Answer:
[tex]\boxed {\boxed {\sf D. \ Sodium}}[/tex]
Explanation:
Valence electrons are electrons in the outermost shell.
One way to find valence electrons is with the electron configuration. This is often found on the Periodic Table. The last number in this configuration is the number of valence electrons, because an electron configuration shows where electrons are located and the last number represents the outer shell.
- Chlorine: 2-8-7 (7 valence electrons)
- Calcium: 2-8-8-2 (2 valence electrons)
- Boron: 2-3 (3 valence electrons)
- Sodium: 2-8-1 (1 valence electron)
In addition, all group 1 or alkali metals have 1 valence electrons. These include lithium, sodium, potassium, rubidium, cesium, and francium.
Even though we already know it is sodium based on valence electrons, here is an explanation of the reaction ratio.
Since sodium has 1 valence electron, it wants to get rid of it to satisfy the Octet Rule (8 valence electrons). Group 17 elements have 7 valence electrons, so they need 1. The two elements will react in a 1 to 1 ratio because the sodium donates its 1 excess electron to the group 17 element that needs 1.
