Answer: The mass number is the sum of the numbers of protons and neutrons present in the nucleus of an atom.
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Oxygen has six valence (outer-shell) electrons so therefore gains two more electrons to form the O-2 ion Its electron configuration is: 1s2 2s2 2p6 or Ne
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1. a. FeS + 2HCl --> FeCl2 + H2S
b. This is a double- replacement reaction. (The elements "switch partners".)
2. a.2Na + F2 --> 2NaF
b. This is a composition/synthesis reaction. (The two reactants are combining or synthesizing to make one product.)
3. a. 2HgO --> 2Hg + O2
b. This is a decomposition reaction. (The single reactant is breaking down or "decomposing" into multiple reactants.)
4. Hydrogen gas reacts with oxygen gas to create water in a composition/synthesis reaction.
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