For an isolated atom, why do we expect the number of electrons present in the atom to be the same as the number of protons in the nucleus of the atom?

Explanation:

As we know , that atom consist of three fundamental sub-particles proton, neutron and electron. Proton has positive charge and electron has negative charge while neutron has no charge. As an atom has a property of neutral, so in order to maintain neutrality, the number of proton (positive charges) and number of electron (negative charges ) are always equal. As these charges are equal in magnitude and have opposite charges. so they cancelled the effect of each other and atom become neutral.

Example:

it's c. elements are placed in rows based on how may valencies electrons have.