1. Chemical Bonds

2. Chemical Chemical Bond Molecules are made up of atoms joined together. The links that joined these atoms are called chemical Bonds. The properties of molecules are determined by its chemical Bonds. The number of bonds per atom and the direction and strength of these bonds are all important in determining the shape and chemical reactions of molecules

3. Octet Rule The noble gases neither gain nor lose electrons easily, therefore they are relatively unreactive, and all the noble gases, except helium, have an octet of electrons in their electron shell. Atom of other elements gain or lose electrons from their outer shell to achieve the noble gas electron arrangement. This suggests that an octet of electrons in the outer shell is particularly stable arrangement in nature. (This is called the octet rule).

4. Octet Rule cont… An atom can gain an octet of electrons in either of two ways: 1. One way is transfer of outer shell electrons from an atom, of one element to an atom of another element, forming (ionic bond), the compound formed in this way is called (ionic compound). 2. The second way is the sharing of outer shell electrons by two or more atoms forming covalent bond. Ionic Bond: it is formed by a strong attraction between negative and positive ions like cations (Na +,Li +,K +,Ca +2 Mg +2,Al +3 ) and anions like (Cl -,F -,Br -,O -2,Se -2 ) all these anions & cations have a stable noble gas electron arrangement in their outer shell.

5. Octet Rule cont… An ionic bond is formed between atoms of wide differences in electronegativity (e.g.) sodium chloride. Alkali earth metal lose two electrons to form cations with positive charge, halogen atom can accept these elections but two halogen atom's are needed. Ex1 beryllium fluoride (BeF2)

6. Ionic Compound Ionic compounds are composed of cation and anions held together by ionic bond, they are solid at room temp usually called salt example, NaCl, Li 2 O some salt have medicinal uses as : Name formula uses Name formula uses 1- Silver Nitrate AgNO 3 Germicide & antiseptic 2- Calicium sulfate CaSO 4 Plaster Casts 3- Potassium permanganate KMnO 4 Antiseptic 4- Zinc sulphate ZnSO 4 use to treat skin conditions such as eczema such as eczema 5- Ferrous sulfate FeSO 4 use to treat simple iron deficiency (Anemia) deficiency (Anemia) 6- Zinc oxide ZnO for skin lotion Ions: Positive ions are called Cations Negative ions are called anions Cations are formed when atoms lose electrons Na +1, Ca +2 Anions are formed when atoms gain electrons Cl -, O -2, S -2

7. Ionic Compound Cont…. The charge on an ion is determined by the number of electrons it must gain or lose to achieve the electron arrangement of nearest noble gas. the physical and chemical properties of ions and their elements are completely different. (ex) the elemental iron is the major component of steel, show metallic luster and good conductor of heat and electricity. Ferric ion on the other hand exist only in combination with an ion in compounds, these compounds have none of the properties associated with metal, the difference between elemental iron & ferrous ion is clearly illustrated by the clinical treatment of anemia. People who are anemic are deficient in ferrous ion, these people are given a compound that contains ferrous ion to cure anemia. They are not given small pieces of iron metal, the body recognize that the element iron (Fe) & ferrous ion (Fe +2 ) are entirely different.

8. Covalent Bond It is formed by sharing electrons to achieve a noble gas electron arrangement for each atom, it is formed between atom of equal electronegativity e.g Some times atom share with more than one electron that is two or three electrons pair can be shared between atoms to form double or treble bond e.g C = C & C  C

9. Covalent Bond Cont….. The tendency to form strongly normal covalent bond is found in element which exist as diatomic molecules like H 2, O 2, F 2, Cl 2, Br 2, these molecules are very stable and energy librated as the form of heat when covalent bond is formed H· + ·H  H 2 + 25 Kcal/g of H 2 exothermic This energy is required to break these bonds and return atoms to their states (endothermic)

10. Polar Covalent Bond Is bond formed by unequal sharing of a pair of electrons. The ability to attract electrons differs from atom to atom, the electronegativity of an atoms is a measure of this ability, the greater the electronegativity of an atom the more likely it to attract electrons. The polar covalent bond formed when there are intermediate differences in electronegativity. e.g the bond in HCl is a polar covalent Bond, the hydrogen & the chlorine atoms share a pair of electrons because chlorine is more electronegativity than hydrogen the electron pair is drawn to word chlorine, giving it a partial negative charge (-) and giving a partial positive charge on hydrogen (+) The electro negativity of H & C are nearly the same then the electron pair in the Cـــــ H as shared about equally them.

11. Bonding Capacities of Atoms Atoms of element H,C,N,O,S & halogens form polar covalent bonds with atoms of other element, the number of bonds formed depending upon the no of electrons it needs to gain a noble gas electron arrangement, e.g the carbon has four electrons in outer shell it must get four electrons to gain noble gas arrangement. one carbon atom combine's with 4H atoms to form molecules CH 4

12. Bonding Capacities of Atoms Also O 2 has six electrons in outer shell, in H 2 O molecule forms bonds with 2 Hydrogen atoms as Hــــــــ O ــــــ H. The number of bonds formed is important in living system, also the structural formula is very useful representation of molecules to know from them the way atoms in molecules are joined together and allowed us to establish a relationship between the structure of molecule & it's chemical properties.

13. Resonance Theory More than one structural foromula can be written to represent certain molecules & ions. The Carbonate ion (CO 3 -2 ) is an example the three following structural formula can be written for carbonate: Here the positions of atoms have not changed only. the locations of bonding electrons have changed. The electron structure of the real molecule or ion is regarded as hybrid of this structure. the hydride structure has chemical & physical properties different from those expected of the contributing structures.