How do we know the number of electrons present in atom, if we have not seen the electrons? If we cannot see electrons then how do we know how many electrons are present in a given atom, for example, how do we know that oxygen has 8 electrons, Nitrogen - 7 electrons, etc?
What i know is that electrons are not possible to be observed, we have seen nucleus of an atom then how come we say that any given atom has this many electrons?
 A: Since about 1913 it was established by the experimental work of Moseley on x-ray emission of atoms (and comparison with the Bohr atomic model) that the atomic number used in the periodic table of elements corresponds to the number of positive elementary charges (proton number) of the atomic nucleus. See Moseleys law: https://en.wikipedia.org/wiki/Moseley%27s_law . It was known that the positive nucleus was surrounded by negatively charged electrons (each with an elementary charge) and that each atom in its normal state was electrically neutral. Thus it was established experimentally that the number of electrons surrounding the nucleus of the atoms has to be equal to the atomic number which is the proton number in the nucleus.  
A: We cannot "see" electrons with our eyes. But there are lots of experiments in which the presence of electrons can be detected and their behavior can be observed. That information can be transformed to create images (for example STM is sensitive to the local density of electrons so can be used for imaging.) The simplest way to know the number of electrons in an atom is based on studying the charge properties.
