All Free Chemistry MCQs with Answers

151. Group II elements are called alkaline earth metals and, compared with group I, they are

• A. less reactive, with higher melting points and harder solids
• B. more reactive in every case
• C. non metallic in character
• D. monovalent in their compounds

Explanation: Group II atoms are smaller and must lose two electrons rather than one, so more energy is needed and they are less reactive than their group I neighbours. Having two delocalised electrons per atom also gives stronger metallic bonding, hence higher melting points and greater hardness. They are divalent, forming ions with a charge of plus 2.

152. The thermal stability of the group II carbonates from magnesium to barium

• A. decreases down the group
• B. is the same for all
• C. increases down the group
• D. first increases then decreases

Explanation: A small, highly charged cation polarises the carbonate ion strongly and weakens the carbon to oxygen bonds, so magnesium carbonate decomposes at a relatively low temperature while barium carbonate needs a much higher one. Polarising power falls as the cation gets larger down the group, so stability rises. The nitrates and hydroxides follow the same pattern for the same reason.

153. Which of the following group II compounds is used in medicine as an antacid?

• A. Barium sulphate
• B. Magnesium hydroxide
• C. Calcium carbide
• D. Beryllium chloride

Explanation: Magnesium hydroxide, sold as milk of magnesia, is a weak insoluble base that neutralises excess stomach acid without being absorbed in quantity. Barium sulphate is also used medically but as a contrast medium for X rays, and it is safe only because it is so insoluble that the toxic barium ions are not released. Beryllium compounds are highly toxic.

154. The elements of group IV show a gradation in character from

• A. metallic carbon to non metallic lead
• B. non metallic carbon through metalloid silicon and germanium to metallic tin and lead
• C. entirely metallic throughout
• D. entirely non metallic throughout

Explanation: Carbon is a non metal, silicon and germanium are metalloids used as semiconductors, and tin and lead are true metals, so group IV shows the clearest metallic gradation of any group. The reason is that ionisation energy falls down the group as the atoms get larger. This trend of increasing metallic character down a group applies across the p block.

155. In group IV, the stability of the plus 2 oxidation state relative to the plus 4 state

• A. decreases down the group
• B. is the same throughout
• C. is zero for all members
• D. increases down the group, so lead two compounds are more stable than lead four

Explanation: This is the inert pair effect: the two s electrons of the outer shell become progressively harder to involve in bonding as the group is descended, so tin favours plus 4 while lead favours plus 2. It follows that lead four compounds such as PbO2 are strong oxidising agents, readily reverting to lead two. The same effect appears in groups III and V.

156. The block of the periodic table to which an element belongs is decided by

• A. the number of shells it has
• B. its atomic mass
• C. the subshell that receives the last electron added
• D. whether it is a metal or a non metal

Explanation: Elements are assigned to the s, p, d or f block according to the subshell into which the differentiating electron enters, so groups I and II form the s block and groups III to VIII the p block. This classification explains why elements in the same block share broad chemical behaviour. Period number, by contrast, is given by the number of the outermost shell.

157. An element with the electronic configuration 1s2 2s2 2p6 3s2 3p4 belongs to

• A. group VI of the p block, period 3
• B. group IV of the p block, period 3
• C. group II of the s block, period 3
• D. group VI of the d block, period 4

Explanation: The outermost shell is the third, giving period 3, and it contains six electrons in total across 3s and 3p, giving group VI, while the last electron entered a p subshell so the element is in the p block. The element is sulphur. Counting only the p electrons and answering group IV is the standard error.

158. Across a period, the first ionisation energy generally

• A. decreases, because atomic size increases
• B. increases, because the atoms get smaller and the nuclear charge increases
• C. remains constant
• D. decreases, because shielding increases sharply

Explanation: A greater nuclear charge acting on a shell at nearly constant distance holds the outer electrons more tightly, so more energy is needed to remove one. The trend is not perfectly smooth, dipping at boron and at oxygen because of the start of the p subshell and the first pairing of p electrons respectively. Those small irregularities are themselves strong evidence for subshell structure.

159. The oxides of the elements across period 3, from sodium to sulphur, change from

• A. acidic through amphoteric to basic
• B. acidic to neutral only
• C. basic through amphoteric to acidic
• D. neutral to basic

Explanation: Sodium and magnesium oxides are basic, aluminium oxide is amphoteric and reacts with both acids and alkalis, and the oxides of phosphorus and sulphur are acidic, dissolving to give acids. The change follows the shift from metallic to non metallic character across the period. Identifying aluminium oxide as the amphoteric turning point is the key fact.

160. Lithium shows a diagonal relationship with

• A. sodium
• B. magnesium
• C. beryllium
• D. aluminium

Explanation: Lithium resembles magnesium because their ions have similar size to charge ratios, so both form nitrides directly with nitrogen, both give carbonates that decompose on heating and both have oxides that are less soluble than those of their own group. Beryllium is diagonally related to aluminium in the same way. Diagonal relationships are strongest in the second and third periods.