On this page you will find clear definitions of elements and compounds along with the related frequently asked questions.


What is an element?

According to modern view, in simpler terms, an element can be defined as a substance made up of atoms of same atomic number.

For example, hydrogen gas, H2 is an element since the atoms in it have the same atomic number i.e. 1. 

Recall that atomic number, Z is the number of protons in an atom. 

Whether elements are pure substances or mixtures?

They may be either pure substances or mixtures!

However, an old definition of an element is - it is a pure substance that cannot not be separated into simpler substances either by physical or by chemical means. But an element may consists of two or more isotopes which can be separated by techniques like mass spectrometry, diffusion etc.

Isotopes are the atoms (or nuclides) with same atomic number but different mass numbers.

For example, hydrogen gas consists of different isotopes like: Protium (1H1), Deuterium (1H2 ), Tritium (1H3), which can be separated, in principle, by diffusion.  

Note that the subscript represents atomic number, Z and the superscript indicates mass number, A = #protons + #neutrons.

Thus an element may be either a pure substance consisting of atoms of same isotope or a mixture of different isotopes. It is not possible to find an element in the nature in its pure form since every element is associated with at least two isotopes.

It is also strictly not true to say that an element contains identical atoms due to above reasons.

What makes an element different from other element?

Atomic number.

Yes. Different elements have different atomic numbers i.e. they differ by number of protons.


The atomic number of hydrogen element is 1. Only one proton.

Whereas, the atomic number of oxygen is 8. There are eight protons in the oxygen atom.

Whether element exists in the free atomic state or in the molecular state?

Noble gas elements, e.g., Helium, Neon, Argon etc., exist in the free uncombined atomic state.

However, most of the elements are in the combined molecular form.

E.g. H2 molecules are present in hydrogen gas. O2 molecules are present in dioxygen gas.

What is the physical state of an element?

It may exist in gaseous or liquid or solid state depending on the strength of intermolecular forces and conditions like temperature, pressure etc. 

How many elements have been discovered until now?

Till to date, 117 elements are reported. In the near future the number may increase. 

You can see the names, symbols and atomic numbers of these elements in the modern long form of periodic table.

How many elements are present in the nature?

According to one view, there are 92 naturally occurring elements and the rest are artificial. 

However, this number is debatable.  According to another view there are only 88 naturally occurring elements since the elements like Technetium, Promethium, Francium and Astatine are present only in very minute quantities and are formed as short lived isotopes during the decay of other radioactive elements.

If above elements are regarded as naturally occurring elements, then one has to consider the short lived Plutonium could occur naturally since traces of this element are discovered in uranium deposits.


A chemical compound consists of two or more different types of atoms in combined state. For example, water is a compound made up of hydrogen and oxygen. Its formula is H2O i.e., there are two hydrogen atoms per every oxygen atom. Each oxygen atom is combined with two hydrogen atoms.

Some other examples of compounds are:

* common salt - NaCl

* lime stone - CaCO3

* carbon dioxide - CO2

* Glucose - C6H12O6

 There is a subtle difference between mixtures and compounds. For example, a few atoms of helium and neon in a container is a mixture. These atoms are not in the chemically combined state.

Author: Aditya vardhan Vutturi