If a neutral atom has 20 electrons, we can determine the number of valence electrons by looking at its position in the periodic table.
In the case of an atom with 20 electrons, it corresponds to the element calcium (Ca), which is in group 2 of the periodic table.
Elements in group 2 have their valence electrons in the outermost s subshell, specifically the 4s subshell. The s subshell can hold a maximum of 2 electrons. Therefore, calcium (Ca) has 2 valence electrons.
It's worth noting that for elements beyond the second period of the periodic table, the valence electrons can also include those in the d or f subshells. However, in the case of calcium, with 20 electrons, only the 4s electrons are considered valence electrons.