the charge is −1

Question 3

Explain what is meant by the term isotopes, and why chlorine-35 and chlorine-37 have the same chemical properties.

Accepted Answer

B1 isotopes are atoms of the same element, with the same number of protons B1 they have different numbers of neutrons; reject 'different mass' alone B1 the two isotopes have the same number of electrons, in the same arrangement or electron configuration B1 chemical reactions involve the electrons, not the nucleus, so the isotopes react in the same way

Question 4

Describe the gold foil experiment carried out by Geiger and Marsden, and the observations it produced.

Accepted Answer

B1 a beam of alpha particles was fired at a very thin sheet of gold foil B1 the alpha particles were detected where they struck a zinc sulfide screen placed around the foil B1 most of the alpha particles passed straight through the foil, undeflected or deflected by a small angle B1 a very small number were deflected through large angles, some by more than 90° back towards the source

Question 5

Explain how the results of the gold foil experiment led to the nuclear model of the atom and ruled out the plum pudding model.

Accepted Answer

B1 most alpha particles passed straight through, so most of the atom is empty space B1 a few were deflected through large angles, so they were repelled by a small region of concentrated positive charge holding most of the mass B1 large deflections were very rare, so this nucleus is very small compared with the whole atom B1 in the plum pudding model the positive charge is spread through the atom, which could not turn an alpha particle through a large angle

Question 6

Compare the atoms ³⁵Cl and ³⁷Cl and the ion ³⁵Cl⁻ in terms of their numbers of protons, neutrons and electrons.

Accepted Answer

B1 all three have 17 protons B1 ³⁵Cl and ³⁵Cl⁻ have 18 neutrons, while ³⁷Cl has 20 B1 ³⁵Cl and ³⁷Cl have 17 electrons, while ³⁵Cl⁻ has 18 B1 ³⁵Cl⁻ differs from ³⁵Cl only in having one more electron, and ³⁷Cl differs from ³⁵Cl only in its nucleus

Question 7

Deduce the identity, the mass number and the charge of a species that has 16 protons, 18 neutrons and 18 electrons, and write its symbol.

Accepted Answer

B1 it is sulfur, because it has 16 protons B1 its mass number is 16 + 18 = 34 B1 its charge is 2−, because it has two more electrons than protons B1 the symbol is ³⁴S²⁻; allow ³⁴₁₆S²⁻

Question 8

Evaluate the claim that isotopes are the radioactive forms of an element.

Accepted Answer

B1 isotopes are defined by the same number of protons and a different number of neutrons; the definition says nothing about radioactivity B1 many isotopes are stable, such as chlorine-35, chlorine-37 and carbon-12 B1 some isotopes are radioactive, such as tritium and carbon-14, so the claim is true of some isotopes only B1 the claim is therefore false as a definition: the ordinary atoms of an element are themselves isotopes

Question 9

Explain why beams of protons, neutrons and electrons, entering the same electric field at the same speed, behave differently between two charged plates.

Accepted Answer

B1 electrons are negative and protons positive, so electrons are attracted to the positive plate and protons to the negative plate B1 the angle of deflection is proportional to charge ÷ mass B1 the charges are equal in size but the electron's mass is 1840 times smaller, so it is deflected about 1840 times as much B1 a neutron has no charge, so the field exerts no force on it and the beam passes straight through

Question 10

State the relative mass and the relative charge of a proton, a neutron and an electron.

Accepted Answer

B1 a proton has a relative mass of 1 and a relative charge of +1 B1 a neutron has a relative mass of 1 and a relative charge of 0 B1 an electron has a relative mass of 1/1840 and a relative charge of −1; allow a relative mass of 0.0005 or 'negligible'

Question 11

Calculate the percentage of the mass of a ¹H atom that is due to its electron, using relative masses of 1 for the proton and 1/1840 for the electron.

Accepted Answer

M1 the relative mass of the atom is 1 + 1/1840 = 1.000543 M1 percentage = (1/1840) ÷ 1.000543 × 100 A1 0.0543% to three significant figures; allow 0.0543% from dividing by 1

Question 12

Explain why heavy water, D₂O, has a higher density than ordinary water, H₂O, although the two take part in the same chemical reactions.

Accepted Answer

B1 a deuterium nucleus holds a proton and a neutron, while a ¹H nucleus is a single proton B1 a D₂O molecule has a relative mass of 20 against 18 for H₂O, while the molecules take up almost the same volume, so the density is higher B1 deuterium has the same number of electrons in the same arrangement as ¹H, and reactions depend on electrons, so the reactions are the same

Question 13

Explain how the line spectrum of hydrogen led Bohr to propose that electrons occupy fixed energy levels.

Accepted Answer

B1 heated hydrogen gives out light at only certain exact wavelengths, not a continuous spectrum B1 each wavelength corresponds to a fixed amount of energy, given out when an electron drops to a lower level B1 the electron can therefore have only certain energies, which means it occupies fixed energy levels

Question 14

Suggest why a sample of chlorine made only of chlorine-37 diffuses through a small hole more slowly than natural chlorine at the same temperature, although its chemical reactions are the same.

Accepted Answer

B1 ³⁷Cl₂ molecules have a relative mass of 74, greater than the mean of about 71 for natural chlorine B1 at the same temperature the molecules have the same mean kinetic energy, so the heavier molecules move more slowly and diffuse more slowly B1 the electrons are the same in number and arrangement, so the chemical reactions are unchanged

Question 15

Calculate the angle through which a beam of ⁴He²⁺ ions is deflected in an electric field that deflects a beam of ¹H⁺ ions, entering at the same speed, through 3.0°, and state its direction.

Accepted Answer

M1 charge ÷ mass is 2 ÷ 4 = 0.5 for ⁴He²⁺ and 1 ÷ 1 = 1 for ¹H⁺ M1 angle = 3.0° × 0.5 ÷ 1 A1 1.5°, to two significant figures, towards the negative plate, the same direction as the ¹H⁺ beam

Question 16

Suggest why the discovery of the neutron in 1932 was needed to explain the mass of a helium atom.

Accepted Answer

B1 a helium nucleus has a charge of +2, so it holds 2 protons, but its relative mass is 4 B1 the extra mass of 2 must come from particles with mass and no charge, which are neutrons