As you move across a period the atomic number increases. Similarly, as you move down a group the atomic number increases. In this way, the atomic number represents exactly where in the periodic table an element stands.
More importantly, and the reason why the ordering of the elements according to atomic number yields elements in groups with similar chemical and physical properties, the atomic number is the same as the number of protons in the nucleus of an atom of an element, and also the same as the number of electrons surrounding the nucleus in a neutral state. Carbon, for example, has six protons and six electrons. (Protons and electrons will be discussed in more detail in the Atomic Structure SparkNote)
Along with protons, an atom also contains neutrons in its nucleus. The atomic mass (also called atomic weight) of an element is the combined number of protons and neutrons in the nucleus.
Atoms of particular elements generally have different "versions," meaning that elements have atoms with different numbers of neutrons in their nucleus. These different versions are called isotopes. The atomic weight displayed is actually the weighted average of the mass numbers of the various isotopes. The atomic weight for Carbon is 12.01 because around 99% of all carbon is the carbon-12 isotope.
The Atomic number increases from the top left to the bottom right. It ascends sequentially across each period.
Weight The atomic weight of the elements generally increases as you move down a group and across a period. Hydrogen, at the top left of the table, is the lightest element. The unnamed element 112 is the heaviest. There are some instances when this rule does not hold true, however. For instance, because it has a high percentage of isotopes with many neutrons, the atomic weight of tellurium (Te) is higher than that for iodine (I), even though iodine has a higher atomic number.