atomic number and atomic weight
all elements are arranged in the periodic table of the elements in order according to their atomic number, which is the number of protons in the nucleus.
atom of hydrogen has one proton and one electron, as shown in fig. 1-2 (a), it atomic number is 1.
atomic weight is approximately the number of protons and neutrons in the nucleus. for example, hydrogen has an atomic number of the one and an atomic weight of one. the atomic number of helium is two, and its atomic weight is four as ahown in fig. 1-2 (b)