Question

why do some atoms react with other atoms to form compounds wher as some do not?

Answer 1

Atoms are made up of Electrons, Protons, Neutrons.

Protons and Neutrons are in the nucleus while electrons orbit around the nucleus. 

Depending on orbit or shell number, each shell holds 2n2 number of electrons (octet rule). So first orbit will hold 2 electrons, 2nd can hold 8, 3rd can hold 18 and so on. 

But when the valence or outermost shell of an atom is not completely full or occupied by electrons, then the atom is considered as Unstable meaning it can be under reaction to exchange electrons to form a  stable out shell. 

For example, 
NaCl. In Sodium there is 1 electron in outermost shell and in Cl, there are 7 electrons in outermost shell. So if Sodium gives 1 electron to Cl, both elements attends stable outermost shell to form NaCl compound.

But in Helium gas, the electronic configuration is 1s2. Its outermost shell (which is first shell) can only hold 2 electron which is does hold. Thus it's outermost shell is stable and cannot take part in electron exchange to form compounds. 

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Answer 2

Explanation:

An atom reacts with another atom in order to completely fill their octet. When the valence or outermost shell of an atom is not completely filled then the atom is considered as unstable.

For example, sodium atom has one extra electron in its valence shell and chlorine has deficiency of one electron. So, chlorine will readily react with sodium atom in order to attain stability.

Whereas atoms of noble gases like helium, neon, argon etc have completely filled valence shells, therefore, they do not react with any other atom because they are stable.