Variation in Physical Properties of Period III

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Variation in Physical Properties of Period III

Below are a few of the physical properties of elements in Period III of the periodic table:

Element Outer electrons Atomic radius (nm) First I.E. Tm K
Na 3s1 0.191 496 371
Mg 3s2 0.160 738 922
Al 3p1 0.143 578 933
Si 3p2 0.118 789 1683
P 3p3 0.110 1012 317
S 3p4 0.102 1000 386
Cl 3p5 0.099 1251 172
Ar 3p6 0.190 1521 84

Changes in atomic radius

Across Period 3, Na to Ar, the number of protons in the nucleus increases: the nuclear charge increases, shielding by inner electrons is unchanged; new electrons enter the same shell; hence atomic radius decreases.

Changes in first ionisation energy

There is a general increase in first I.E. as atomic number increases because:

  • a) The nuclear charge increases, due to increase in number of protons and electrons.
  • b) The radius of atom decreases with increasing atomic number, hence more energy required to remove outer electrons.

Changes in melting points

To understand these changes we must understand the nature of the bonding within the elements.

Three types of structure occur as we cross period 3.

Metallic Structures, Na, Mg, Al

The bonding in metals is most simply explained as: pseudo cations in a delocalised sea of electrons.

The factor which determine metallic bond strength are:

  • a) The number of outer electrons per atom; the larger the number the greater the bond strength.
  • b) The charge of the metal ion; the larger the charge the greater the bond strength.
  • c) The radius of the metallic ion; the smaller the radius the greater the metallic bond strength.

These factors explain why melting point increases from Na to Mg to Al.

Giant Molecular, Si

In these structures large numbers of atoms are held together by strong covalent bonds. To melt these structures it is necessary to break the actual covalent bond.

Melting points are determined by number of covalent bonds and their strength. Bond strength is related to bond length and hence determined by the radius of the atoms involved in forming the covalent bonds. Generally, bond strength increase as bond length decreases.

Molecular Structures, P4, S8, Cl2 and Ar

To melt these substances it is only necessary to break weak van der Waal's forces of attraction. These forces increase as molecular surface increases. Hence, the following pattern in melting points.

P4 < S8 > Cl2 > Ar

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