10.2Chapter 10: Structure, Bonding, and Properties

Electrons in atoms — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 9 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision

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10.2 Electrons in atoms — Electron Shells and Electronic Configurations — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define electron configuration and write and draw the electron configurations of the first 20 elements.Neutral atom — it has the same number of …

Chemical properties
Filling order
Maximum numbers
Same column
Full outer shell
Non-metals

Part 1 of 6Electrons in atoms

Neutral atom

Neutral atom it has the same number of protons and electrons, so hydrogen has 1 of each, sodium has 11 and silver has 47.

Chemical properties

Chemical properties the number and arrangement of electrons give an element its chemical properties.
Silver and sodium have similar physical properties (shiny when freshly cut, conduct electricity) but different chemical properties because their atoms have different numbers of electrons.
Sodium reacts vigorously with water but silver does not.

Part 2 of 6Electron shells

Shells

Shells electrons are arranged in shells (energy levels) around the nucleus.

Filling order

Filling order the shell closest to the nucleus has the lowest energy and fills first.

Maximum numbers

Maximum numbers the first shell holds up to 2 electrons, the second up to 8 and the third up to 18.
For the first 20 elements the third shell fills to 8 before the fourth shell starts.

Electron configuration

Electron configuration the arrangement of the electrons in an atom.

Sodium has 2 electrons in the first shell, 8 in the second and 1 in the outer shell, written 2,8,1.

Part 3 of 6Writing a configuration

Find the atomic number from the periodic table, which is the number of electrons.

Fill the first shell with up to 2 electrons.

Fill the second shell with up to 8.

Fill the third shell with up to 8, then place any remaining electrons in the fourth.

Add the numbers to check the total equals the atomic number.

Examples

Examples lithium (3) is 2,1;
- carbon (6) is 2,4;
- sodium (11) is 2,8,1;
- aluminium (13) is 2,8,3;
- chlorine (17) is 2,8,7;
- calcium (20) is 2,8,8,2.

Drawing

Drawing draw concentric circles around a nucleus labelled with its symbol and place electrons as crosses on each circle, counting carefully so that there are not too many shells.

Part 4 of 6Links to the periodic table

Links to the periodic table

Period

Period the period number equals the number of occupied shells.

Group

Group for Groups 1 to 7 the group number equals the number of outer-shell electrons.
Group 1 atoms have one outer electron and Group 2 atoms have two.

Same column

Same column all elements in the same column have the same number of outer-shell electrons and therefore similar chemical properties.

Group 0

Group 0 the noble gases have full outer shells, which explains their lack of reactivity.

Element	Atomic number	Configuration	Group	Period
Oxygen	8	2,6	6	2
Magnesium	12	2,8,2	2	3
Argon	18	2,8,8	0	3

Part 5 of 6Why outer electrons matter

Reactions

Reactions chemical reactions involve the outer-shell electrons.

Full outer shell

Full outer shell atoms react to achieve a full outer shell by losing, gaining or sharing electrons.

Metals

Metals they tend to lose outer electrons.

Non-metals

Non-metals they tend to gain or share electrons.

Part 6 of 6Electron configurations and ions

1Sodium

2,8,1 loses its single outer electron to reach the stable 2,8 arrangement of neon.

2Chlorine

2,8,7 gains one electron to become 2,8,8, like argon.

3Two or three outer electrons

Two or three outer electrons elements such as magnesium and aluminium lose them.

4Six or seven outer electrons

Six or seven outer electrons elements tend to gain electrons.

5Four outer electrons

Four outer electrons elements such as carbon usually share electrons.

6Atoms and ions

Atoms and ions comparing the numbers of protons and electrons shows whether a particle is an atom or an ion, for example 2,8,8 could be argon, K⁺ or Cl^-.

Key terms

Term	Meaning
Electron shell	an energy level where electrons are found around the nucleus.
Electron configuration	the arrangement of electrons in shells.
Outer shell	the shell farthest from the nucleus that holds electrons.
Noble gas	an unreactive Group 0 element with a full outer shell.
Group	a vertical column of the periodic table.
Period	a horizontal row of the periodic table.