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To understand electronegativity, it's important first to understand what a "bond" is. Any two atoms in a molecule that are "connected" to each other on a molecular diagram are said to have a bond between them. This means that they share a set of two electrons with each atom contributing  one electron to the bond. The exact reasons for why atoms share electrons and bond are a little beyond the scope of this article. If you want to learn more, try this article on the bond basics or WikiHow's own How to Study the Nature of the Chemical Bond (Chemistry). When two atoms share a set of two electrons in a bond, they don't always share them equally. When one atom has higher electronegativity than the atom it's bonded to, it pulls the two electrons in the bond closer to itself. An atom with very high electronegativity may pull the electrons all the way to its side of the bond, barely sharing them at all with the other atom. For example, in the molecule NaCl (sodium chloride), the chloride atom has a fairly high electronegativity and the sodium has a fairly low one. Thus, the electrons will get pulled towards the chloride and away from the sodium. An electronegativity table of the elements has the elements arranged exactly like in a periodic table, except that each atom is labeled with its electronegativity. These can be found in a variety of chemical textbooks and technical articles as well as online.  Here is a link to an excellent electronegativity table. Note that this uses the Pauling electronegativity scale, which is most common. However, there are other ways to measure electronegativity, one of which will be shown below. If you don't have an electronegativity table handy, you can still estimate the strength of an atom's electronegativity compared to the strength of another element's atom based on where it is located on a normal periodic table. Although you will not be able to calculate a number value, you can evaluate the difference between the electronegativities of 2 different elements. As a general rule:  An atom's electronegativity gets higher as you move to the right in the periodic table. An atom's electronegativity gets higher as you move up in the periodic table. Thus, the atoms in the top right have the highest electronegativities and the atoms in the bottom left have the lowest ones. For example, in the NaCl example from above, you can tell that chlorine has a higher electronegativity than sodium because it's almost all the way in the top right. On the other hand, sodium is far to the left, making it one of the lower-ranking atoms.
Understand that chemical bonds occur when atoms share electrons. Understand how electronegativity affects the electrons in the bond. Use an electronegativity table as a reference. Remember electronegativity trends for easy estimations.