Alkanes are hydrocarbons that only contain single bonds in them. A carbon can bond with up to 4 atoms, even with another carbon atom. So, in a C-C bond, 3 more H atoms can bond to each of the C atom. Generally, the chemical formula for alkanes is CₓH₂ₓ₊₂. So for butane, there are 4 C atoms. The corresponding H atoms are 2(4) + 2 = 10. That's why it's chemical formula is C₄H₁₀.
