Which of the following elements has the fewest valence electrons in the ground state? A) Boron, B) Chlorine, C) Sulfur, D) Nitrogen, or E) Neon.
For an atom, the valence electrons are those in the outer shell. So, if we had two electrons in the 1s shell only, those would be the valence electrons. But if we also had two electrons in the 2s subshell, those would be the valence electrons because they are in a higher shell. To count the number of valence electrons, we find the highest-value shell, in this case three, and we add up all the electrons in that shell. The question’s asking which of the five elements has the fewest valence electrons in the ground state, by which it means we have to look at the atoms for those elements and then count up the valence electrons for those atoms and then find the atom with the fewest valence electrons.
We get a lot of information for free when we look at the periodic table. If we find the group for all these elements, we’ll be able to predict the number of valence electrons in their atoms. On the periodic table, elements are arranged in two groups. The element boron is in group 13. For this area of the periodic table, the P block, we can work out the number of valence electrons for atoms of these elements by taking the group and subtracting 10. Boron is in group 13, so an atom of boron has three valence electrons. The element chlorine is in group 17, so an atom of chlorine has seven valence electrons. Sulfur is in group 16, so we subtract 10 to give six valence electrons per atom.
Nitrogen is in group 15, so atoms of nitrogen have five valence electrons. And finally, neon is in group 18, and atoms of neon have eight valence electrons. Out of these five elements, the element whose atoms have the fewest valence electrons in their ground state is boron. To reassure yourself, we can go to the periodic table and get the atomic number for each element. The atomic number is equal to the number of protons in an atom’s nucleus. And since we need as many electrons as we have protons to make a particle neutral, it’s also the number of electrons the atom has. So, for boron, we have five electrons per atom, chlorine 17, sulfur 16, nitrogen seven, and neon 10.
This means, for boron atoms in their ground state, we have an electron configuration of 1s², 2s², 2p¹ with clearly a total of three valence electrons. For a chlorine atom, the configuration is 1s², 2s², 2p⁶, 3s², 3p⁵, seven electrons in the valence shell. For sulfur, we can see six valence electrons. For nitrogen, we can see five. And for neon, we can see eight. So, there we have two different ways to determine the number of valence electrons per atom for an element. And in this case, of the five elements given, the one whose atoms have the fewest valence electrons in their ground state is boron.