Reading the Periodic Table for SPM Chemistry: Periods, Groups, and Trends
The periodic table is not a random chart — it is a map of chemical behaviour. Elements are arranged by proton number, and their properties repeat in patterns. Understanding the structure of the table and the trends within it lets you predict reactions you have never seen, which is exactly what SPM examiners reward.
How the Table Is Organised
Each row is a period and each column is a group. The period number equals the number of electron shells; the group number (for main-group elements) equals the number of valence electrons. So an element in Period 3, Group 2 has three shells and two valence electrons — that is magnesium. Knowing this single rule lets you read the electron configuration from the position alone.
Trends Across a Period
Moving left to right across a period, proton number increases, the nuclear charge grows stronger, and electrons are pulled closer to the nucleus. As a result, atomic radius decreases, electronegativity increases, and metallic character decreases. The change from metals to non-metals happens around Groups 14-15, which is why Period 3 runs from sodium (a soft reactive metal) to chlorine (a toxic gas).
Trends Down a Group
Moving down a group, atoms gain a new electron shell each time, so atomic radius increases. The outermost electrons are farther from the nucleus and more shielded by inner shells, so they are held more loosely. Electronegativity decreases, and metallic character increases. This explains why Group 1 metals become more reactive down the group: the outer electron is lost more easily.
| Property | Across a period | Down a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
| Reactivity (Group 1) | — | Increases |
| Reactivity (Group 17) | — | Decreases |
Group 1: The Alkali Metals
Lithium, sodium, potassium, rubidium, and caesium are soft enough to cut with a knife, have low melting points, and are stored under oil because they react violently with water and air. With water they produce hydrogen gas and a metal hydroxide; the reaction grows more vigorous down the group. Lithium fizzes gently; sodium melts into a ball and skitters; potassium ignites spontaneously. This trend follows directly from the increasing ease of losing the single valence electron.
Group 17: The Halogens
Fluorine, chlorine, bromine, and iodine are toxic non-metals that exist as diatomic molecules. Down the group, they change from gas to liquid to solid, their colour deepens, and their reactivity decreases. They react by gaining one electron to form a -1 ion. A more reactive halogen will displace a less reactive one from its salt solution: chlorine displaces bromine from potassium bromide, but bromine cannot displace chlorine from potassium chloride.
Transition Elements
The block in the middle of the table holds the transition elements. Compared with main-group metals, they have higher densities, higher melting points, and form compounds with characteristic colours — copper(II) sulfate is blue, iron(III) chloride is yellow-brown. They often act as catalysts: iron in the Haber process, nickel in hydrogenation. They also form ions with different charges, such as Fe²⁺ and Fe³⁺.
Why This Topic Underpins Everything
Once you can read position and predict trend, you can reason about chemical equations, bonding, reactivity series, and even electrolysis. Many SPM Chemistry Paper 2 questions test these ideas indirectly, asking you to explain why one element reacts faster than another or why a displacement does or does not occur. Mastering the table earns marks across the whole syllabus.