Chemistry Compounds

Describe the difference between a mixture and a compound.

Suppose that you have a pure substance. How can you tell whether it is a compound or an element? What is the difference between an ionic and a covalent bond?

Explain why ionic compounds are formed when a metal from the left side of the periodic table reacts with a nonmetal from the right side. Give two examples of such compounds.

Explain why covalent bonds are formed when nonmetals from the right side of the periodic table bond with each other. Give two examples of such compounds

1. In every case encountered in nature a mixture of substances is inhomogeneous and a compound is  homogeneous. In other words the observable difference between them is in the physical properties, although there are also significant chemical differences.
2. There are no general guidelines in telling that a substance is a compound or an element. In telling a substance is an element always remember that the vast majority of chemical elements (around 4/5 of them) are metals and metals have characteristic physical (electrical and thermal conductivity, high density, almost all of them are solids with high melting point, etc) and chemical properties. From the rest of the elements 6 of them are rare gases (always), 5 of them (F, Cl, Br, I, At) have characteristic chemical properties (specific smell, etc),, another two (O, N) form the vast majority of the atmosphere and the rest (C, P, Se) have also some characteristic physical and chemical properties.
3. The difference between an ionic and a covalent bond lies in the place where the outer electrons are located. In an ionic bond always the electrons are belonging to a single atom (they literally stay next to the atom), one of the atoms of the ionic bond is giving its valence electrons and becomes positively ionized and the other atom is receiving electrons on its outer shell and becomes negatively ionized. The electrons given stay literary next to the negative ion. In a covalent bond the outer electrons (bonding electrons) belong to both atoms, in other words they are shared between 2 atoms and belong equally to both atoms. The valence electrons forms a new bonding orbital which is common to both atoms. This is best described by the quantum mechanics.
4. 5. By definition there is no pure ionic or covalent bond. Strong ionic bonds are formed between elements from far left and right side of the periodic table (group 1 and 7), whereas strong covalent bonds are formed between elements that lies in the middle of the periodic table (group 4).
This happens BY DEFINITION and is because the elements from the far left and right of the periodic table have a strong tendency to ionize (to form ions – loose or gain electrons to be stable on the outer electron shell) and the elements from the center of the periodic table have the tendency to put together electrons (as is demonstrated by the quantum mechanics).
Example of almost pure ionic substances are NaCl and LiF.
Example of almost pure covalent substance is the Carbon to Carbon bond (which happens in monomers or polymers) and the Si-Si bond which happens in semiconductors. Also less pure covalent bonds are in the semiconductors AlP and GaAs.