Electronegativity is a measure of how strongly an atom in a molecule rejects extra electrons away from itself. This occurs because an atom has a positively charged centre (made up of protons and neutrons) which is continuously orbited by electrons. Outside electrons are rejected because the charge distribution of an atom can be thought of as a positive centre surrounded by negativity. This negativity rejects any outside electrons (which also have a negative charge). Electronegativity increases as the number of electrons and orbital shells around the nucleus increases; they shield the outside electrons from the postive nucleus, which attacts them, making their rejection easier. Generally, electronegativity increases across a horizontal row of the periodic table, as well as up a vertical column. Fluorine, a member of the Halogen group of elements, is the most electronegative known element. A group of elements is a vertical column, and the Halogens are group 17. Group 18, the Noble gases, are the most unreactive elements known. This is due to the fact that they have a complete octet of electrons in their valence shell.