Free Reactions of Group I Elements MCQs with Answers

1. Down group I from lithium to caesium, the reactivity of the metals

• A. decreases, because atomic radius increases
• B. stays constant
• C. decreases, because ionisation energy falls
• D. increases, because the outer electron is further from the nucleus and more easily lost

Explanation: Each element down the group has an extra shell, so the single outer electron is further from the nucleus and better shielded, ionisation energy falls and the metal loses that electron more readily. This is why potassium ignites the hydrogen it releases from water while lithium merely fizzes. In the halogens the same reasoning gives the opposite trend, since there reactivity depends on gaining an electron.

2. The elements of group I are called the alkali metals because they

• A. are found free in nature
• B. react with water to form hydroxides that are strongly basic
• C. are all radioactive
• D. form acidic oxides

Explanation: Sodium and its neighbours react with water to give hydrogen and a hydroxide solution that is strongly alkaline, which is the origin of the family name. Because they are so reactive they are never found uncombined in nature and are stored under oil. Their oxides are basic, not acidic, which is typical of metals.

3. When sodium reacts with excess oxygen on burning, the main product is

• A. sodium oxide, Na2O
• B. sodium superoxide, NaO2
• C. sodium peroxide, Na2O2
• D. sodium hydroxide, NaOH

Explanation: Lithium forms the simple oxide, sodium forms mainly the peroxide and potassium, rubidium and caesium form superoxides, the larger cations being better able to stabilise the larger anions. Sodium peroxide reacts with water to give hydrogen peroxide and alkali. Sodium hydroxide is the product of reaction with water rather than with oxygen.

4. The flame colour produced by potassium compounds is

• A. golden yellow
• B. brick red
• C. apple green
• D. lilac

Explanation: Heating excites the outer electron to a higher level and the colour is emitted as it falls back, the energy gap being characteristic of the element, so potassium gives lilac, sodium golden yellow, calcium brick red and barium apple green. The flame test is the simplest analytical use of atomic emission spectra. Sodium contamination easily masks the potassium colour, so it is viewed through blue glass.