Reading the periodic table

By UK GCSE Chemistry · independent study · myAudience

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Reading the periodic table

The modern periodic table orders elements by increasing atomic number. A period is a horizontal row and a group is a vertical column. Repeating outer-electron arrangements produce repeating chemical behaviour, so the layout is a predictive tool rather than merely a list.

Elements in a main group have similar chemical properties because they have related outer-shell structures. Sodium and potassium each have one outer electron in the GCSE model; chlorine and bromine each have seven. Going down a group adds occupied shells, increasing the distance between outer electrons and the nucleus.

GCSE uses Group 0 for noble gases and Group 7 for halogens. International numbering calls these Groups 18 and 17 respectively. Always establish the numbering convention; modern IUPAC Group 7 refers to a different transition-metal column.

Worked example: Compare sodium 2,8,1 with potassium 2,8,8,1.

Both have one outer electron.

Potassium has one more occupied shell.

Predict related chemistry but not identical reactivity.

Answer: They are in the same main group but different periods.

Check your understanding: Do elements in a period all have the same outer-electron count?

Answer: No.

Why: Moving across a period generally adds electrons to the outer energy level in the simple main-group model.