H2 Chem Periodic Table - Period 3

What is not a trend from left to right across the elements of the third period of the Periodic Table?

• The radii of the atoms decrease.

• The oxides of the elements change from basic to acidic.

• The melting points of the elements decrease steadily.

• The compounds of the elements change from ionic to covalent.

Answer explanation

Across the third period, melting points of the elements do not decrease steadily but varies.

The melting points increase from Na to Si, then decrease to Ar.

What is observed when magnesium chloride is added to water?

• solubility in water: dissolves

  pH of resulting solution: 2

• solubility in water: dissolves

  pH of resulting solution: 6.5

• solubility in water: insoluble

  pH of resulting solution: 7

• solubility in water: very slightly soluble

  pH of resulting solution: 8

Answer explanation

MgCl₂ is an ionic compound which dissolves in water with slight hydrolysis to give a slightly acidic solution of pH = 6.5.

The diagram shows part of the Periodic Table.

Which element in the diagram has the largest atomic radius?

• Br

• K

• Kr

• Sc

Answer explanation

Across a period, atomic radius decreases as proton number increases.

This is due to increase in nuclear charge and weak screening effect as successive electrons are added to the same quantum shell.

Phosphorus is an element in the third period, Na to Ar, of the Periodic Table.

What is true for phosphorus and none of the other elements in this period?

• Phosphorus has the highest melting point of the elements in the period.

• Phosphorus is the only element in this period which forms two acidic oxides.

• Phosphorus is the only element in this period with exactly four atoms in its molecule.

• Phosphorus is the only element in this period whose chlorides react with water to form acidic solutions.

Answer explanation

In the third period of the Periodic Table,

A - Si has the highest melting point, not phosphorus.

B - Phosphorus forms two acidic oxides (P₄O₆ and P₄O₁₀)

C - Molecules are P₄, S₈ and Cl₂.

D - Chlorides of period 3 elements from Al to S react with water to give acidic solutions.

Which compound is not a product of the reaction between an oxide of a third period element and water?

• NaOH

• H₂SiO₃

• H₃PO₄

• H₂SO₄

For the elements in the third period of the Periodic Table, which property decreases consistently from sodium to chlorine?

• electrical conductivity

• ionisation energy

• melting point

• radius of the atom

Answer explanation

A: Electrical conductivity increases from Na to Al, then decreases to Si. P, S and Cl are non-conductors as there are no mobile electrons in their structures.

B: In general, 1st IE increases across the period due to increasing nuclear charge and decreasing atomic radius.

C: Melting point increases from Na to Si, then decreases from Si to Cl.

D: Atomic radius decreases across the period due to increasing effective nuclear charge.

Element X is in Period 3 of the Periodic Table. The following four statements were made about the properties of element X or its compounds.

Three statements are correct and one is false.

Which statement does not fit with the other three?

• Adding NaOH(aq) to the solution resulting from reaction of XCl₃ with water produces a white precipitate which is soluble in excess NaOH(aq)

• Element X forms a chloride XCl₃, which reacts with more chlorine to give XCl₅.

• Element X is a solid at room temperature.

• The oxide of X reacts with water to give an acidic solution.

The following represents the electronic configuration of both a Group 2 cation and a Group 17 anion.

1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶

The radius of the anion is approximately twice that of the cation.

Which reasons explain the difference in size?

1 - The cation has a greater nuclear charge than the anion.

2 - There is more electron shielding in the anion than in the cation.

3 - On forming the anion from its atom, the extra electron repulsion makes the ion much larger.

• 1, 2 and 3 are correct.

• 1 and 2 only are correct.

• 2 and 3 only are correct.

• 1 only is correct.

Answer explanation

Before forming cation, the electronic configuration of Group 2 element is

1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶5s²

(38 protons and 38 electrons)

After forming cation, the particle has 38 protons and 36 electrons.

Before forming anion, the electronic configuration of Group 17 element is

1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁵

(35 protons and 35 electrons)

After forming anion, the particle has 35 protons and 36 electrons.

The smaller size of cation is due to greater nuclear charge, resulting in same number of electrons being attracted more strongly by the positively charged nucleus.

What factors explain why a solution of aluminium chloride is acidic?

1 - Chloride ions react with water to form hydrochloric acid.

2 - Aluminium ions have a large charge/surface area ratio.

3 - The H-O bonds are weaker in [Al(H₂O)₆]³⁺ than in H₂O.

• 1, 2 and 3 are correct.

• 1 and 2 only are correct.

• 2 and 3 only are correct.

• 1 only is correct.

Which statement concerning only the elements in the third period, sodium to argon, is correct?

• The element that has exactly four atoms in its molecule is sulfur.

• The element with the highest electrical conductivity is aluminium.

• The element with the highest melting point is aluminium.

• The element with the largest anion is chlorine.

Answer explanation

A - Sulfur exists as S₈ molecule.

B - Na, Mg and Al are all good electrical conductors. Al has the highest electrical conductivity as it has the largest number of delocalised electrons.

C - Si has the highest melting point in period 3.

D - Chlorine has the smallest anion in period 3.