The number of protons in an atom never changes (for our purposes!) but the number of electrons changes all the time. Every time a bond is formed or broken, or we go from a neutral atom to an ion (ion = a charged species) we are changing the distribution of electrons around that atom.
Electrons are the currency of chemistry. Just like how every transaction you make at a store involves a flow of currency from buyer to seller, every chemical reaction you’ll learn involves a flow of electrons.
What’s the #1 guide to help get a grip on the important trends in organic chemistry? Electronegativity. Think of it like an atom’s “greed” for electrons (there’s the currency analogy again). More than any other factor the electronegativity of an element determines its chemical personality.
It’s easy to remember the periodic trend here: Electronegativity increases UP and TO THE RIGHT in the periodic table. That makes fluorine the greediest atom of all (4.0), and cesium (1.0) the least.
In the next few days I’ll highlight a few important ways in which electronegativity has an impact on organic chemistry.
In the meantime, having a model in your head for determining which atom is more electronegative than another is going to be KEY. Here’s a handy mnemonic. Imagine two guys watching TV, the phone rings and one of them answers…
Remembering the order of electronegativity of atoms will help in figuring out the electron density of molecules. More on that soon.