PERIODIC CLASSIFICATION OF ELEMENTS
Making Order Out of Chaos - The Modern Periodic Table
Trends in the Modern Periodic Table
Valency : As you know, the valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
Atomic size: The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom. The atomic radius of hydrogen atom is 37 pm (picometre, 1 pm = 10–12m).
Let us study the variation of atomic size in a group and in a period.
Atomic radii of the elements of the second period are given below:
Period II elements : B Be O N Li C
Atomic radius (pm) : 88 111 66 74 152 77
Arrange them in decreasing order of their atomic radii.
We will see that the atomic radius decreases in moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.
Group 1 Elements : Na Li Rb Cs K
Atomic Radius (pm) : 86 152 244 262 231
We will see that 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 in spite of the increase in nuclear charge.
Metallic and Non-metallic Properties
As we can see, the metals like Na and Mg are towards the left-hand side of the Periodic Table while the non-metals like sulphur and chlorine are found on the right-hand side. In the middle, we have silicon, which is classified as a semi-metal or metalloid because it exhibits some properties of both metals and non-metals.
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 and are called metalloids or semi-metals.
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 is decreasing 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-metals, on the other hand, are electronegative. They tend to form bonds by gaining electrons. Let us learn about the variation of this property.
As the trends in the electronegativity show, non-metals are found on the right-hand side of the Periodic Table towards the top.
These trends also help us to predict the nature of oxides formed by the elements because it is known to you that the oxides of metals are basic and that of non-metals are acidic in general.
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