Atomic Structure — Part 2: Electron shells, chemical activity and isotopes: visual lesson, ICSE Class 9 Chemistry (Science Paper 2)

Lesson

Filling the shells

Part 2 — how are electrons shared out among the shells, and why does the outer shell decide how an atom behaves?

In this lesson you will

Write the electronic configuration of the first 20 elements using the 2n squared rule
Explain chemical reactivity in terms of completing a duplet or an octet
Define isotopes and represent the isotopes of hydrogen, carbon and chlorine

Concept

The maximum number of electrons in the shell with number n is 2n². The outermost shell never holds more than 8 electrons (2 for the K shell).

Concept

Fill the shells in order: the K shell first, then the L shell. Hydrogen and helium have only the K shell.

Table of the electronic configurations of the first ten elements
Z	Element	Configuration	Outer electrons
1	Hydrogen	1	1
2	Helium	2	2
3	Lithium	2, 1	1
4	Beryllium	2, 2	2
5	Boron	2, 3	3
6	Carbon	2, 4	4
7	Nitrogen	2, 5	5
8	Oxygen	2, 6	6
9	Fluorine	2, 7	7
10	Neon	2, 8	8