Question

Smita studied about the Modern Periodic Table. She placed hydrogen, lithium, sodium, and calcium in the same group in the periodic table. However, her teacher told her that she has placed one element incorrectly in the periodic table.(Atomic numbers of $\mathrm{H}$, $\mathrm{Li}$, $\mathrm{Na}$, and $\mathrm{Ca}$ are 1, 3, 11, and 20 respectively) Which element has been placed incorrectly by Smita?

Answer 1

Basis of modern periodic table:

1. It is basically a tabular or the organized display of the chemical elements.
2. As these chemical elements are displayed in an organized way in a modern periodic table so it's so easy to locate any chemical element and their characteristics too.
3. So basically, the elements are arranged in the modern periodic table on the basis of the increasing ‘atomic number’ which is generally depicted by Z

Characteristic of group in term of valency:

1. The valency of an element is basically defined as the number or valence electrons that are present in the outermost shell of an atom.
2. In the modern periodic table a group is defined as the vertical way or the column in the table that consists of chemical elements.
3. So, the chemical elements which are arranged in the group will have the same number of valence electrons present in their outermost shell.
4. Even if the atomic size increases from top to bottom in a group, the valency for all the elements present in a group would be same.

Electronic configuration:

1. It is actually an arrangement of the electrons according to or on the basis of the energy levels around the atoms.
2. Electrons are arranged in the atomic or the molecular orbital of the atoms or the molecules.
3. And those elements which will have the same number of valence electrons would be there in the same group only.
4. So, Electronic configuration of $\mathrm{H}:1$
5. Electronic configuration of $\mathrm{Li}:2,1$
6. Electronic configuration of $\mathrm{Na}:2,8,1$
7. Electronic configuration of $\mathrm{Ca}:2,8,8,2$
8. As, except Calcium ($\mathrm{Ca}$) all the elements are having one valence shell electron which means Calcium won't be there in the same group.

Therefore, Calcium ($\mathrm{Ca}$) is the element which has been placed incorrectly by Smita.