Question

For each reaction, show how electrons are gained and lost:
2Na (s) + Br2(l) 🡪 2NaBr (s)

2Ca (s) + O2(g) 🡪 2CaO (s)

Answer 1

Both are done the same way. You must have a working knowledge of the oxidation states of the elements in the reaction. The first one is:
2Na(s) + Br2(l) 🡪 2NaBr(s)
Na is zero on the left and 1+ EACH on the right for total of 2+.
Br is zero on the left and 1- EACH on the right for a total of 2-.
So Na going from zero on the left to 2+ on the right is a lost of 2 electrons.
Br going from zero on the left to 2- on the right is a gain of 2 electrons.