Fig. 1.1 shows part of the Periodic Table.
Answer the following questions using only the elements in Fig. 1.1. Each symbol of the element may be used once, more than once or not at all.
Give the symbol of the element that:
(a)(i) produces a lilac colour in a flame test
(a)(ii) has an atom with only two occupied electron shells
(a)(iii) is an unreactive gas
(a)(iv) forms an ion that gives a white precipitate after the addition of excess sodium hydroxide
(a)(v) forms an ion with a charge of 2-
(a)(vi) is added to iron to make stainless steel.
(b) Stainless steel is a mixture. State two characteristics of a mixture.
▶️ Answer/Explanation
(a)(i) K (Potassium)
Potassium is well known for producing a lilac (pale purple) color in flame tests, which is a characteristic test for this element.
(a)(ii) F (Fluorine)
Fluorine has an atomic number of 9, with electron configuration 2,7. This means it has electrons in only two shells (K and L shells).
(a)(iii) Ar (Argon)
Argon is a noble gas in Group VIII (18) which is chemically unreactive due to its complete outer electron shell.
(a)(iv) Ca (Calcium)
Calcium forms Ca²⁺ ions which react with sodium hydroxide to form white calcium hydroxide precipitate, Ca(OH)₂, which remains insoluble even in excess NaOH.
(a)(v) S (Sulfur)
Sulfur forms S²⁻ ions (sulfide ions) with a 2- charge, as it gains two electrons to achieve a stable electron configuration.
(a)(vi) Ni (Nickel)
Nickel is commonly added to iron to make stainless steel, which improves its corrosion resistance and mechanical properties.
(b) Any two of:
- Contains two or more different substances not chemically bonded
- Does not have a fixed amount of each substance in it (has variable composition)
- Substances in mixture can be separated by physical methods
- Substances in mixture keep their own chemical properties
These are fundamental characteristics that distinguish mixtures from compounds. In stainless steel, the iron and nickel (and often chromium) retain their individual properties and can be separated by physical means.
Fig. 1.1 shows part of the Periodic Table.
Answer the following questions using only the elements in Fig. 1.1. Each symbol of the element may be used once, more than once or not at all.
(a) Give the symbol of the element that:
(i) is 78% of clean, dry air
(ii) forms an ion with a charge of 3+
(iii) has an atom with only five occupied electron shells
(iv) forms an ion that gives a light green colour in a flame test
(v) is used in food containers because of its resistance to corrosion
(vi) is the metal with the lowest reactivity.
(b) Helium is a monatomic gas.
(i) State the meaning of the term monatomic.
(ii) Explain in terms of electronic configuration why helium is unreactive.
▶️ Answer/Explanation
(a)
(i) N (Nitrogen makes up about 78% of Earth’s atmosphere)
(ii) Al (Aluminum forms Al³⁺ ions by losing 3 electrons)
(iii) I (Iodine is in period 5, meaning its electrons occupy 5 shells)
(iv) Ba (Barium ions (Ba²⁺) produce a characteristic light green flame color)
(v) Al (Aluminum is used in food containers due to its corrosion-resistant oxide layer)
(vi) Au (Gold is the least reactive metal among the options)
(b)
(i) Monatomic means consisting of single atoms (not molecules). Helium exists as individual He atoms rather than as molecules like O₂ or N₂.
(ii) Helium is unreactive because it has a complete outer electron shell (2 electrons in its only shell, following the duplet rule). This stable configuration means it doesn’t need to gain, lose, or share electrons to become more stable.
Detailed Explanation:
For part (a), we need to carefully examine the periodic table fragment provided and recall key properties of the elements:
- Nitrogen (N) is the main component of air (78%)
- Aluminum (Al) is in group III and forms 3+ ions
- Iodine (I) is in period 5, meaning its electrons fill 5 shells (K, L, M, N, O)
- Barium (Ba) gives a light green flame test color
- Aluminum (Al) forms a protective oxide layer making it corrosion-resistant
- Gold (Au) is the least reactive metal shown (noble metal)
For part (b), helium’s monatomic nature and inertness are due to its complete valence shell (1s² configuration), which makes it extremely stable and unreactive under normal conditions.