Atomic and Physical Properties of the Period 3 Elements

This topic describes and explains the trends in atomic and physical properties of the Period 3 elements from sodium (Na) to argon (Ar). It covers ionization energy, atomic radius, electronegativity, electrical conductivity, melting point, and boiling point.

Electronic Structures

In Period 3 of the Periodic Table, the 3s and 3p orbitals are filling with electrons. The shortened electronic configurations for the eight elements are as follows:

Element	Electron Configuration
Na	[Ne] 3s¹
Mg	[Ne] 3s²
Al	[Ne] 3s² 3p¹
Si	[Ne] 3s² 3p²
P	[Ne] 3s² 3p³
S	[Ne] 3s² 3p⁴
Cl	[Ne] 3s² 3p⁵
Ar	[Ne] 3s² 3p⁶

In each case, [Ne] represents the complete electronic structure of a neon atom. The number of electron shells remains constant across the period, but the effective nuclear charge increases from left to right, pulling the electron cloud closer to the nucleus and reducing the atomic radius.

Trends in Atomic Radius

We know that the number of shells in all elements of a given period remains the same, but the effective nuclear charge increases from left to right. The increased effective nuclear charge pulls the electron cloud closer to the nucleus, thus decreasing the size of the atoms and ions across the period.

Element	Atomic Radius (pm)
Na	186
Mg	160
Al	143
Si	118
P	110
S	104
Cl	99
Ar	71

Trends in Electronegativity

Electronegativity measures an atom’s tendency to attract a bonding pair of electrons. The Pauling scale assigns fluorine a value of 4.0, the highest, with values decreasing to cesium and francium at 0.7. Across Period 3, electronegativity increases due to the decreasing atomic size and increasing nuclear charge.

Element	Electronegativity
Na	0.93
Mg	1.31
Al	1.61
Si	1.90
P	2.19
S	2.58
Cl	3.16
Ar	–

Electrical Conductivity

The physical properties section covers electrical conductivity and the melting and boiling points of elements, emphasizing the importance of understanding structure. The structures of Period 3 elements change across the period: the first three (Na, Mg, Al) are metallic, silicon is giant covalent, and the rest (P, S, Cl, Ar) are simple molecules.

Structures of the Elements

Metallic Structures:

Sodium, magnesium, and aluminum have metallic structures, with varying numbers of electrons involved in metallic bonding: one for sodium, two for magnesium, and three for aluminum. Sodium atoms are 8-coordinated, while magnesium and aluminum atoms are 12-coordinated, resulting in less wasted space and stronger bonding.

Giant Covalent Structure:

Silicon, with its giant covalent structure, has all its atoms linked in a three-dimensional network by strong covalent bonds, leading to high melting and boiling points.

Simple Molecular Structures:

Phosphorus, sulfur, chlorine, and argon are simple molecular substances held together by weaker van der Waals dispersion forces. Sodium, magnesium, and aluminum are good conductors of electricity due to free electrons, while silicon is a semiconductor. Phosphorus, sulfur, chlorine, and argon do not conduct electricity as they lack delocalized electrons.

Trends in Melting and Boiling Points

Trends in melting and boiling points across the period show that silicon has a high melting point due to its giant covalent structure, while molecular substances have lower melting and boiling points due to weaker intermolecular forces.

Element	Melting Point (K)	Boiling Point (K)
Na	371	1156
Mg	923	1363
Al	933	2740
Si	1683	2628
P	317.1	553.5
S	388.2	717.6
Cl	172	239
Ar	83.7	87.2

Summary of Period 3 Element Trends

A table summarizing the general trends observed for atomic radius, electronegativity, ionization energy, melting point, and boiling point across Period 3 elements (from left to right).

Property	Trend Across Period 3
Atomic Radius	Decreases
Electronegativity	Increases
Ionization Energy	Increases (with exceptions for Al and S)
Melting Point	Generally increases to Si, then decreases
Boiling Point	Generally increases to Si, then decreases

Additional Considerations

Electrical Conductivity:

Sodium, magnesium, and aluminum are good conductors of electricity due to delocalized electrons. Silicon is a semiconductor. Phosphorus, sulfur, chlorine, and argon do not conduct electricity as they are simple molecular substances without free electrons.

Exceptions:

The trends discussed are general observations. Exceptions like the lower ionization energy of Al due to a stable electron configuration upon ionization highlight the importance of understanding electron configurations for a complete picture.