As you move to larger atoms (with higher atomic numbers), the energy of the 1s orbital becomes more negative (lower in energy) due to the increasing nuclear charge. This is because the nucleus contains more protons, which increases the positive charge attracting the electrons closer to the nucleus, thus lowering the energy of the 1s electrons. However, this effect is moderated by the addition of more electron shells, which increases the distance of the 1s electrons from the outer electrons and introduces electron-electron repulsions (shielding effect), slightly reducing the effective nuclear charge felt by the 1s electrons. The key structural change causing this trend is the increase in nuclear charge (number of protons) as you move to larger atoms, which pulls the 1s electrons closer and lowers their energy. At the same time, the addition of inner electron shells increases shielding, which can offset this effect to some extent but does not overcome the dominant influence of the increasing nuclear charge on the 1s orbital energy
. In summary, the hypothesis for the observed trend in 1s orbital energies as atomic size increases is:
- The increasing nuclear charge in larger atoms causes a stronger electrostatic attraction between the nucleus and the 1s electrons, lowering their energy.
- The addition of inner electron shells increases shielding and electron-electron repulsion, slightly reducing the effective nuclear charge experienced by the 1s electrons.
- Overall, the dominant effect of increased nuclear charge leads to a decrease in 1s orbital energy (more negative, more tightly bound) as atomic number increases.
This explains why 1s electrons in larger atoms are more tightly bound despite the presence of more electron shells
