provide an hypothesis for the observed trend in energies for the 1s orbitals as you move to larger atoms. what change to the structure of the atom causes this trend?

As you move to larger atoms, the energy of the 1s orbital becomes lower (more negative, more stabilized). The key structural change causing this trend is the increasing nuclear charge (more protons in the nucleus), which increases the effective nuclear charge felt by the 1s electrons.

What happens to the 1s orbital energy?

• Each step to a heavier element adds a proton to the nucleus.
• The 1s electrons are very close to the nucleus and are only weakly shielded by other electrons.
• As a result, the positive charge “seen” by the 1s electrons increases.
• Stronger attraction → electrons pulled closer → lower energy (more tightly bound).

So the hypothesis for the observed trend:

As atomic number increases, the nuclear charge increases faster than any shielding affecting the 1s electrons, so the effective nuclear charge on the 1s orbital increases, causing the 1s orbital to contract toward the nucleus and drop in energy (become more negative).

Structural change in the atom

The structural change is:

• Increase in nuclear charge (number of protons Z).
• Only a modest increase in electron–electron shielding for 1s, because additional electrons mostly go into higher shells (2s, 2p, 3s, …), which shield 1s electrons only partially.
• This net increase in effective nuclear charge ZeffZ_{\text{eff}}Zeff​ on the 1s electrons is what drives the trend.

One-sentence version

As you go to larger atoms, the nucleus gets more positively charged while 1s electrons remain poorly shielded, so the effective nuclear charge they feel increases, pulling them in closer and lowering their energy.