Magnesium (proton number 12) has three naturally occurring isotopes. Table 12.1 gives their nucleon numbers and abundances. Table 12.1 Isotope of magnesium ...
Question 1 Report
Magnesium (proton number 12) has three naturally occurring isotopes. Table 12.1 gives their nucleon numbers and abundances.
Table 12.1
| Isotope of magnesium | Nucleon number | Abundance / % |
|---|---|---|
| magnesium-24 | 24 | 79 |
| magnesium-25 | 25 | 10 |
| magnesium-26 | 26 | 11 |
(a) State, in terms of subatomic particles, how an atom of magnesium-24 differs from an atom of magnesium-26. [2]
(b) Calculate the relative atomic mass of magnesium using all three isotopes. Give your answer to two decimal places and show your working. [3]
(c) Write the full electronic configuration of a magnesium atom. [1]
(d) Fig. 12.1 shows the electron arrangement of the magnesium ion.
Fig. 12.1
(i) Deduce the charge on the magnesium ion and write its symbol. [2]
(ii) Explain, in terms of electrons and protons, how the magnesium atom forms this ion and why the ion is smaller than the atom. [3]
(e) Magnesium is in Group II and Period 3. State what the group number and period number each tell you about a magnesium atom. [2]
(f) Magnesium burns in oxygen. Write a balanced symbol equation for this reaction and state the type of oxide formed (acidic or basic). [3]
Answer Details
(a) Both magnesium-24 and magnesium-26 have 12 protons and 12 electrons [1]; they differ only in neutrons, magnesium-24 having 12 neutrons and magnesium-26 having 14 neutrons [1]. Isotopes are atoms of the same element with different numbers of neutrons.
(b) Relative atomic mass is the weighted mean of the three isotope masses [method 1]:
\[ A_r = \frac{(24\times 79) + (25\times 10) + (26\times 11)}{100} \] \[ = \frac{1896 + 250 + 286}{100} = \frac{2432}{100} \quad [1] \] \[ = 24.32 \quad [1] \](c) Magnesium has 12 electrons, so its full configuration is 2,8,2 [1].
(d)(i) The ion drawn shows only 10 electrons (2,8), but the nucleus still holds 12 protons, so the charge is 2+ and the symbol is \(\text{Mg}^{2+}\) [2+, 1; symbol, 1].
(d)(ii) The atom (2,8,2) loses its 2 outer electrons [1]; it then has 12 protons but only 10 electrons, giving a 2+ charge [1]. The ion is smaller than the atom because the outermost occupied shell has been removed, and the 12 protons now pull the remaining 10 electrons in more strongly [1].
(e) Group II tells you there are 2 electrons in the outer shell [1]; Period 3 tells you there are 3 occupied electron shells [1].
(f) Magnesium burns in oxygen to form magnesium oxide [formulae 1; balanced 1]:
\[ 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \]Magnesium is a metal, so its oxide is a basic oxide [1] (metal oxides are basic and react with acids to form a salt and water).
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