During which period did mammals first appear?

The period between the extinction of the dinosaurs and the present day is called the Age of Mammals or Cenozoic. Mammals appeared on the earth long before the extinction of the dinosaurs; in fact, dinosaurs and mammals originated within 10 million years of each other, in the late Triassic about 200 million years ago.

The proton has a positive charge while the neutron has no charge they both make up the nucleus of the atom while the elections have a negative charge and surround the nucleus of the atom. the protons and electrons are used to calculate the atom's mass whereas the neutrons don't affect the mass