Classification of Elements – Periodic Table
The paper tests the historical development of the periodic table, its periods and groups, and the trends in atomic radius, metallic character, ionisation energy and valency.
From Mendeleev to the modern table
Mendeleev arranged elements by atomic mass and left gaps for undiscovered elements, but the argon–potassium and cobalt–nickel pairs broke his order. Moseley showed that atomic number is the better basis. The modern periodic law states that the properties of elements are a periodic function of their atomic number. Elements are arranged in 7 horizontal periods and 18 vertical groups.
Trends you must state
- Atomic radius decreases across a period because nuclear charge rises while the shell number stays the same; it increases down a group because new shells are added.
- Metallic character decreases across a period and increases down a group.
- Ionisation energy and electronegativity increase across a period and decrease down a group.
- Valency equals the group number for groups 1 to 4, and 8 minus the group number for groups 15 to 17; noble gases show zero valency.
- Group 1 alkali metals, group 2 alkaline earth metals, group 17 halogens, group 18 noble gases. Hydrogen is placed in group 1 but resembles halogens in some ways.
Worked example
Rank Na, Mg, Al and Si by atomic radius. All are in period 3, so the radius falls as Z rises: Na > Mg > Al > Si. The same three shells are present, but extra protons pull the electrons in more tightly. Similarly, the valency of Mg (group 2) is 2 and of Cl (group 17) is 8 − 7 = 1.
Traps
Do not say atomic mass decides position in the modern table; across a period the radius decreases although mass increases; down a group metal reactivity increases but non-metal reactivity decreases.