Final answer:
Sodium (Na) must lose one electron to achieve stability, while strontium (Sr) needs to lose two electrons. Sulfur (S) aims to gain two electrons for stability, and astatine (At), not As, needs to gain one electron, generally following the trends of other halogens in group 17.
Step-by-step explanation:
To determine how many electrons an atom needs to gain or lose for a stable outer electron configuration, we refer to the octet rule which states that atoms tend to adopt a noble gas electron configuration, usually eight valence electrons, to become more stable.
For sodium (Na) with the electron configuration of 1s²2s²2p¶3s¹, it must lose one electron to achieve the stable electron configuration of neon, becoming Na+. Strontium (Sr) is in group 2 and has two valence electrons; thus, it will tend to lose two electrons to achieve a noble gas configuration, becoming Sr2+. For sulfur (S), which typically has six valence electrons, it will gain two electrons to fill its 3p orbital, resulting in the stable S2- anion.
Lastly, there seems to be a typo in your question for astatine - the correct chemical symbol is At, not As (which stands for arsenic). Astatine (At) falls within group 17 (halogens), which means it needs to gain one electron to complete its outer shell, similar to other halogens. However, given that astatine's behavior is less well-documented due to its radioactivity and rarity, predictions are more speculative compared to other group 17 elements.