The periodic table: development, groups, periods and blocks: WASSCE Chemistry

The periodic table is chemistry's most useful map. Knowing just an element's position tells you whether it is a metal, how many electrons it uses in bonding, the formula of its oxide and how it is likely to react. This lesson shows how the table was built and how it is organised.

Learning objectives

By the end of this lesson you should be able to:

describe the contributions of Döbereiner, Newlands, Mendeleev and Moseley
state the periodic law
describe groups, periods and blocks, and name the main families of elements
locate metals, non-metals and metalloids
find an element's position from its atomic number or electron configuration

Development of the periodic table

Scientist	Idea	Limitation
Döbereiner (1829)	Triads: groups of three similar elements in which the middle element's atomic mass is about the average of the other two	Worked for only a few groups
Newlands (1864)	Law of octaves: arranged by atomic mass, every eighth element had similar properties, like notes in music	Broke down after calcium
Mendeleev (1869)	Arranged elements by atomic mass in groups of similar properties; left gaps for undiscovered elements and predicted their properties	A few pairs (such as Ar and K) were out of order by mass
Moseley (1913)	Showed that atomic number (number of protons), not mass, is the true basis	(Gave the modern table)

Worked example

A Döbereiner triad. Calcium (40), strontium (88) and barium (137) are similar metals.

\text{average of Ca and Ba} = \dfrac{40 + 137}{2} = 88.5

\begin{aligned}&\text{average of Ca and Ba} \\ &= \dfrac{40 + 137}{2} \\ &= 88.5\end{aligned}

This is close to strontium's relative atomic mass of 87.6.

Mendeleev's boldest prediction was "eka-silicon", a missing element below silicon. When germanium was found in 1886, its mass, density and oxide formula matched his predictions closely, which convinced chemists his table was right.

Definition

Modern periodic law: the properties of elements are a periodic function of their atomic numbers.

Structure of the modern table

Outline of the periodic table showing groups 1 to 0, periods 1 to 4, and the s-block, d-block and p-block, with the first 20 elements labelled

Figure 1. Groups, periods and blocks. WASSCE Tutor diagram.

Groups are vertical columns. Elements in a group have the same number of outer electrons, so they have similar chemical properties.
Periods are horizontal rows. Elements in a period have the same number of electron shells.
Blocks are named after the sub-shell being filled: s-block (groups 1, 2), p-block (groups 3 to 0), d-block (transition metals) and f-block (lanthanides and actinides).

Families of elements

Group	Name	Outer electrons	Examples	Character
1	Alkali metals	1	Li, Na, K	Very reactive metals; form M⁺ ions
2	Alkaline earth metals	2	Be, Mg, Ca	Reactive metals; form M²⁺ ions
3	Boron group	3	B, Al	Al forms Al³⁺
4	Carbon group	4	C, Si	Mainly covalent bonding
5	Nitrogen group	5	N, P	Non-metals at the top
6	Oxygen group (chalcogens)	6	O, S	Form X²⁻ ions
7	Halogens	7	F, Cl, Br, I	Very reactive non-metals; form X⁻ ions
0 (18)	Noble gases	8 (He: 2)	He, Ne, Ar	Very unreactive
d-block	Transition metals	—	Fe, Cu, Zn	Coloured compounds; variable valency; catalysts

Metals, non-metals and metalloids

Metals are on the left and centre of the table, about three-quarters of all elements.
Non-metals are at the top right, plus hydrogen.
Metalloids (B, Si, Ge, As) lie along the staircase between them and have properties of both. Silicon is used in solar cells and computer chips.

Finding an element's position

You know	Period	Group
Shell arrangement 2, 8, 6	3 shells → period 3	6 outer electrons → group 6
Configuration [Ar] 4s¹	n = 4 → period 4	1 outer electron → group 1

Worked example

Element X has atomic number 17.

Arrangement: 2, 8, 7.
Three shells, so period 3; seven outer electrons, so group 7.
X is a halogen (chlorine). It forms the ion X⁻ and the compound NaX with sodium.

Uses of the periodic table:

predicting the properties of an element from its group
predicting formulas of compounds (group 2 + group 7 gives MX₂)
classifying elements as metals or non-metals
understanding trends in reactivity (next lesson)

In Sierra Leone

Many of Sierra Leone's mineral resources sit in particular parts of the table. Iron (Tonkolili, Marampa) and titanium (rutile) are d-block transition metals. Aluminium (bauxite) is in group 3. Gold is a d-block metal, and diamond is pure carbon from group 4.

Common mistake

Elements in the same group have similar properties, not elements in the same period. Across a period, properties change steadily from metal to non-metal.

Key points

Döbereiner (triads), Newlands (octaves), Mendeleev (gaps and predictions) and Moseley (atomic number) built the table.
Modern periodic law: properties are a periodic function of atomic number.
Groups share outer electrons and properties; periods share the number of shells; blocks are s, p, d and f.
Main families: alkali metals, alkaline earth metals, halogens, noble gases, transition metals.
Find period from the number of shells and group from the number of outer electrons.

WASSCE examination focus

WAEC tests this

Expect to describe Mendeleev's contribution (gaps and predictions), to state the periodic law, to locate elements from atomic number or configuration, and to name groups and their characteristics. Questions often give an unknown letter, such as "element Y with Z = 12", and ask for its group, period and the formula of a compound.

Quick check

State the modern periodic law.
What did Mendeleev do that convinced chemists his table was right?
Element Q has atomic number 12. Give its period, group and the formula of its chloride.
Where are the non-metals found in the periodic table?

Answers:

The properties of elements are a periodic function of their atomic numbers.
He left gaps for undiscovered elements and correctly predicted their properties, e.g. germanium.
2, 8, 2: period 3, group 2; chloride QCl₂.
At the top right of the table (plus hydrogen).

The periodic table: development, groups, periods and blocks