Here's how it works:
1. Identify the more electronegative element: The element with higher electronegativity is written first.
2. Exceptions for metals: Metals are generally less electronegative than nonmetals. When a metal and nonmetal combine, the metal is written first.
3. Common exceptions: There are a few exceptions to this rule, mainly due to historical reasons. For example, hydrogen (H) is sometimes written first in binary compounds, even when it's less electronegative than the other element.
Examples:
* NaCl: Chlorine (Cl) is more electronegative than sodium (Na), so it's written first.
* H₂O: Oxygen (O) is more electronegative than hydrogen (H), but hydrogen is written first due to convention.
* CO₂: Oxygen (O) is more electronegative than carbon (C), so it's written first.
* Fe₂O₃: Oxygen (O) is more electronegative than iron (Fe), but iron is written first because it's a metal.
Understanding Electronegativity:
* Higher electronegativity means an atom has a stronger pull on electrons.
* The periodic trend of electronegativity is generally:
* Increases across a period (from left to right).
* Decreases down a group (from top to bottom).
In summary, the element with the higher electronegativity is usually written first in a binary compound, unless the compound involves a metal, in which case the metal is written first.
