An atom has two major parts in its structure. They are the nucleus and the electron cloud.
The nucleus is formed by protons and neutrons. Protons are positively charged, but they are bonded together through neutrally charged neutrons. The atomic number of an atom is the number of protons in the nucleus. The number of neutrons in an atom is usually equally to the number of protons or greater than that. So even a single type of atom can have different numbers of neutrons in their nuclei. This leads to the existence of isotopes, which have same atomic number but different atomic weight. But Hydrogen has no neutrons in its nucleus, it consists of only one proton.
Atomic mass of an atom is the total mass of protons, neutrons and electrons. It is measured by amu (atomic mass unit) or dalton. One dalton is approximately equal to the mass of a proton or neutron.
The number of electrons in the electron cloud is the same as the number of protons in its nucleus. The electrons are orbiting around the nucleus in different orbit levels or different energy levels, The arrangement and number of electrons in its outer most shell determines the chemical properties of an atom.