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ChemistryEdu Logo Periodicity | Trends in chemical properties#

In this article, we will discuss oxidation states, Anomalous Properties of Second Period Elements and chemical reactivity.

Periodicity of valence or oxidation states#

  • Valence of an atom is generally equal to its valence electrons or (8 - valence electrons). Though, it is not always true.
  • Valence is popularly known as oxidation state.
  • Oxidation state of an atom is not fixed. It depends on the element with which it has made chemical bond. For example, in Na2O, oxidation state of O is -2 while in OF2, oxidation state of O is +2 because it is bonded with more electronegative atom F.
  • Oxidation state can be positive, negative or zero.
  • The transition elements and actinoids exhibit variable valence.
Group 1 2 13 14 15 16 17 18
Number of valence electrons 1 2 3 4 5 6 7 8
Valence 1 2 3 4 3, 5 2, 6 7, 1 0, 8

Anomalous properties of second period#

  • The elements of the second period (in group 1, 2 and 13-17) differs in properties from rest of the members of their respective groups.
  • For example, lithium unlike other alkali metals, and beryllium unlike other alkaline earth metals, form compounds with pronounced covalent character; the other members of these groups predominantly form ionic compounds.
  • It is observed that lithium shows similarity with magnesium and beryllium shows similarity with aluminium, i.e, they show similarity with second and third element of next group respectively. This is known as diagonal relationship.
  • The reasons for their anomalous behaviour are small size, large charge/radius ratio and high electronegativity.
  • Covalency is the number of covalent bonds that a particular atom can make with other atoms in forming a molecule. For example, in [AlCl(H2O)5]2+, the covalency of Al is 6 while its oxidation state is +3.
  • Chemical reactivity is high at left and right side of the periodic table (except noble gases).
  • Alkali metals and alkaline earth metals readily lose electrons to attain stable electronic configuration while chalcogens and halogens attain noble gas configuration by either sharing electrons or gaining electron(s).
  • Chemical reactivity is low in middle of the periodic table.
  • Among alkali metals and alkaline earth metals, chemical reactivity generally increases down the group due to decrease in ionization enthalpy.