The periodic table: classification of elements practice questions.

Moving left to right across a period, the first ionisation energy generally

• decreases
• increases
• stays constant
• becomes negative

Answer: increases

Nuclear charge increases while electrons enter the same shell, so the outer electrons are held more tightly and are harder to remove.

Which of these Period 3 oxides gives the most acidic solution in water?

• Na₂O
• MgO
• Al₂O₃
• SO₃

Answer: SO₃

Non-metal oxides such as SO₃ are acidic (SO₃ + H₂O → H₂SO₄). Metal oxides on the left (Na₂O, MgO) are basic, and Al₂O₃ is amphoteric.

State the block of the periodic table in which an element with the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³ is found.

• s-block
• p-block
• d-block
• f-block

Answer: p-block

The highest-energy electrons occupy the 4p subshell, so the element (arsenic) belongs to the p-block.

An element is in Group 16 and Period 3. Determine its identity from its position.

• Selenium
• Sulfur
• Phosphorus
• Chlorine

Answer: Sulfur

Period 3, Group 16 is sulfur (electron configuration ending 3s² 3p⁴, six outer electrons).

Determine the number of the period and the group of an element whose ground-state electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴.

• Period 3, Group 14
• Period 3, Group 16
• Period 2, Group 16
• Period 4, Group 6

Answer: Period 3, Group 16

The highest occupied shell is n = 3 (Period 3), and there are 6 electrons in 3s+3p, placing it in Group 16.

Down Group 17 (the halogens), the elements change from gases to a liquid to solids. Explain the increase in melting and boiling points.

• The covalent bonds within the halogen molecules themselves become much stronger down the group
• Molecules gain more electrons, so London dispersion forces increase down the group
• Metallic bonding becomes stronger down the group
• Electronegativity increases down the group

Answer: Molecules gain more electrons, so London dispersion forces increase down the group

Larger halogen molecules have more electrons and a larger electron cloud, giving stronger London dispersion forces and therefore higher melting and boiling points down the group.