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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Third Row Elements Page 1

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1.

Intoduction

 

The elements sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulphur (S), chlorine (Cl), and argon (Ar) are the third row elements. In the third period, the metallic character of the elements decreases with the increase in the atomic number. The first three elements Na, Mg and Al are metals, Si is a metalloid and P, S and Cl are non-metals. Ar is a noble gas.

 

 

2.

Position and Electronic Configuration of the third Row Elements:

 

All the elements from Na to Ar belong to Group IA, IIA, IIIA, IVA, VA, VIA, VIIA, and zero group respectively. The electronic configuration in the first and second shells of all the elements is 1s2, 2s2, 2p6. Na and Mg have 1 and 2 electrons respectively in the 3s subshell and Al to Ar have 1, 2, 3, 4, 5 and 6 electrons repectively in the 3p subshell. Thus, Na and Mg, are called 's' block elements and Al to Ar are called 'p' block elements. Since, Ar has a completely filled outer shell it is called a noble gas.

 

 

3.

Periodic Trends

 

  • Ionisation Potential
    The energy required to remove an electron from an atom in the gaseous phase is called ionisation potential. With an increase in atomic number, atomic size decreases. Thus, the effective nuclear charge increases and the ionisation potential (I.P.) increases.
    Na has the lowest I.P. Mg has completely filled 3s orbital while, Al has partially filled 3p orbital. Thus, the I.P. of Mg is more than Al. The half-filled 3p orbital of P is stable than the partially filled 3p orbital of S. Hence, the I.P. of P is more than that of S. As Cl requires only one electron to become stable, it has a high I.P while the I.P of Ar is the highest as it has a stable electronic configuration.
    The I.P value shows the following order:
    Na < Al < Mg < Si < S < P < Cl < Ar.
  • Electropositivity and Electronegativity
    The tendency of an atom to donate electrons from its outermost shell is called electropositivity.
    The tendency of an atom to attract electrons towards itself from a covalent bond is called electronegativity.
    As the nuclear charge increases from Na to Ar, the electropositivity (metallic character) decreases and electronegativity (non-metallic character) increases. Ar is neither electropositive nor electronegative as it has a completely filled valence shell.
  • Oxidising and Reducing Properties
    The removal of an electron is called oxidation, while the addition of an electron is called reduction.
    As the atomic number increases from Na to Cl, the ionisation potential increases and the reducing property decreases. Na, Mg, and Al have low I.P. and hence are strong reducing agents, Si, P and S have high I.P., and hence are weak reducing agents. while Cl has the highest I.P., hence it is a strong oxidising agent. Ar does not show any oxidising or reducing property.

 

 

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