What are Compounds and Molecules?

1. What are Compounds and Molecules? INTRODUCTION:Since there are only some ninety elements, but over two million known compounds, most substances are not found as independent atoms but as groups of atoms known as molecules in compounds. The compounds may be divided into two groups. Those molecules that consist of charged ions with opposite charges are called IONIC. These ionic compounds are generally solids with high melting points and conduct electrical current. The other type of molecules are called COVALENT and do not consist of ions. Covalent compounds have low melting points and do not conduct electric current.

2. COMPOUNDS • COMPOUNDS contain two or more different elements in a chemically combined form. • The simplest form of most matter, other than simple elements which still retains specific properties, is in the form of compounds. The simplest form of a compound is a molecule.

3. MOLECULE • A MOLECULE contains two or more atoms. • Since a compound contains two or more different elements, it also contains two or more different atoms, hence the simplest form of a compound is a molecule. As an example, a molecule of water contains two hydrogen atoms and one oxygen atom, (written as H2O).

4. diatomic molecule When an element consists of two identical atoms it is called DIATOMIC molecule. The simplest form of some elements is a molecule. For example oxygen in air consists of molecules containing two atoms of oxygen, (written as O2).    

5. COMPOSITION OF COMPOUNDS • Compounds contain atoms of different elements combined in whole number ratios as stated as the Law of Definite Composition. The Law of Definite Composition states that two or more elements combine to form a compound in a fixed proportion by weight without regard to method of preparation. • For example water is always 88.9% by weight oxygen and 11.1% by weight hydrogen. Through a fairly complicated process the percentages by weight are ultimately converted into simple whole number ratios. Water molecules contain a ratio of two hydrogen for every one oxygen atom . This is expressed in the form of a formula. • Water is always H2O and not HO or H3O.

6. COMPOSITION OF COMPOUNDS • A good contrast to the compound of water is the compound hydrogen peroxide, a very similar, but very distinct compound with one extra oxygen atom. The ratio is two hydrogen atoms to two oxygen atoms. The formula for hydrogen peroxide is H2O2. • The composition of compounds is indicated by a formula using symbols for the elements. The number of atoms in a formula is given by a subscript. If no subscript is given, then the number of atoms is assumed to be one.

7. COMPOSITION OF COMPOUNDS • The composition of compounds is also indicated by the names of compounds. For example carbon monoxide (CO) consists of one carbon and one oxygen atom ("mono" = one) . While carbon dioxide (CO2) consists of one carbon and two oxygen atoms ("di" = two). • As you study chemistry, there will be constant references to both formulas and names of compounds. You must learn these as they are the language of chemistry.

8. Ionic Bonding • Introduction to Ionic Bonding: • Ionic bonding is best treated using a simple electrostatic model . The electrostatic model is simply an application of the charge principles that opposite charges attract and similar charges repel. • An ionic compound results from the interaction of a positive and negative ion, such as sodium and chloride in common salt.

10. Ionic Bonding • The IONIC BOND results as a balance between the force of attraction between opposite plus and minus charges of the ions and the force of repulsion between similar negative charges in the electron clouds. In crystalline compounds this net balance of forces is called the LATTICE ENERGY. • Lattice energy is the energy released in the formation of an ionic compound. • DEFINITION: The formation of an IONIC BOND is the result of the transfer of one or more electrons from a metal onto a non-metal.

11. Ionic Bonding • Metals, with only a few electrons in the outer energy level, tend to lose electrons most readily. The energy required to remove an electron from a neutral atom is called the IONIZATION POTENTIAL. • Energy + Metal Atom ---> Metal (+) ion + e- • Non-metals, which lack only one or two electrons in the outer energy level have little tendency to lose electrons - the ionization potential would be very high. Instead non-metals have a tendency to gain electrons. The ELECTRON AFFINITY is the energy given off by an atom when it gains electrons. • Non-metal Atom + e- ---> Non-metal (-) ion + energy •  The energy required to produce positive ions (ionization potential) is roughly balanced by the energy given off to produce negative ions (electron affinity). The energy released by the net force of attraction by the ions provides the overall stabilizing energy of the compound.

12. Introduction to Covalent Bonding • Bonding between non-metals consists of two electrons shared between two atoms. In covalent bonding, the two electrons shared by the atoms are attracted to the nucleus of both atoms. Neither atom completely loses or gains electrons as in ionic bonding. • There are two types of covalent bonding: • Non-polar bonding with an equal sharing of electrons. • Polar bonding with an unequal sharing of electrons. The number of shared electrons depends on the number of electrons needed to complete the octet.

13. NON-POLAR BONDING • NON-POLAR BONDINGresults when two identical non-metals equally share electrons between them. • One well known exception to the identical atom rule is the combination of carbon and hydrogen in all organic compounds.

14. IODINE • Iodine forms a diatomic non-polar covalent molecule. The graphic on the right shows that iodine has 7 electrons in the outer shell. • Since 8 electrons are needed for an octet, two iodine atoms EQUALLY share 2 electrons.

15. OXYGEN • Molecules of oxygen, present in about 20% concentration in air are also a covalent molecules . See the graphic on the right the Lewis symbols. • There are 6 electrons in the outer shell, therefore, 2 electrons are needed to complete the octet. The two oxygen atoms share a total of four electrons in two separate bonds, called double bonds. • The two oxygen atoms equally share the four electrons.

16. POLAR BONDING • POLAR BONDING results when two different non-metals unequally share electrons between them. One well known exception to the identical atom rule is the combination of carbon and hydrogen in all organic compounds. • The non-metal closer to fluorine in the Periodic Table has a greater tendency to keep its own electron and also draw away the other atom's electron. It is NOT completely successful. As a result only partial charges are established. One atom becomes partially positive since it has lost control of its electron some of the time. The other atom becomes partially negative since it gains electron some of the time. 

17. WATER • Water, the most universal compound on all of the earth, has the property of being a polar molecule. As a result of this property, the physical and chemical properties of the compound are fairly unique. • Hydrogen Oxide or water forms a polar covalent molecule. The graphic on the left shows that oxygen has 6 electrons in the outer shell. Hydrogen has one electron in its outer energy shell. Since 8 electrons are needed for an octet, they share the electrons. • However, oxygen gets an unequal share of the two electrons from both hydrogen atoms. Again, the electrons are still shared (not transferred as in ionic bonding), the sharing is unequal. The electrons spends more of the time closer to oxygen. As a result, the oxygen acquires a "partial" negative charge. At the same time, since hydrogen loses the electron most - but not all of the time, it acquires a "partial" charge. The partial charge is denoted with a small Greek symbol for delta.  

20. CompareIonic, Polar, and Non-polar Bonds • Whereas non-polar bonding involves the equal sharing of electrons between identical non-metal atoms, POLAR BONDING is the unequal sharing of electrons between two different non metal atoms. A proper understanding of polar bonding is gained by viewing the types of bonding on a continuum as in the diagram on the top left. Ionic bonding is on one extreme with a complete transfer of electrons forming charged ions. Non-polar covalent bonding with equal sharing of electrons is at the other extreme. Somewhere in the middle but favoring the covalent side is polar bonding with unequal sharing of electrons and partial but incomplete transfer of electrons.

21. Comparison of Lewis Diagrams of Ionic, Polar and Non-Polar Bonding: • The best way to show and represent the unequal sharing of electrons would be by comparison with NaCl and HCl, and H2 using Lewis diagrams. • The captions below correspond to the graphic on the bottom left. • IONIC: Complete transfer of electrons, therefore Na becomes positive (lost e-) and Cl becomes negative (gained e-). • POLAR: Unequal sharing. Chlorine has a greater tendency to keep its own electron and also draw away hydrogen's electron. It is NOT completely successful. As a result only partial charges are established. Hydrogen becomes partially positive since it has lost control of its electron some of the time (H +). Chlorine becomes partially negative since it gains hydrogen's electron some of the time (Cl -).

23. polar bond In summary, a polar bond results when different atoms share electrons. One atom will attract the bonding electrons more strongly than the other atom and will acquire more than a half share of these electrons. This leaves the other atom with less than a half share and makes the electron distribution unsymmetrical. On a time-average basis the electrons spending more time with one atom and cause it to have a partial negative charge. The other atom, deficient in electrons, acquires a partial positive charge.

24. NON-POLAR • Equal Sharing. Neither atom can dominate the other, therefore the electrons are shared equally between them.