Atomic mass generally increases across a period in the periodic table.
A period in the periodic table is a horizontal row that represents elements with the same number of electron shells. As you move from left to right across a period, the atomic number increases, indicating the number of protons in the nucleus of an atom. Since the number of protons determines the atomic mass, the atomic mass generally increases across a period.
However, it's worth noting that there can be some irregularities in the trend due to the arrangement of electrons in the atoms and the influence of isotopes. Isotopes are atoms of the same element that have different numbers of neutrons, resulting in different atomic masses. In some cases, the presence of isotopes can lead to a slight variation in atomic mass within a period.