A group is a vertical column of elements with the same number of valence electrons and similar chemical properties. A period is a horizontal row of elements with the same number of occupied electron shells.
The number of valence electrons gives the group and the number of shells gives the period.
This page covers a single Form 4 content standard: groups and periods in the periodic table. It is one of the most useful skills in the whole chapter, because once you can read a group and period straight from an electron arrangement, you can predict how an element behaves without memorising every element. This is examined in almost every paper, so it is worth mastering completely.
What a group is
A group is a vertical column in the periodic table. The most important fact to remember is that all the elements in the same group have the same number of valence electrons (electrons in the outermost shell). Because chemical properties are decided by the valence electrons, elements in the same group have similar chemical properties and react in similar ways. There are 18 groups. In the KSSM syllabus we use the numbering Group 1, Group 2, then Group 13 to Group 18 for the main groups, with the transition elements in between.
- Group 1 (alkali metals) all have 1 valence electron.
- Group 2 all have 2 valence electrons.
- Group 17 (halogens) all have 7 valence electrons.
- Group 18 (noble gases) have a full outer shell (8 valence electrons, except helium which has 2).
What a period is
A period is a horizontal row in the periodic table. All the elements in the same period have the same number of occupied electron shells. There are 7 periods.
- Period 1 elements have 1 occupied shell (hydrogen and helium).
- Period 2 elements have 2 occupied shells (lithium to neon).
- Period 3 elements have 3 occupied shells (sodium to argon).
As you move across a period from left to right, the number of valence electrons increases by one each time, and the properties change gradually from metallic (on the left) to non-metallic (on the right), ending with a noble gas.
Reading group and period from an electron arrangement
Two simple rules connect the electron arrangement to the table:
- Number of occupied shells = the period.
- Number of valence electrons = the group (for Groups 1 and 2 directly; for Groups 13 to 18, group number = number of valence electrons + 10).
For example, an element with the arrangement 2.8.1 has 3 shells (Period 3) and 1 valence electron (Group 1), it is sodium. An element with 2.8.7 has 3 shells (Period 3) and 7 valence electrons (Group 17), it is chlorine.
Worked example
Question. An element has the electron arrangement 2.8.2. State its group and its period, and predict whether it is a metal or a non-metal.
Step 1, Count the shells. The arrangement 2.8.2 has three numbers, so the atom has 3 occupied shells. Therefore the element is in Period 3.
Step 2, Count the valence electrons. The outermost shell contains 2 electrons, so there are 2 valence electrons.
Step 3, Find the group. Two valence electrons places the element in Group 2 (for Groups 1 and 2 the group number equals the number of valence electrons).
Step 4, Predict metal or non-metal. Group 2 is on the left of the table, so the element is a metal. (This element is magnesium.)
Answer. The element is in Group 2, Period 3, and it is a metal.
Practice question
Question. An element X has the electron arrangement 2.8.5. (a) State the period of X. (b) State the group of X. (c) State whether X is a metal or a non-metal.
Answer. (a) Three occupied shells, so X is in Period 3. (b) Five valence electrons, so the group is 5 + 10 = Group 15. (c) Group 15 is on the right-hand side of the table, so X is a non-metal. (This element is phosphorus.)
Exam tip
The number of shells and the number of valence electrons are all you need, do not confuse them. A quick memory aid: the period is how many numbers are in the electron arrangement, and the group comes from the last number. Watch the group numbering for the right-hand side of the table: 3 valence electrons is Group 13, not Group 3, because you add 10 for Groups 13 to 18. When a question gives you a proton number instead of an electron arrangement, write the arrangement first, then read off the group and period. Getting this right also lets you predict the charge of the ion an element forms, which links straight into the Chemical Bond chapter.
Why elements in the same group are similar
The reason elements in the same group behave alike is simple but important, and examiners like to test it. Every member of a group has the same number of valence electrons, and it is these valence electrons that take part in chemical reactions. So each member tends to form an ion with the same charge and reacts with the same substances in a similar way. For example, lithium, sodium and potassium (all Group 1) each have 1 valence electron, so all three lose one electron to form an ion of charge +1 and react with water in a similar way. The same pattern explains why fluorine, chlorine and bromine (all Group 17) share similar chemical properties. When you are asked to explain why two elements are in the same group, always tie the answer back to having the same number of valence electrons, that is the reasoning the marking scheme looks for.
Moving down a group, the atomic size increases because the number of occupied shells increases, even though the number of valence electrons stays the same. This change in size explains why reactivity changes down a group, an idea you will use directly when you study the Group 1 alkali metals and the Group 17 halogens in the next standards. So the two skills, locating an element and understanding the reason behind its properties, work together throughout this chapter.
Where this fits
This standard sits at the heart of the The Periodic Table of Elements chapter and connects directly to the groups you study next: Group 1, Group 17 and Group 18. Revise the definitions with the revision notes and drill group-and-period identification with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check that you never mix up group with period and that you handle the +10 rule for Groups 13 to 18 correctly, because those are the two slips that cost easy marks here. Since this skill is reused throughout SPM Chemistry, securing it now saves time in every later chapter.
Quick recap
- A group is a vertical column; elements share the same valence electrons and similar chemical properties.
- A period is a horizontal row; elements share the same number of shells.
- Number of shells = period; number of valence electrons = group (add 10 for Groups 13 to 18).
- Across a period, properties change from metallic to non-metallic.
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