Chemical reactivity of an element

A chemist would first of all like to know how many electrons there are in the outermost energy level. This tells them about the chemical nature of the element. So while studying chemistry, pay attention to how many electrons there are in the outermost energy level.

How does it help to know how many electrons there are in the outermost?

You have seen the use of Sodium - its shiny surface becomes clouded when kept exposed to the air. But why?

We do not find the element Hydrogen as an atom on Earth. We get it in the form of a H₂ molecule. We find this H₂ in air in the form of a gas. H₂ means two atoms of hydrogen combine together. Similarly, Hydrogen exists in the form of other compounds as well, in which it combines with atoms of other elements, for example - HCl, H₂O, NH₃, etc.

Why is it so?

Oxygen is also not found in the atomic form either on Earth - that is, only in the form of O. That too is found in the form of O₂. It combines with various other elements and is also found in the form of compounds, for example, H₂O, SO₂, Na₂O, CaO, etc.

Now take water, H₂O, for example. As you can see, it is a compound made of Hydrogen and Oxygen.

We find Sodium in the form of (natural salt or table salt) NaCl or in the form of other compounds.  In NaCl too, the Sodium atom is combined with the Chlorine atom. Why are Sodium or Chlorine not able to stay independently?

Iron also is not found in the form of pure Iron. It is mostly found as Iron Oxide.

The same is true of Aluminium - Aluminium is mostly found as Aluminium Oxide.

Why is this so?

In order to understand the answer to this question, we need to understand the chemical reactivity (chemical nature) of elements vis-à-vis one another. Chemical Reactivity depends on the number of electrons present in the outermost energy level of the atom of an element. Lets try and understand it a bit more.