How come an atom become an ion?

Answer 1

Normally in an atom the number of positively charged protons and negatively charged electrons is the same. Thus an atom as a whole is electrically neutral since the charges of protons and electrons cancel each other (neutron has no charge).

But an atom can acquire charge. Such an atom having a net charge is called an ion.

The charge may be positive or negative depending on how it is formed.

Let us see how an ion is formed.

Consider a hydrogen atom. At neutral state it has 1 proton and 1 electron (We don't have to bother about neutrons when discussing about ions and hydrogen atom contain no neutrons). Assume that during a chemical reaction the hydrogen atom lost the one electron. Now it has one proton and no electron. So the charge of proton dominates and we say that the hydrogen atom has become an ion having a positive charge of one (since it has lost one electron). We can represent the ion as: H+

Now let us look into the formation of a negative ion.

Consider an oxygen atom ( 8 protons; 8 electrons; 8 neutrons). Since number of protons = Number of electrons, oxygen atom is neutral. Suppose it has took part in a chemical reaction and accepted two electrons. Now the oxygen atom contain 8 protons and 10 electrons. So the charge of electrons dominates and we say that the oxygen atom has become an ion havig a negative charge of two ( since it has 2 additional electrons ). We can represent the ion as: O2-

Answer 2

Atoms form ions so that they can have a full valence shell of electrons. Many atoms will lose electrons and become positive ions, so that their electron configuration is like that of a noble gas. Certain atoms can gain electrons, becoming negative ions to reach a noble gas configuration.