Tuesday, December 6, 2011
There are three sorts of changes in matter - nuclear, physical, and chemical. Nuclear changes occur in the nucleus, or center, of atoms. Physical changes occur when a substance undergoes change, but retains its chemical composition. When chemical changes occur, there is an actual chemical change in the substance. The original atoms remain, but a new substance is created, with a different chemical composition. This is how items such as plastic are created. The substances used to make it cannot be separated, which is what distinguishes between these and mixtures, such as muddy water. If given enough time, mud and water will separate. In a compound, however, the valence electrons (the electrons in the outer energy level) are lost, gained, or shared between the different elements to create unique substances. They are held together by bonds, either ionic or covalent. I'll talk about ionic bonds today.
If an atom looses a negatively charged electron, it becomes a positive ion, because it has more positively charged protons than negatively charged electrons. If the opposite happens, and the atom looses a positively charged proton, it becomes a negatively charged ion. A positive ion and a negative ion will attract. Sodium has only one valence electron. Chlorine is an atom that is missing only one electron in order to become stable. If both come close together, the sodium's valence electron will join the chlorine atom, completing an ionic bond and forming sodium chloride, or table salt. This sort of bond is common in chemistry and has given rise to the Octet rule, which states that atoms tend to gain, loose, or share electrons in order to acquire a full set of valence electrons.
Ionic bonds share similar properties, which include high melting points, a tendency to be brittle, and dissolve in water.