The Periodic Table: Chemical Periodicity I

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In this chapter we would concentrate on the elements found in Period 3 of the Periodic Table. They are : Na, Mg, Al, Si, P, S, Cl and Ar

1) Variation of atomic radii across Period 3
Atomic radii decreases across the period. Across the period, electron are added to same electron shell (Principal quantum number =3). Shielding effect is similar across the period. However nuclear charge increases across the period due to the increase in number of protons. Hence effective nuclear charge increases and atomic radii decreases.

2) Variation of ionic radii across Period 3
Ionic radii decreases across the period from Na+ to Si4+. Across the period, the outer most electron of the ion is in the 2p subshell, hence the outer most electron experiences the same shielding effect. However nuclear charge increases across the period due to the increase in number of protons. Hence effective nuclear charge increases and ionic radii decreases.

Note that we have to leave out P, S, Cl and Ar in the comparison as their outermost electron is found in the 3p subshell.

3) Variation of melting point across the period
The melting point of an element depends on the bonds present and the structure of the element.

Metals
Na, Mg and Al are metals. The bonds present are metallic bonds defined as the electrostatic forces of attraction between the positive metal ions and sea of delocalised electrons. The metal ions are pack together to form a metallic lattice. Hence the melting point of metals are high and increases from Na to Al as the nuber of delocalized electrons increase.

Si
The atoms of Si are bonded to each other by covalent bonds in a giant molecular structure. When Si melts, all the covlent bonds have to be broken. This is unlike metals where some metallic bonding still remains in the liquid metal. Hence Si has a melting point that is very much higher than metals

P, S, Cl and Ar
Phosphorous, sulphur and chlorine has simple molecular structures. Within the molecule, the atoms are bonded together by strong covalent bonds. Van der Waals interactions exists between molecules. These intermolecular interactions are weak, thus P,S and Cl have low melting points.

4) Variation in the first ionization energy
First IE is the energy required to remove the outermost electron from one mole of atoms in the gas phase. First IE of the elements in period 3 increases across the period. Across the period, electron are removed from the same electron shell (Principal quantum number =3). Shielding effect is similar across the period. However nuclear charge increases across the period due to the increase in number of protons. Hence effective nuclear charge increases and first IE increases.

5) Reaction with oxygen
Na reacts very vigorously with oxygen to form sodium oxide.
Mg reacts very vigorously with oxygen to form MgO.
Al reacts vigorously with oxygen to form aluminium oxide.
P reacts vigourously with oxygen to form phosphorous (V) oxide.
S reacts slowly with oxygen to form sulphur dioxide and sulphur trioxide.

6) Reaction with chlorine
Na reacts very vigorously to giveNaCl.
Mg reacts vigorously to form magnesium chloride.
Al reacts vigorously to form aluminium chloride.
Si reacts slowly to form silicon chloride.
P reacts slowly to form phosphorous (V) chloride.

The Cambridge International Examinations Website has put up the questions for A Levels Nov 2006 Paper 2. These questions are available for download. Students can download the question paper and try question 3. This question is about chemical periodicity. You can try it and check your answers against the mark scheme provided.

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