 Why is electronegativity important? Well here's an example. The water molecule H2O is made of two atoms that have quite different electronegativities. This means that one end of the molecule, the oxygen end, has much more electron density than the other end. Here's a simulation from FET that illustrates the situation. The red colour around the oxygen indicates greater electron density. Because the oxygen is more electronegative the bonding electrons are spending more time around it than they are around the hydrogen atoms. Because electrons are negatively charged the oxygen itself effectively takes on a slight negative charge. While the hydrogens which are a bit electron deficient because the electrons keep zipping off to spend time with the very attractive oxygen have a slight positive charge. This feature a molecule with two oppositely charged ends which we call a dipole means that water molecules stick to one another quite well like little magnets. This in turn leads to water having a much higher boiling point than other molecules of its size like methane which means that it is liquid at normal atmospheric temperatures and pressures. And that means that on earth we have oceans and it means that life was able to evolve in those oceans in a largely watery form. What I'm saying is that this little difference in the balance of nuclear charge and electron levels between two types of atom is responsible for life on earth as we know it. So if you're finding learning your periodic trends a bit tedious just sit back for a minute and consider that you are probing some of the details of atomic structure that have after about four billion years of evolution given rise to you.