The ordering of elemental symbols in a compound follows certain conventions and is not solely based on atomic numbers. The general guidelines for writing chemical formulas are as follows:
The element with the lower atomic number usually comes first. For example, in H2O, hydrogen (H) has a lower atomic number than oxygen (O), so it is written first.
Hydrogen is typically written first, except when it appears as a negative ion (hydride). For example, in NaH (sodium hydride), hydrogen is written last because it is acting as a negatively charged ion.
Polyatomic ions are often written in a specific order based on their charge and/or convention. For example, in NaOH (sodium hydroxide), the hydroxide ion (OH-) is written last because it acts as a negatively charged unit.
It's important to note that these guidelines are not absolute rules and there can be exceptions depending on the specific compound or context. For complex compounds, the ordering may also be influenced by other factors such as the structural arrangement of atoms.
In summary, the ordering of elemental symbols in a compound is generally based on conventions and considerations such as atomic number, the role of hydrogen, and the nature of polyatomic ions.