8.2 - Modern Periodic Table of Elements

Structure and Arrangement of Elements

• The Modern Periodic Table of Elements contains 118 elements.
• Each element entry shows its proton number, symbol, name and relative atomic mass.
• Elements are classified as metals, semi-metals or non-metals; their physical states are shown as solid, liquid or gas.

Feature	Description
Group	A vertical column; there are 18 groups.
Period	A horizontal row; there are 7 periods.
Lanthanoid series	Elements with proton numbers 57–71, placed separately below the main table.
Actinoid series	Elements with proton numbers 89–103, placed separately below the main table.

Named groups and region

• Group 1: alkali metals
• Group 2: alkaline earth metals
• Groups 3–12: transition metals
• Group 17: halogens
• Group 18: inert gases

Changes from left to right across a period

• Metallic properties decrease; non-metallic properties increase.
• Properties of oxides change from basic oxides to acidic oxides.
• Electrical conductivity decreases.

Principle of arrangement

• Elements are arranged in ascending order of proton number, from left to right and from top to bottom.
• Proton number is the number of protons in the nucleus of an atom.

Electron Arrangement of Elements

• For a neutral atom:

$\text{number of protons}=\text{number of electrons}$

• Electrons are arranged around the nucleus in electron shells.
• The shell nearest to the nucleus is filled first. A new shell is occupied only after the preceding shell has been filled.
• Electron shell capacities:

Electron shell	Maximum number of electrons
First	2
Second	8
Third	18

• For atoms with proton numbers 1–20, fill 2 electrons in the first shell, 8 electrons in the second shell and 8 electrons in the third shell before starting the fourth shell.

Example: sulphur

• Proton number = 16; hence a neutral sulphur atom has 16 electrons.
• Distribution: 2 in the first shell, 8 in the second shell, 6 in the third shell.
• Electron arrangement: 2.8.6.

Electron arrangements of the first 20 elements

Elements	Proton numbers	Electron arrangements
H, He	1–2	1; 2
Li–Ne	3–10	2.1; 2.2; 2.3; 2.4; 2.5; 2.6; 2.7; 2.8
Na–Ar	11–18	2.8.1; 2.8.2; 2.8.3; 2.8.4; 2.8.5; 2.8.6; 2.8.7; 2.8.8
K, Ca	19–20	2.8.8.1; 2.8.8.2

Formation of Positive and Negative Ions

• Most atoms have an unstable electron arrangement and tend to form a stable electron arrangement.
• A neutral atom forms an ion by losing outermost-shell electrons to another atom or gaining outermost-shell electrons from another atom.
• This loss or gain allows the atom to achieve a stable duplet or octet electron arrangement.

Stable arrangement	Meaning
Duplet	2 electrons in the first shell.
Octet	8 electrons in the outermost shell.

Ion formed	Process
Positive ion	A metal atom loses electrons.
Negative ion	A non-metal atom gains electrons.

Magnesium ion, $\mathrm{M}{\mathrm{g}}^{2+}$

• Magnesium atom: 12 electrons; electron arrangement 2.8.2.
• Losing 2 electrons is easier than gaining 6 electrons.
• It forms a magnesium ion with electron arrangement 2.8, achieving a stable octet.

$\mathrm{Mg}(\mathrm{2.8.2})⟶\mathrm{M}{\mathrm{g}}^{2+}(2.8)+2{\mathrm{e}}^{-}$

Chloride ion, $\mathrm{C}{\mathrm{l}}^{-}$

• Chlorine atom: 17 electrons; electron arrangement 2.8.7.
• It gains 1 electron and forms a chloride ion with electron arrangement 2.8.8, achieving a stable octet.

$\mathrm{Cl}(\mathrm{2.8.7})+{\mathrm{e}}^{-}⟶\mathrm{C}{\mathrm{l}}^{-}(\mathrm{2.8.8})$

• Corrosion of a metal occurs when metal atoms form positive ions; rusting is therefore linked to ion formation.