Across a period in the periodic table, the trend in atomic radius is generally a decrease from left to right. This trend is due to the increasing number of protons in the nucleus, which leads to a stronger attraction between the positively charged nucleus and the negatively charged electrons in the outer energy levels.
As the atomic number increases across a period, the number of electrons increases, occupying the same energy level. However, the number of energy levels (or shells) remains the same. As a result, the increased nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus. This contraction of the electron cloud results in a decrease in atomic radius.
It's important to note that there may be some exceptions or irregularities in this trend due to factors such as electron-electron repulsions or the influence of electron configuration. However, the general trend of decreasing atomic radius across a period is observed in most cases.