Atomic Structure

1. What is the smallest particle of an element that can exist?

Answer: An atom.

2. For each of the following compounds, give their name and state what elements they contain: NaCl, MgO, H2S, AlF3 , CuI2 .

Answer: NaCl (sodium chloride): sodium and chlorine ; MgO (magnesium oxide): magnesium and oxygen ; H2S (hydrogen sulfide): hydrogen and sulfur ; AlF3 (aluminium fluoride): aluminium and fluorine ; CuI2 (copper iodide): copper and iodine

3. Separating mixtures relies on differences in physical properties of the components in the mixtures. For each of the following mixtures, choose either X or Y to describe how the mixture is separated.

X: differences in solubility       Y: differences in boiling points

a) a solid precipitate of lead iodide from a solution of potassium nitrate

b) a mixture of cyclohexane (a liquid hydrocarbon) and water

c) a solution of ethanol dissolved in water

Answer: a) X , b) X , c) Y

4. Name one technique used to separate a soluble solid from its solution.

Answer: Evaporation or crystallisation.

5. Define the terms ‘atomic number’ and ‘mass number’.

Answer: Atomic number is the number of protons in the nucleus of an atom. Mass number is the number of protons and neutrons in the nucleus of an atom.

6. Give the approximate diameter of a typical atom, in metres, using standard form.

Answer: 1 x 10-9 m  (1 nm)

7. Give the term for atoms with the same atomic number but different mass numbers.

Answer: Isotopes.

8. Define the term ‘relative atomic mass’.

Answer: The average mass of atoms of an element that takes into account the mass and amounts of each isotope the element contains.

9. The electronic structure of a sodium atom can be stated as 2-8-1. Give the electronic structures of an oxygen atom and a calcium atom.

Answer: Oxygen atom: 2-6 ; Calcium atom: 2-8-8-2

10. When sodium reacts with water, its products are sodium hydroxide and hydrogen gas.

a) Write a word equation for the reaction between sodium and water.

Answer: sodium + water  →  sodium hydroxide + hydrogen

b) The balanced symbol equation for the reaction between sodium and water is: 2Na + 2H2O  →  2NaOH + H2
Explain why hydrogen is an element, whilst water is a compound.

Answer: Hydrogen contains only one type of atom whilst water contains two types of atoms (hydrogen and oxygen), chemically bonded together in a mixed ratio.

c) State the ratio of hydrogen to oxygen atoms in water.

Answer: H : O ratio is 2 : 1

11. The figure below shows the apparatus used to separate a mixture of substances.

a) Name the separation process being carried out in this figure.

Answer: Distillation.

b) Describe the role of the water-cooled condenser in this separation process.

Answer: The condenser causes the water vapour to condense so that it can be collected as a liquid.

c) Ethanol and water are two liquids that could be separated using this technique. When a mixture of these two liquids is separated, the ethanol is the first liquid to arrive in the conical flask. Explain why this occurs using ideas about boiling points.

Answer: As the mixture of ethanol and water is heated the substance with the lowest boiling point will boil and leave the mixture first. Ethanol must have a lower boiling point than water.

12. Every element has its own type of atoms. For instance, neon atoms are different to magnesium atoms. Use a periodic table to answer the following questions.

a) Explain, in terms of sub-atomic particles, how neon atoms are different to magnesium atoms.

Answer: Ne atoms have 10 protons, whilst Mg atoms have 12; Ne atoms have 10 neutrons whilst Mg atoms have 12; and Ne atoms have 10 electrons, whilst magnesium atoms have 12.

b) Explain why a neon atom has no overall charge.

Answer: Ne atoms have the same number of protons as electrons. Protons are positively charged and electrons are negatively charged. Because there is the same number of each, their charges cancel.

c) When magnesium atoms react they lose their two outer-shell electrons. Determine the charge on the magnesium ions that form. Explain your answer.

Answer: The charge on the magnesium ions that form is 2+. If a Mg atom loses two electrons it will have two more protons than electrons, so their charges will no longer cancel out.

13. When magnesium burns in air, a small proportion of the magnesium reacts with nitrogen to form magnesium nitride. The ratio of magnesium to nitrogen particles in magnesium nitride is 3 : 2

a) Give the chemical formula for magnesium nitride.

Answer: Mg3N2

b) Most nitrogen atoms have an atomic number of 7 and a mass number of 14. The nitrogen particle in magnesium nitride is the nitride ion, N3-. Draw the electronic configuration of the nitride ion.


c) Give the number of protons, neutrons and electrons in a nitride ion, N3-.

Answer: 7 protons, 7 neutrons, 10 electrons

14. Food colouring is often a mixture of different coloured substances dissolved in water. Some students separated a sample of brown food colouring. This is the method they used:

  1. Draw a pencil line 1 cm from the base of a piece of chromatography paper.
  2. Place a small spot of the brown food colouring onto the pencil line.
  3. Fill a beaker with a 1.5 cm depth of water.
  4. Stand the chromatography paper in the beaker.
  5. Leave the paper in the beaker until the water has nearly reached the top of the paper.
  6. Remove the paper and allow it to dry.

a) The students’ experiment did not work. Suggest one improvement to step 3 that would ensure the experiment does work.

Answer: Fill the beaker with water to a depth less than 1 cm.

b) Using an improved method, the students’ chromatography experiment showed that the brown food colouring was made up of three different coloured substances. Describe how chromatography causes the brown food colouring to be separated into its three components.

Answer: During chromatography the three substances move up the paper at different speeds. Substances which are more soluble in the solvent travel further up the paper so the substances are separated across the paper.

c) In another chromatography experiment, three more food colourings (A, B and C) were analysed. The chromatography paper is shown in the figure below. Use this information to compare the three food colourings.

Answer: Colouring A contains three different substances. B and C contain only two different substances. A and B contain the same red substance. B and C contain the same green substance. A and C contain the same blue substance. Only A contains the purple substance.

15. The following particles all have the same electronic arrangement of 2-8-8: K+, Ca2+ , Cl, Ar, S2−. Use a periodic table to answer the following questions.

a) Which particle has the same number of protons as electrons? Which particle is an atom?

Answer: Ar has the same number of protons as electrons, and is also an atom.

b) Which particle has 20 protons but only 18 electrons? Which particle has one more electron than proton?

Answer: Ca2+ has 20 protons but only 18 electron.

c) An atom of phosphorus has an atomic number of 15 and a mass number of 31. Give the number of protons, neutrons and electrons in an atom of phosphorus.

Answer: protons: 15, neutrons 16, electrons 15

d) Describe how the work of Ernest Rutherford, Neils Bohr and James Chadwick changed the ‘plum pudding’ model of the atom to the actual model of the atom.

Answer: The plum-pudding model of the atom described atoms as spheres of positive charge with negatively charged electrons embedded throughout them. Ernest Rutherford’s work proved that the atom consisted of a small central nucleus that was positively charged, with electrons around the nucleus (the nuclear model). Neils Bohr’s work later showed that the electrons orbited the nucleus at certain distances from it, called energy levels (or shells). Finally, James Chadwick discovered neutrons that were the last sub-atomic particle to be discovered.

16. Potassium and ˞fluorine combine in a ratio of 1:1 to form potassium ˞fluoride.

a) Give the chemical formula for potassium ˞fluoride.

Answer: KF

b) Write a balanced symbol equation for the reaction between potassium and ˞fluorine (F2).

Answer: 2K + F2 → 2KF

c) During this reaction, potassium atoms become potassium ions, K+. Give the half equation, including electrons for this change.

Answer: K → K+ + e-

d) During this reaction ˞uorine molecules (F2) become ˞uoride ions, F -. Give the half equation, including electrons for this change.

Answer: F2 + 2e- → 2F -

17. Silicon’s three most stable isotopes are 28Si, 29Si and 30Si.

a) Define the term ‘isotope’.

Answer: An isotope is an atom with the same number of protons and electrons, but a different number of neutrons to other atoms of the same element.

b) Silicon has a relative atomic mass of 28.1 . Choose the most abundant isotope from the list of silicon’s three most stable isotopes.

Answer: Relative atomic mass is the average mass of isotopes taking into account their relative abundances. The relative atomic mass is 28.1, so 28Si must be the most abundant to cause the average mass to be so close to 28.

c) Chlorine has two stable isotopes with mass numbers of 35 and 37. The relative atomic mass of chlorine is 35.5 . Estimate the relative abundances of these two isotopes and choose the most accurate option from the list below:

  • 25% 35Cl and 75% 37Cl
  • 50% 35Cl and 50% 37Cl
  • 55% 35Cl and 45% 37Cl
  • 75%35Cl and 25% 37Cl

Answer: 75% 35Cl and 25% 37Cl

18. The three most common isotopes of sulfur, and their relative abundance, are shown in the table below.

a) 32S and 34S are the two most common isotopes of sulfur. Describe the similarities and differences between these two isotopes.

Answer: Both isotopes have the same number of protons and electrons. The 34S isotope has two more neutrons than the 32S isotope.

b) Calculate the relative atomic mass of sulfur atoms.

Answer: Ar = [(32 × 95) + (33 × 0.8) + (34 × 4.2)] ÷ 100 = 32.092 = 32.1