Final answer:
The metalloid in period 3 that lies between sodium (group 1, 11 protons) and chlorine (group 17, 17 protons) is Silicon (group 14, 14 protons), which is also a semiconductor.
Step-by-step explanation:
The metalloid in period 3 that has more protons than sodium and fewer valence electrons than chlorine is Silicon (Si). Sodium (Na) has 11 protons and is located in group 1 of the periodic table, while Chlorine (Cl) has 17 protons and is located in group 17.
Silicon, which is right in between these two elements, has 14 protons and is situated in group 14. It is known to be a semiconductor and exhibits properties that are intermediate between those of metals and nonmetals. This makes Silicon unique and highly valuable in industries, especially in electronics, due to its semiconductor properties.