The Atom

28 Grade 10 Chemistry questions on The Atom, with worked marking schemes. 28 questions are available on this topic in full.

1. Question 1

   1 mark

   Magnesium has atomic number 12. When magnesium ribbon burns in air it forms the ion Mg2+. The number of electrons in one Mg2+ ion is

   A. 10

   B. 24

   C. 14

   D. 12

   (1 mark)

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2. Question 2

   3 marks

   Potassium has atomic number 19. A learner writes its electron arrangement as 2.8.9. Explain why this is wrong and write the correct arrangement.

   (3 marks)

   Marking scheme

   Electrons fill the energy levels starting with the one nearest the nucleus, and the third energy level holds a maximum of 8 electrons for the first twenty elements (1). Therefore 9 electrons cannot be placed in the third energy level; the nineteenth electron enters the fourth energy level (1). Correct arrangement: 2.8.8.1 (1).

3. Question 3

   2 marks

   Suggest why elements in the periodic table are arranged in order of atomic number rather than in order of mass number.

   (2 marks)

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4. Question 4

   5 marks

   Explain the term valency and determine the valency of the elements whose electron arrangements are 2.8.2 and 2.8.7, giving a reason in each case.

   (5 marks)

   Marking scheme

   Valency is the combining power of an element, that is, the number of electrons an atom loses, gains or shares in order to attain a stable electron arrangement (1). 2.8.2: valency is 2 (1) because the atom loses the two electrons in its outermost energy level to attain the stable arrangement 2.8 (1). 2.8.7: valency is 1 (1) because the atom gains one electron to fill its outermost energy level to 8 and attain the stable arrangement 2.8.8 (1).

5. Question 5

   2 marks

   State two differences between a proton and an electron.

   (2 marks)

   Marking scheme

   Any TWO of the following, 1 mark each: a proton carries a positive charge while an electron carries a negative charge (1); a proton has a relative mass of 1 while the mass of an electron is negligible, about $\frac{1}{1840}$ that of a proton (1); a proton is found inside the nucleus while an electron occupies the energy levels outside the nucleus (1). Maximum 2 marks.

6. Question 6

   1 mark

   Isotopes of the same element take part in chemical reactions in exactly the same way because they have the same

   A. density

   B. number of electrons in the outermost energy level

   C. number of neutrons in the nucleus

   D. mass number

   (1 mark)

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7. Question 7

   1 mark

   Which model of the atom described it as a ball of positive charge in which electrons were embedded, rather like seeds spread through the flesh of a pawpaw?

   A. Bohr's model

   B. Dalton's model

   C. Rutherford's model

   D. Thomson's model

   (1 mark)

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8. Question 8

   4 marks

   Energy must be supplied to remove an electron from a sodium atom, and less energy is needed to remove the outermost electron than an electron from the second energy level. Explain these observations.

   (4 marks)

   Marking scheme

   The electrons are negatively charged and are attracted by the positively charged nucleus (1); energy must therefore be supplied to overcome this attraction and pull an electron away (1). The outermost electron is further from the nucleus, so the force of attraction on it is weaker (1). It is also screened (shielded) from the nucleus by the inner energy levels of electrons, so less energy is required to remove it (1).

9. Question 9

   1 mark

   Which property is the same for every atom of a given element and is different for the atoms of every other element?

   A. The number of neutrons in the nucleus

   B. The number of protons in the nucleus

   C. The number of electrons in the outermost energy level

   D. The mass number of its atoms

   (1 mark)

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10. Question 10

    6 marks

    In a classic experiment, fast-moving positively charged particles were directed at a very thin sheet of gold foil.

    (a) State three observations that were made.

    (b) State the conclusion drawn from each observation.

    (6 marks)

    Marking scheme

    (a) Observations, 1 mark each: most of the particles passed straight through the foil undeflected (1); a small number were deflected through small angles (1); a very few were deflected through large angles or bounced straight back (1). (b) Conclusions, 1 mark each, matched to the observation above: the atom is largely empty space (1); the centre of the atom carries a positive charge that repels the positively charged particles passing near it (1); the positive charge and almost all the mass of the atom are concentrated in a very small, dense nucleus (1).

11. Question 11

    3 marks

    A teacher tells the class that naturally occurring helium consists of helium-3 and helium-4. Explain what this statement means and state which of the two has more neutrons.

    (3 marks)

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12. Question 12

    1 mark

    An element has the electron arrangement 2.8.3. In the periodic table, this element is placed in

    A. Group III, period 4

    B. Group III, period 2

    C. Group III, period 3

    D. Group II, period 3

    (1 mark)

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13. Question 13

    2 marks

    An atom of sodium is written in the form 23 over 11 Na. State what each of the two numbers represents.

    (2 marks)

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14. Question 14

    1 mark

    Sulphur has atomic number 16. When a sulphur atom reacts with a metal it forms an ion whose electron arrangement and charge are

    A. 2.8.8 and 2−

    B. 2.8.4 and 2−

    C. 2.8.8 and 2+

    D. 2.8.6 and 2+

    (1 mark)

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15. Question 15

    1 mark

    Which statement about the nucleus of an atom is correct?

    A. It has a negligible mass compared with the rest of the atom

    B. It contains protons and electrons held together by attraction

    C. It occupies most of the volume of the atom

    D. It contains protons and neutrons and carries a positive charge

    (1 mark)

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16. Question 16

    2 marks

    Explain why an atom of an element is electrically neutral while an ion of the same element carries a charge.

    (2 marks)

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17. Question 17

    3 marks

    Argon, which is used to fill some electric bulbs, has the electron arrangement 2.8.8. Account for the fact that argon does not readily form compounds.

    (3 marks)

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18. Question 18

    3 marks

    An atom of phosphorus contains 15 protons, 16 neutrons and 15 electrons.

    (a) State its atomic number.

    (b) Determine its mass number.

    (c) State the number of electrons in its outermost energy level.

    (3 marks)

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19. Question 19

    3 marks

    The relative atomic mass of chlorine is given in data booklets as 35.5. State what is meant by relative atomic mass and account for this value not being a whole number.

    (3 marks)

    Marking scheme

    Relative atomic mass is the average mass of the atoms of an element compared with one twelfth of the mass of one atom of carbon-12 (1; accept 'average mass of one atom of an element compared with $\frac{1}{12}$ of an atom of carbon-12'). Chlorine exists as two isotopes, chlorine-35 and chlorine-37, whose atoms have different masses (1). The value 35.5 is the weighted average of the masses of the isotopes taking account of their relative abundances, so it need not be a whole number (1).

20. Question 20

    4 marks

    Sodium has atomic number 11. Describe, in terms of electrons, how a sodium atom changes into a sodium ion, and state two ways in which the ion differs from the atom.

    (4 marks)

    Marking scheme

    Electron transfer: the sodium atom has the arrangement 2.8.1 and loses the single electron in its outermost energy level to another atom (1); it is left with the arrangement 2.8, which is full and stable (1). Differences, any TWO, 1 mark each: the atom is electrically neutral while the ion carries a charge of 1+ (1); the atom has 11 electrons while the ion has 10 electrons (1); the ion is smaller than the atom because it has one fewer energy level (1); the atom is reactive while the ion is stable (1). Maximum 4 marks overall.

21. Question 21

    1 mark

    Chlorine has atomic number 17. The electron arrangement of a chlorine atom is

    A. 2.8.7

    B. 2.7.8

    C. 2.8.8

    D. 8.8.1

    (1 mark)

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22. Question 22

    1 mark

    Which of the following pairs of particles are isotopes of the same element?

    A. A particle with 17 protons and 18 neutrons, and a particle with 18 protons and 17 neutrons

    B. A particle with 6 protons and 6 neutrons, and a particle with 6 protons and 8 neutrons

    C. A particle with 11 protons and 11 electrons, and a particle with 11 protons and 10 electrons

    D. A particle with 20 protons and 20 neutrons, and a particle with 18 protons and 22 neutrons

    (1 mark)

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23. Question 23

    1 mark

    The maximum number of electrons that can occupy the second energy level of an atom is

    A. 2

    B. 10

    C. 8

    D. 18

    (1 mark)

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24. Question 24

    1 mark

    An atom of argon is represented as argon-40 with atomic number 18. The number of neutrons in this atom is

    A. 40

    B. 22

    C. 58

    D. 18

    (1 mark)

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25. Question 25

    1 mark

    Copper used in electrical cables consists of two isotopes: 69% of atoms have mass number 63 and 31% have mass number 65. The relative atomic mass of copper, correct to one decimal place, is

    A. 65.0

    B. 63.6

    C. 64.0

    D. 63.0

    (1 mark)

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26. Question 26

    1 mark

    An ion of an element X is represented as X2−. The ion contains 18 electrons and an atom of X contains 16 neutrons. The mass number of X is

    A. 34

    B. 32

    C. 18

    D. 30

    (1 mark)

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27. Question 27

    5 marks

    Elements X and Y have atomic numbers 12 and 17 respectively.

    (a) Write the electron arrangement of each element.

    (b) State the charge on the ion formed by each element.

    (c) Identify, giving a reason, which of the two elements is a metal.

    (5 marks)

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28. Question 28

    1 mark

    The mass of an atom is taken to be equal to its mass number, and the electrons are left out of the count. This is because

    A. electrons keep moving rapidly around the nucleus

    B. an atom contains very few electrons

    C. the negative charge of the electrons cancels the positive charge of the protons

    D. the mass of an electron is negligible compared with that of a proton

    (1 mark)

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