92 naturally occurring elements, of these 25 are essential to life. The rest are called trace elements. Small units of matter are called atoms. Their mass is measured in units called Daltons . Different atomic forms caused by varying the number of neutrons.
Some isotopes are radioactive, they undergo a transformation to gain a stable condition.
This transformation is called the half-life of the isotope. In 10,000 years we will have 25g. etc. All electrons have the same mass and charge. They differ in the amounts of potential energy they possess. Electrons closer to the nucleus contain less potential energy.