When hydrogen atoms combine with highly electro-negative elements such as fluorine , oxygen , chlorine, and nitrogen to form covalent compounds , they form palarity or bipolar. When the Paella molecules formed in this way come close to each other, the positive hydrogen edge of one molecule is particularly attracted to the negative edge of the other molecule, creating a weak bond. This weak attraction is called hydrogen bonding. The hydrogen bond is expressed by the dot dot "......" sign.