What is the electron configuration trend as you move across a period in the periodic table?

As you move across a period in the periodic table, the trend is that electrons are added to the outermost shell, which corresponds to an increase in atomic number. Each element in a period has one more proton and one more electron than the preceding element. The additional electrons enter the same principal energy level (or outermost shell) of the atom, specifically filling the available subshells (such as s, p, and potentially d subshells in later periods). This pattern is fundamental to understanding how elements behave chemically, as the outermost electrons, often referred to as valence electrons, play a crucial role in bonding and chemical reactivity.

The other options mentioned do not accurately describe the trend. For instance, electrons from inner shells are not lost; instead, the focus is on the addition of electrons to the outer shell, as these inner electrons remain constant during the progression across a period. Additionally, although d subshells are filled in transition metals and later periods, the general trend for the initial elements in a period pertains to filling the s and p subshells. The number of protons actually increases as one moves right across a period, not decreases. Thus, the correct understanding of the trend is focused on the addition of