In the periodic table, as we move from left to right within a period, the atomic number of elements increases. The atomic number represents the number of protons in an atom's nucleus and determines the element's identity. Each element has a unique atomic number.
However, the mass of an atom, represented by its atomic mass or atomic weight, can vary even within a period. Atomic mass takes into account the total number of protons and neutrons in the nucleus, as well as the electrons' contribution (which is relatively negligible compared to protons and neutrons).
The reason for the increase in atomic mass as we move across a period is primarily due to the increasing number of neutrons in the nucleus. Neutrons are electrically neutral particles found in the nucleus alongside protons. While the number of protons (and hence the atomic number) remains constant within a period, the number of neutrons can vary. Isotopes of an element have the same number of protons but different numbers of neutrons.
Elements within the same period often exhibit a gradual increase in the number of neutrons from left to right. Since the atomic mass is determined by the combined mass of protons and neutrons, the increase in neutron count leads to a corresponding increase in atomic mass.
It's important to note that the pattern of increasing atomic mass within a period can be disrupted by certain factors, such as nuclear stability or the presence of isotopes. Some elements may have stable isotopes with lower atomic masses compared to adjacent elements. This is why the atomic mass listed on the periodic table is often an average value, considering the abundance of different isotopes found in nature.
One notable exception to the general trend of increasing atomic mass within a period is helium (He). Helium is the second element in the periodic table, but it has an atomic mass of around 4, which is significantly higher than hydrogen (H), the first element. This discrepancy is because helium has two protons and two neutrons, while hydrogen has only one proton and no neutrons, making helium relatively heavier than its position in the periodic table would suggest.