Why do atoms combine?

Hint: Atoms are the smallest particles and have incomplete octets in its outermost valence shell and have strong tendency to combine to attain stable electronic configuration. Now you can easily answer the statement.

Complete answer:
First of all, let’s discuss the atoms. An atom is the smallest particle of an element. It is made up of positively charged particles i.e. the protons and the negatively charged particles i.e. the electrons.
The positively charged particles are present in the center of the atom i.e. in the nucleus of the atom and the negatively charged particles i.e. electrons are revolving in the atom around the nucleus in fixed circular orbits. ( the circular path around the nucleus is termed as the orbits).
Atoms combine among themselves to form molecules and then, molecules combine among themselves to form compounds. It is so because the atoms in its outermost valence shell have a maximum of eight electrons and in order to complete this octet, they have a strong tendency to combine among themselves to acquire the stable electronic configuration of the nearest noble gas. Example: oxygen has six electrons in its valence shell and has a strong tendency to combine to acquire the stable electronic configuration of the nearest noble gas.
So, thus from the above it is clear that atoms combine to acquire stable fully filled electronic configuration of nearest noble gas.

Note: The number of electrons present in an atom is always equal to the number of the protons in the nucleus to make electrical neutrality and constitutes the atomic number and the number of protons and the neutrons in the nucleus constitutes its mass number.