Trends in the Modern Periodic Table

Trends in the modern periodic table: In this table, some properties show a regular trend when we move along a period from left to right or in a group from top to bottom.

1. Valency: It is the combining capacity of an atom of an element to acquire noble gas configuration and depends upon the number of valence electrons, i.e. the electrons present in the outermost shell of its atom. For the elements of group 1, 2, 13 and 14, valency = number of valence electron(s), whereas for the elements of group 15 onwards, valency = 8 - valence electrons. Thus, the valencies of elements of different groups are as follows:

Valency of group 1 elements is 1; Valency of group 2 elements is 2

Valency of group 13 elements is 3; Valency of group 14 elements is 4

Valency of group 15 elements is 3; Valency of group 16 elements is 2

Valency of group 17 elements is 1; Valency of group 18 elements is 0

Variation along a Group: In a group, outer electronic configuration is same for all the elements, so all have the same number of valence electrons and the valency. E.g. All the elements of group 1 have valency = 1.Similarly, for the elements of group 17, valency = 1(8 - 7= 1)

Variation along a Period: The valency increases from 1 to 4 (till group 14) and then decreases to zero from group is to 18.

2.    Atomic Size: It refers to the radius of an atom it may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom. It is measured in picometres: [1 pm (picometre)=  .  Like this, atomic radius of hydrogen atom is 37 pm (or 37 X  m)

Variation along a Group: The atomic size increases down the group. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases inspite of the increase in nuclear charge.

Variation along a Period: The atomic radius decreases on moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the valence electrons closer to the nucleus and reduces the size of the atom.

3.    Metallic and Non-metallic Properties: Elements, having a tendency to lose one or more electrons and form positive ions are called metals. These are present on the left side as well as in the centre of the periodic table. The tendency of these elements to lose electrons is called their metallic character. Because of the formation of positive ions, these are also called electropositive elements. Non-metals are the elements which have a tendency to gain one or more electrons to form negative ions. Thus, these are electronegative elements. These are present on the right side in the periodic table. The non-metallic character of the elements is due to their electron accepting tendency.There are some elements which exhibit the properties of both metals and non-metals. These are called metalloids. In the modern periodic table, a zig-zag line separates metals from non-metals. The borderline elements—boron, silicon, germanium, arsenic, antimony, tellurium and polonium are intermediate in properties so they are called metalloids or semi-metals.

Variation along a Period and a Group: As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease. Down the group, the effective nuclear charge experienced by valence electrons decreases because the outermost electrons are farther away from the nucleus. Therefore, these can be lost easily. Hence, metallic character decreases across a period and increases down a group. Non-metallic character, however increases across a period and decreases down a group. E.g. in case of elements of third period, metallic and non-metallic character vary in the following manner.

However in groups, the order of variation of metallic and non-metallic character is shown:

Reactivity of metals increases on moving down a group while that of non-metals decreases.

4.    Nature of Oxides: Oxides of the metals are of basic nature while those of non-metals are acidic. This means that along a period, the basic character of the oxides of the elements decreases while their character increases.

On going down in a group of the periodic table, the order is reversed, i.e. basic character of oxides increases and acidic character of oxides decreases. E.g. in case of oxides of elements of group 2, the nature varies as:

Note: Metal oxides dissolve in water to form metal hydroxide.

Oxides are formed when metal is burned in air.

5.  Electronegativity: It may be defined as the relative electron attracting tendency of an atom for a shared electron pair in a covalent bond with other atom.

Variation along a Period and a Group: The electronegativity of the elements increases along a period since the non-metallic character increases. Similarly, it decreases down the group since the non-metallic character decreases.