(a) Why does the He atom have a smaller radius than the H atom? (b) Why is the He

atom smaller than the Ne atom?

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Think about the number of shells and the positive charge on the outside electrons.

(a) The reason why the He atom has a smaller radius than the H atom is due to their electron configuration.

To explain this, we need to consider the position of the electrons in the atoms. The atomic radius generally increases as you move down a group in the periodic table. Since helium (He) is located below hydrogen (H) in the periodic table, we expect it to have a larger atomic radius. However, this is not the case.

In the case of hydrogen, it consists of only one electron in its outermost shell. This electron occupies the 1s orbital. On the other hand, helium has two electrons and both of them occupy the 1s orbital. Due to the presence of two electrons, the electron-electron repulsion between them causes the 1s orbital to contract, making the atomic radius smaller.

(b) The reason why the He atom is smaller than the Ne atom can also be attributed to their electron configuration.

Neon (Ne) belongs to the noble gas group and has a full electron shell with an electron configuration of 1s² 2s² 2p⁶. This means that it has a total of 10 electrons filling up its energy levels.

On the other hand, helium (He) only has two electrons filling up its energy level, with the electron configuration of 1s². Since helium has a smaller number of electrons than neon, it experiences stronger attractive forces between the positively charged nucleus and the negatively charged electrons. This pulls the electrons closer to the nucleus, resulting in a smaller atomic radius for helium compared to neon.