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Three different types of bonds in chemistry exist: covalent, ionic, and polar. All matter, in whatever state it exists, is held together by the interaction of that substance's molecules or atoms. Covalent bonds are created when atoms or molecules share electrons; Ionic bonds are created when atoms or molecules transfer electrons; Polar, or Hydrogen bonds, are created when a difference in electrical charge occurs across a molecule. A molecule of water, (H2O) comprised of two hydrogen and one oxygen atom, exemplifies a Covalent Bond -- the oxygen and hydrogen share their outer electrons so that the molecule has a stable configuration. Adding another oxygen to that molecule creates hydrogen peroxide, (H2O2) which is also covalently bonded, but a much less stable configuration than water. The weakly held extra oxygen tends to break from the molecule, transforming hydrogen peroxide into water and oxygen gas. Common table salt, or sodium chloride (NaCl) is an example of the stronger Ionic Bond - a positively charged sodium atom (Na+) bonds with a negatively charged chlorine atom (Cl-) and the electrostatic charge keeps the molecule together. The characteristic properties of melting and boiling points of a substance give some indication of the bonds that must be overcome or formed for that substance to change its state. The melting point and boiling point for sodium chloride are 801 C and 1465 C. However, sodium fluoride (NaF) is also held together by an Ionic bond, but here the bonding of the atoms is so strong that the melting point and boiling point are 993 C and 1700 C. Finally, water can also exhibit the formation of Hydrogen or Polar Bonds between its molecules; one end of a given water molecule has a slight positive electrical charge, whereas the other end has a slight negative electrical charge. These intramolecular bonds give water its unusual but well-known melting and boiling points of 0 C and 100 C. In contrast, Hydrogen sulfide (H2S), even if similar in construct to water (H2O) has no polar bonds between its molecules, and exhibits melting and boiling points of -85 C and -60 C. In other words, it changes state from solid to liquid and liquid to gas with far less energy than water due to its lack of Polar Bonds.
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