properties of elements and their trends in groups and in periods

Valence: valence (or) valency of an element was defined as the combining power Of an element with respect to hydrogen, oxygen or indirectly any other element through hydrogen and oxygen.
         Valence of an element with respect to hydrogen is the number of hydrogen atoms with which one atom of that element chemically combines.valence of an element with respect to oxygen is twice the number of oxygen atoms with which one atom of that element combines.
       For example one atom of 'na' chemically combines with one atom of 'H' to give NaH. Therefore,the valence of Na is 1. One atom of 'Ca' combines with one atom of 'O' to give CaO. So the valency of Ca is 2.
  In general,the valence of an element with respect to hydrogen is it's traditional group number.if the element is in the group V or above,it's valence is 8- group number.for example, chlorine valence is 8-7=1.
Generally every period Starts with valence '1' ends with '0'.

Valence of atomic radii in group: atomic radii increase from top to bottom in a group of the periodic table.As we go down in a group,the atomic number of the element increases.therefore to accommodate more number of electrons, more shells are required.As a result the number of shells in a group top to bottom increases.the distance between the nucleus and the outer shell of the atom increases.hence the atomic size increases with atomic number when we go down the group.

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