Why would you expect strontium to be chemically more similar to calcium? A; They are in the same family/group.
B; They are in the same period.
C; They have the same atomic number.
D; They have the same atomic mass.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.

instead of copying and pasting things off of tests or work done on computer which im sure you did, try and read over the question carefully