Elements and Compounds


Elements are substances composed of only one type of atom. There are around 100 naturally occurring elements, which can be seen on a periodic table.

Elements consist of atoms that all have the same number of protons. An example of an element is nitrogen, which has a proton number of 7. Each element is represented by its own chemical symbol and atomic number (the number of protons).

Atoms of elements are represented by one or two-letter chemical symbols. For example, carbon is also referred to as C. These can be found in the periodic table.

Take hydrogen as an example. The atomic symbol ‘H’ tells us that this is hydrogen.

Each element has an atomic number and a mass number. The atomic number tells us how many protons (and electrons) are in the atom and the mass number refers to the number of protons + neutrons that are in the atom.

The same element can exist in different forms, known as isotopes. An example of this is carbon-12 and carbon-13, which both have the same number of protons, but a different number of neutrons.

  • Isotopes have different mass numbers and the same atomic numbers.

Carbon isotopes

These are carbon isotopes:

As you can see by the image above, one is carbon-12 and the other is carbon-13. They both have the same number of protons (and electrons) but a different number of neutrons.


Compounds are substances formed when atoms join together. Once compounds form, it is often hard to separate out the elements that formed the compound.

Formulas are used to show which atoms were used to form a compound. For example, H2O, which is water, is made up of two hydrogen atoms and one oxygen atom.

  • Another example is CO2. Carbon dioxide is made up of one carbon atom and two oxygen atoms.

Below are a few helpful formulas.

Name of CompoundMolecular Formula
Carbon dioxideCO2
Sodium chlorideNaCl
Carbon monoxideCO
Hydrochloric acidHCl
Calcium chlorideCaCl2
Sodium carbonateNa2CO3
Sulfuric acidH2SO4

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