In modern chemistry, we follow the Hill notation (a system devised by Edwin Allston Hill in 1900) for covalent compounds similar to BODMAS rule for writing mathematical equations. read more
According to the convention the in a molecular formula of a compound positive ion is written on left side and negative ion on the right side. In H2O hydrogen is positive (H+) and Oxygen is negative(O2-). As positive is written on left and negative on the right molecular formula will be H2O. read more
Traditionally, it is written so that the cation is mentioned first, and the anion is second. In this case, H is the cation (plus charge) and the O is the anion (negative charge). This goes with most of the compounds you see. read more
In fact, you can write any compound in any order-- for example, you could call ethylenediamine--which in the book is NH2CH2CH2NH2-- N2C2H8, or C2N2H8, or even H8N2C2, but the reason why it's written the way it is in the book, is to emphasize the order of bonding when drawing Lewis structures, or to clarify concepts. read more