Electron Configurations & the Periodic Table (DP IB Chemistry): Revision Note
Electron configurations & the Periodic Table
Electron configuration tells you how electrons are arranged in an atom
It can be used to find the position of an element in the periodic table
This uses information about:
Period number - this indicates:
The main energy level (shell) the valence electrons occupy
How many occupied energy levels the atom has
For example, if an element is in Period 4, its outermost electrons are in the 4th shell
Group number
This indicates the number of valence electrons
For example, Group 4 means the atom has 4 valence electrons
Examiner Tips and Tricks
You should know the following information about groups without the Periodic Table:
Group 1 – Alkali metals
Group 17 – Halogens
Group 18 – Noble gases
Groups 3 - 12 – Transition elements
Block
The periodic table is split into four blocks: s, p, d, and f
In the p-block, the element’s position from left to right tells you how many electrons are in the p subshell
For example, the 2nd element in the p-block has 2 electrons in the p-subshell
Writing electronic configuration
We can write electronic configurations using the notation below:
This tells us how the electrons in an atom or ion are arranged in their shells, sub-shells and orbitals
Hydrogen
Hydrogen has 1 single electron
The electron is in the s orbital of the first shell
Its electron configuration is 1s1
Potassium
Potassium has 19 electrons
The first 2 electrons fill the s orbital of the first shell, 1s2
They then continue to fill subsequent orbitals and sub-shells in order of increasing energy
The 4s orbital is lower in energy than the 3d subshell, so it is therefore filled first
The full electron configuration of potassium is 1s2 2s2 2p6 3s2 3p6 4s1
To simplify this electronic structure, we can use shorthand electronic configuration
The nearest preceding noble gas to potassium is argon
This accounts for 18 electrons of the 19 electrons that potassium has
The shorthand electron configuration of potassium is [Ar] 4s1
Worked Example
Write down the full and shorthand electron configuration of the following:
Calcium
Gallium
Mg2+
Answer 1:
Calcium has has 20 electrons so the full electronic configuration is:
1s2 2s2 2p6 3s2 3p6 4s2
The 4s orbital is lower in energy than the 3d subshell and is therefore filled first
The shorthand version is [Ar] 4s2 since argon is the nearest preceding noble gas to calcium which accounts for 18 electrons
Answer 2:
Gallium has 31 electrons so the full electronic configuration is:
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p1
The shorthand electronic configuration is:
[Ar] 3d10 4s2 4p1
Even though the 4s is filled first, the full electron configuration is often written in numerical order. So, if there are electrons in the 3d sub-shell, then these will be written before the 4s
Answer 3:
A magnesium atom has 12 electrons so its electronic configuration would be
1s2 2s2 2p6 3s2
To form a magnesium ion, it loses its two outer electrons so the electronic configuration for the ion is:
1s2 2s2 2p6
Using the shorthand, the electronic configuration is:
[Ne]
Interpreting the electron configuration
An element has the electron configuration 1s2 2s2 2p6 3s2 3p5
Period number:
The highest number directly before a block is 3
This element is in period 3 (Na - Ar)
Group:
This element has 3s and 3p electrons, totalling 7 electrons
This element is in Group 7 (17)
Block:
The last term (3p5) containing p means that this element is in the p-block
p5 indicates that the element is the 5th element along in the p-block
Period 3, Group 7 and p5 all indicate that the element is chlorine
Worked Example
Identify the element with the electron configuration of [Ar] 4s2 4p2.
Answer:
The period number is 4
The element has 4s and 4p electrons, totalling 4 electrons
This element is in Group 4 (14)
The last term (4p2) containing p means that this element is in the p-block
p2 indicates that the element is the 2nd element along in the p-block
Period 4, Group 4 (14) and p2 all indicate that the element is germanium
Worked Example
Element Z is in Period 4 and Group 5 of the periodic table. Which statement is correct?
A. Z has 5 occupied energy levels.
B. Z can form ions with 3– charge.
C. Z is a transition element.
D. Z has 4 valence electrons.
Answer:
The correct option is C
Groups 3–12 elements are transition metals. Vanadium, in Group 5 and Period 4, is a transition element
A is incorrect: Period 4 has 4 occupied energy levels, not 5
B is incorrect: Group 5 elements in Period 4 (like vanadium) are metals and typically form positive ions, not a 3- charge
D is incorrect: Group 5 elements have 5 valence electrons, not 4
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