Answer: I think the answer is Cesium (Cs)
Explanation:
A half-filled 6s subshell would be 6s^1
Sodium<span> has one naturally occurring </span>isotope<span>. The </span>nucleus<span> of this isotope contains 11 </span>protons<span> and 12 </span>neutrons<span> and is not radioactive. There are 18 other known isotopes of sodium. All are radioactive. Sodium-22, sodium's most stable radioactive isotope, has a </span>half-life<span> of 2.6 year</span>
(4) PCl5--> PCl3+ Cl2 is the correct answer because we both have 1 P atom and 5 Cl atoms on both sides, and thus the masses of the elements don't change.
Hope this would help~
