In this blog, we are going to learn about the periodic classification of elements. We all know metals around us are present in the form of elements, compounds, and mixtures. Elements are made up of only one type of atom. Till now, we know about more the 114 elements. As different elements were being discovered scientists gathered more and more information about the properties of these elements.

They found it difficult to arrange all that was known about the elements. They started looking for some pattern in their properties so that they could study such a large number of elements with ease.

In 1913 Henry Mosley arranged the elements in increasing order of their atomic number such that the properties of elements repeat themselves after some fixed interval. This arrangement is known as the modern periodic table.

The modern periodic law table states that- �Properties of elements are a periodic function of their atomic number.�

Position of elements in the modern periodic table

The elements when arranged in order of increasing atomic number lead us to the classification known as Modern Periodic Table. Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic numbers. In this blog, we will learn about the position of elements in the modern periodic table

The modern periodic table has 18 vertical columns known as �groups� The modern periodic table has 7 horizontal rows known as �periods�. The elements in a group have the same number of valence electrons such as all the elements of group 1 have one electron in valence shell that means they have �1 valence electron Elements of group II have 2 valence electrons.

Similarly, Elements of group 17 have 7 valence electrons. Hence we can say that the group in the periodic table signifies an identical outer shell electronic configuration and show similar properties. The number of shells increases as we go down the group When we go from left to right in a period for example in the second period, the number of valence electrons increases by one unit as the atomic number increases by one unit.

Although the elements in this period do not have the same number of valence electrons they have the same number of shells So we can say that atoms of different elements with the same number of occupied shells are placed in the same period.

But each period marks a new electronic shell getting filled. As the valence electrons determine the kind and the number of bonds formed by an element in the periodic table, it tells us about its chemical reactivity, how many elements are there 1, II, III, and IV periods, and a maximum number of electrons in a shell.

We know that the maximum number of electrons that can be accommodated in a shell depends on the formula 2n2 where n is the number of the given shell from the nucleus In K shell maximum number of electrons are 2 hence the first period has 2 elements.

In the L shell, the maximum number of electrons is 8 hence the second period has 8 elements. In the �M� shell the maximum number of electrons is 18 but the outermost shell can have only 8 electrons, as the third period also has only 8 elements

Trends in the modern periodic table

Let's learn about the trends of different characteristics of elements in the modern periodic table

Valency

We know that the valence of an element is the combining capacity of the element which is determined by the number of valence electrons present in the outermost shell of its atom.

Let�s take the elements of the second period and first group.
Now write down the atomic number of all the elements.
Now write the electronic configuration of all the elements.
Now circle the valence electronics in all of them.
Now write down the valence of all. What do you observe? How does valence vary in a period going from left to right? How does valence vary in going down a group?
Going from left to right, the valence of the element increases and valence electrons reach close to 8. But in a group, the number of valence electrons does not change

Atomic size

Distance between the center of the nucleus and the outermost electron shell of an isolated atom is known as atomic size. It refers to the radius of an atom. It is measured in �picometer� (1pm = 10-12 m) e.g. atomic radius of the hydrogen atom is 37 pm. Again consider the elements of the second period and first group. Now note their atomic sizes.

Atomic radius decrease in moving from left to right along a period. This is due to an increase in effective nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom. But going down in a group atomic size increases.

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, thereby increasing the atomic size in spite of the increase in nuclear charge. Very important to note that the effect of the addition of a new shell outweighs the effect of the increased nuclear charge.

Metallic properties

We know that metals are electropositive in nature as they tend to lose electrons while forming bonds. Now again consider the elements of the second group and 1st period. As the effective nuclear charge acting on the valence shell electrons increase across a period the tendency to lose electrons decreases. On moving from left to right in a period, the metallic character of the element decreases.

While down to the group, the effective nuclear charge experienced by valence electrons decreases as the outermost electrons move further apart from the nucleus and as a result, the element can easily lose the electron. Hence metallic character increases down a group.

Non-metallic characters

We all know that nonmetals are electronegative and tend to gain electrons while forming bonds. Now again consider elements of the second period and first group. As the effective nuclear charge acting on the valence shell electrons increase across a period, the tendency to gain electrons will increase and so the non-metallic character also increases.

But as we go down the group, the effective nuclear charge experienced by valence electrons decreases as the outermost electrons are farther away from the nucleus, hence the non-metallic character decreases.

Read More: Mendeleev's Periodic Table: Achievements and Limitations