CHEMICAL BONDS. Tellez Carmona José Manuel 1

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1 CHEMICAL BONDS 1

2 CHEMICAL BONDS; THE "GLUE" THAT HOLDS MOLECULES TOGETHER. Hydrogen "H" usually exists as the molecule (H 2 ). A hydrogen molecule is more stable than two hydrogen atoms, therefore, energy must be expended in order to "break" the hydrogen molecule into its component atoms. Atoms are "most stable" when their outermost orbitals are filled. Two hydrogen atoms, each of which has one electron, can "share" the electrons so that each effectively has two electrons ion the 1s orbital. Thereby completing it and establishing the most stable arrangement. As the atoms approach each other, each nucleus begins to attract the electron held by the other nucleus. Eventually, the electron clouds overlap and fuse into one "molecular orbital". Like an atomic orbital, a molecular orbital is most stable when filled by a pair of electrons. This shared orbital acts ad a "chemical bond" between the two atoms and resembles a strong "spring", in its properties. It can be compressed, stretched and bent to a certain extent without breaking, it can also spin like and axil or vibrate. 2

3 An atom can form as many bonds as there are unpaired electrons in its outermost orbital. The bond between two atoms of hydrogen is called a "covalent bond". One biologically important element that hydrogen can also bond to is carbon. Carbon has a total of six electrons. Two in its 1s orbital and four electrons in the outermost (second) orbital. This second energy level (orbital) can accommodate a maximum of eight electrons. Therefore, carbon is looking for four additional electrons to fill its 2p orbitals, and give it maximum stability. The unfilled orbitals of four hydrogen can form four covalent bonds by a sharing of pairs of electrons between carbon and hydrogen. 3

4 Covalent bonds Double covalent bonds The kinds of bonds in methane (CH 4 ) are "single bonds", meaning only one pair of electrons is shared between two atoms. But two atoms can share two or three pairs of electrons forming "double or triple" bonds. Carbon atoms often form double bonds. For example Ethylene. 4

5 IONIC BONDS Where covalent bonds involve shared electrons, "ionic bonds" are formed when one atom gives up an electron from an outer shell (orbital) and the other atom adds the free electron to its outer most orbital, thereby holding the atoms together in an energetically stable unit. When an atom loses an electron it would have one more positively charged proton (+) then electrons, therefore, the atom would be carrying an overall net charge of (1+). When an atom gains and electron it contains one more electron than protons and therefore would be carrying a net charge of (1-). Atoms which have gained or lost electrons are called ions. Ions are charged, atoms or molecules. Anions carry a negative charge eg. (Cl - ) while cations carry a positive charge (Na + ). 5

6 IONIC BONDS 6

7 IONIC BONDS Ionic bonds are formed when atoms become ions by gaining or losing electrons. Chlorine is in a group of elements having seven electrons in their outer shells. Members of this group tend to gain one electron, acquiring a charge of -1. Sodium is in another group with elements having one electron in their outer shells. Members of this group tend to lose that outer electron, acquiring a charge of +1. Oppositely charged ions are attracted to each other, thus Cl - (the symbolic representation of the chloride ion) and Na + (the symbol for the sodium ion, using the Greek word natrium) form an ionic bond, becoming the molecule sodium chloride,. Ionic bonds generally form between elements in Group I (having one electron in their outer shell) and Group VIIa (having seven electrons in their outer shell). Such bonds are relatively weak, and tend to disassociate in water, producing solutions that have both Na and Cl ions. 7

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9 HYDROGEN BONDS Hydrogen bonds, result from the weak electrical attraction between the positive end of one molecule and the negative end of another. Individually these bonds are very weak, although taken in a large enough quantity, the result is strong enough to hold molecules together or in a three-dimensional shape. 9

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11 Molecules are compounds in which the elements are in definite, fixed ratios. Those atoms are held together usually by one of the three types of chemical bonds discussed above. For example: water, glucose, ATP. Mixtures are compounds with variable formulas/ratios of their components. For example: soil. Molecular formulas are an expression in the simplest wholenumber terms of the composition of a substance. For example, the sugar glucose has 6 Carbons, 12 hydrogens, and 6 oxygens per repeating structural unit. The formula is written C 6 H 12 O 6. 11