Edexcel IGCSE Chemistry · Spec 1.18-1.19
The Periodic Table & Electronic Configuration
How the periodic table is arranged and how to write electronic configurations.
Chemistry revision video
The Periodic Table & Electronic Configuration
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Explained
Reading the table, and filling the shells
The elements are listed in order of increasing atomic number, which is the number of protons. Everything else about the table follows from that ordering.
The vertical columns are groups, and the horizontal rows are periods. Elements in the same group have similar chemical properties, which is the whole reason the table is arranged this way rather than as a simple list.
Electronic configuration
Electrons occupy shells around the nucleus, and the shells fill from the inside out. The first holds up to 2 electrons, the second up to 8, and the third up to 8 at this level.
Sodium has 11 electrons, so it is 2,8,1. Chlorine has 17, so it is 2,8,7. Calcium has 20, so it is 2,8,8,2.
Fill each shell before starting the next, and check that your numbers add up to the atomic number.
Configuration tells you the position
This is the part worth understanding rather than memorising, because it works in both directions.
The number of electrons in the outer shell is the group number. Sodium has 1 in its outer shell, so it is in Group 1. Chlorine has 7, so it is in Group 7.
The number of shells used is the period number. Sodium uses three shells, so it is in Period 3.
So given an element's configuration you can state its group and period without looking at the table, and given its position you can write its configuration. Questions ask both ways round.
Why groups behave alike
Chemical reactions involve the outer electrons, so elements with the same number of outer electrons react in similar ways.
Group 1 metals all have one outer electron to lose, so they all form ions with a 1 plus charge. Group 7 elements all have seven and need one more, so they all form ions with a 1 minus charge. Group 0 elements have full outer shells, so they have no tendency to gain or lose electrons at all, which is why they are unreactive.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme awards four separate marks for a single element: its mass number, its group number, its period number and its electronic configuration. For boron the answers are 11, 3, 2 and 2,3. Four marks from one atom, all of them recall, all of them available if you can count electrons.
On a drawing question, another mark scheme gives one mark for a diagram with an inner configuration of 2,8 and a second for an outer shell of 8, then states that writing 2,8,8 with no diagram scores one mark only. When the question says draw, the numbers on their own are worth half.
A third note is worth carrying beyond this topic. Asked to describe the relationship between relative atomic mass and atomic number, the mark scheme credits saying that as the atomic number increases so does the relative atomic mass, and then rejects directly proportional, proportional and linear.
Those three words claim something stronger than the data shows. A quantity can rise steadily without rising in proportion, and using the precise term when you mean only that one goes up with the other loses the mark.
Metals, non metals and the transition elements
Metals are on the left and non metals on the right, separated by a rough diagonal. Metals lose electrons to form positive ions; non metals gain them to form negative ions.
The transition elements sit in the block between Groups 2 and 3. They are metals with high melting points and densities, they often form coloured compounds, they can form ions with more than one charge, and many are useful catalysts. Those four properties are the set worth learning, because a question asking how they differ from Group 1 metals is asking for them.
Spec 1.18-1.19
What you need to know
- Describe how the periodic table is arranged
- Work out the electronic configuration of an atom
- Link that configuration to an element's group and period
Active recall
Quick check
Answer each question before opening the answer.
What do the group number and period number tell you about an atom?
The group number gives the number of electrons in the outer shell; the period number gives the number of occupied electron shells.
Give the electronic configuration of a sodium atom (11 electrons).
2, 8, 1.
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