No, atom properties are not determined solely by the size of valence electrons. While the size of valence electrons plays a role, it's just one piece of the puzzle.

Here's why:

* Number of Valence Electrons: The number of valence electrons is a major factor in determining an atom's chemical reactivity, bonding behavior, and ability to form ions.

* Effective Nuclear Charge: The effective nuclear charge (the net positive charge experienced by valence electrons) influences the size of the atom and the attraction between the nucleus and valence electrons. A higher effective nuclear charge results in a smaller atom.

* Electron Configuration: The arrangement of electrons in different energy levels and sublevels influences an atom's properties. For example, the presence of d-orbitals in transition metals affects their reactivity and bonding characteristics.

* Shielding Effect: Inner electrons shield valence electrons from the full attraction of the nucleus, affecting the size and energy of the valence electrons.

In summary:

While the size of valence electrons is a factor, other aspects, such as the number of valence electrons, effective nuclear charge, electron configuration, and shielding effect, all contribute significantly to determining an atom's properties.