2.2.1 (d) Deducing Electronic configurations of atoms and ions

Given the Atomic Number of an atom you know how many electrons it has.

You could simply start from 1s²

Then add 2s²

Then 2P⁶ etc until your electronic configuration adds up to the Atomic Number (Z)

This will work up to Z = 18 =Neon.

eg ₁₂Mg = 1s²2s²2p⁶3s²

Or, you can look at the Periodic Table

Providing you recall that Groups 1 and 2 are the s-block and the rest are p-block, you can read the electronic configuration directly.

eg Na is 1 element into Period 3's s-block

So, its configuration ends with 3s¹

So it must be 1s² 2s² 2p⁶ 3s¹

^{Beyond element 18 it is less obvious, although you can do the same for most elements.}

eg Ga is 1 element into Period 4's p block making it 1s² 2s² 2p⁶ 3s²3p⁶ 4s¹ 3d¹⁰ 4p¹

eg I is 5 elements into Period 5's p-block making it 1s² 2s² 2p⁶ 3s²3p⁶ 4s¹ 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵

The exceptions that you need to recall are Cr and Cu - see 2.2.1 (a,b,c)

For the first 20 elements the easiest way to find the electronic configuration of an ion is to first find the configuration of the atom and then adding electrons if it is a (negative) anion, or subtracting electrons if it is a (positive) cation.

eg What is the electronic configuration of Iodide (I^-) ions?

Electronic configuration of Iodine = 1s² 2s² 2p⁶ 3s²3p⁶ 4s¹ 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵

Electronic configuration of Iodide (I^-) = 1s² 2s² 2p⁶ 3s²3p⁶ 4s¹ 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ - (one extra electron)

eg What is the electronic configuration of Calcium (Ca²⁺) ions?

Electronic configuration of Calcium atoms= 1s² 2s² 2p⁵ 3s²3p⁶ 4s²

Electronic configuration of Calcium ions (Ca²⁺) = 1s² 2s² 2p⁵ 3s²3p⁶ - (two fewer electrons)

If you can already write a full electronic configuration for a transition metal atom - see 2.2.1 (a,b,c) then all you have to remember is that Transition Elements always lose their 4s electrons first - despite them being at lower energy!

Annoying, eh!

Copper atoms are 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰

Cu⁺ ions are 1s² 2s² 2p⁶ 3s² 3p⁶ 4s⁰ 3d¹⁰

Cu²⁺ ions are 1s² 2s² 2p⁶ 3s² 3p⁶ 4s⁰ 3d⁹

1. (a) Explain why certain elements in the Periodic Table are classified as p-block elements. Illustrate your answer with an example of a p-block element and give its electronic configuration.

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1. (a) Elements in the p block have their outer electron(s) in p orbital(s) or levels or sub-shells (1)

example of element (1)

correct electronic configuration (1)