The rather steady increase of atomic masses through the periodic table was explained when physicists managed to split atoms into three component particles.
The exploration of atomic structure began in 1911, when Ernest Rutherford, a New Zealander who worked in Canada and England, discovered that atoms had a dense central nucleus that contained positively charged particles, which he named protons. (See Table 1.) It was soon established that each chemical element was characterized by a specific number of protons in each atom. A hydrogen atom has 1 proton, helium has 2, lithium has 3, and so forth through the periodic table. The atomic number is the number of protons for each element.
Except for the simplest hydrogen atom with a single proton as its entire nucleus, all atoms contain neutrons (particles that are electrically neutral) in addition to protons. For most of the light elements, the numbers of protons and neutrons in the nucleus are nearly equal. Table 2 shows the most common nucleus for each element with the atomic mass rounded to the nearest integer. You can see that the rounded‐off atomic masses are the sum of the protons and neutrons for each atom. The sum of the protons and neutrons is the mass number of an atom.
John Dalton's idea that atomic masses were multiples of hydrogen mass was premature, but near the truth. The series of elements of increasing atomic masses is generated by adding nucleons, the two types of particles comprising the nucleus, which are the protons and neutrons.