Chemical Bonding free essay sample

The main types of chemical bonds are ionic bonding, covalent bonding and metallic bonding. Covalent Bonding A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, and other covalent bonds. Covalent bonding consists of many kinds of interaction, including ? -bonding, ? -bonding, metal to metal bonding, agostic interactions, and three-centre two-electron bonds. Picture 01: Diagram showing covalent bonding. Covalent bond means the atoms share the same valence. E. g. in the molecule H2, the hydrogen atoms share the two electrons through covalent bonding.The properties of the compounds that are formed in covalent bonding are that they can either be a liquid or gas at room temperature, they have both low melting and boiling temperatures and low heats of fusion and vaporisation, some are soluble in water while others are also soluble in non-polar organic solvents and they do not conduct electricity when molten. They can either be a liquid or gas at room temperature because they have small molecules and the attraction of forces between them are weak. They have low melting and boiling temperatures because of the weakness of intermolecular forces. Some dissolve in water while others dissolve in non-polar organic solvents because the non-polar molecular substances dissolve in non-polar solvents while polar substances dissolve in both types of solvents. They do not conduct electricity when molten due to the fact that they don’t have ions. Ionic Bonding An ionic bond is a type of chemical bonding which involves a metal and non-metal ion. This bond is formed by two oppositely charged ions. Examples of some ionic bonds are sodium chloride, magnesium chloride, calcium chloride, potassium oxide and potassium fluoride. pic] Picture 02: Diagram showing the formation of magnesium chloride In picture 2 it shows that the 2 valence electrons from the magnesium atom entered the chlorine atom. The properties of the compounds formed by ionic bonding are they all form crystals, they tend to have high melting and heats of fusion, they are very hard and brittle, they conduct electricity well when they dissolve in water or molten, they d on’t conduct electricity in the solid state, most dissolve easily in water and they react readily with each other in solutions. Dative(co-ordinate) Covalent BondingA dative (co-ordinate) covalent bond is a bond where one atom provides both of the electrons. This kind of bond is represented by an arrow starting from the donor towards the acceptor atom. Examples of dative (co-ordinate) covalent bonds are sulphur dioxide, sulphur trioxide and ammonia boron tri fluoride. Dative (co-ordinate) covalent) bonds will experience some of the same properties and observations as covalent bonds. Metallic Bonding When an atom undergoes the strong electrostatic force of attraction between stationary positive ions and mobile electrons it is called metallic bonding. pic] Picture 03: Diagram showing an electron sea model Examples of metallic bonds are copper, zinc, sodium, lithium and cobalt. The properties of compounds formed from metallic bonding are that they tend to have high boiling and melting points, they are good conductors of electricity, they are good conductors of heat, they are malleable and ductile, they are shi ny when clean and the hardness of the metal depends grain boundaries. They have high boiling and melting points because of the strength of the metallic bond. The strength of the bond depends on the type of metal it is.They are good conductors of electricity because of the mobile electrons. They are good conductors of heat because the end of the rod which is hotter will have more kinetic energy than the cooler end and since the particles are packed closely together they energy will be passed on by collisions. They are malleable and ductile because when the metal is beaten the layers lap each other . The layers can move relatively to one another without losing cohesion because of the free electrons. They are shiny because they reflect light of all wave lengths and they are usually solid and hard because they contain strong electrostatic bonding. Ionic Crystals An ionic crystal is a crystal in which the lattice-site occupants are charged ions held together primarily by their electrostatic interaction. They have at least two atoms in their base which are ionized. It has a neutral charge and so we always need ions with opposing charge. Ionic crystals can be described as an ensemble of hard spheres which try to occupy a minimum volume while minimizing electrostatic energy at the same time. In an ionic crystal there are no free electrons since they are insulators. A crystal in which the lattice-site occupants are charged ions held together primarily by their electrostatic interaction.E. g. 1 Sodium chloride -Each atom has six nearest neighbours, with octahedral geometry. This arrangement is known as cubic close packed Light blue = Na+ Dark green = Cl- Picture 02: Diagram showing arrangement of sodium chloride. E. g. 2 Cesium chloride- This has a two atom basis, where both atoms have eightfold coordination The chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the centre of the cubes. Light green = Cs+ Dark yellow = Cl- Picture 03: Diagram showing arrangement of cesium chloride. Simple Covalent molecules The simplest molecules are formed from two atoms.At a simple level a lot of importance is attached to the electronic structures. Small covalent molecules are often volatile liquids with low boiling points, can be easily vaporised, or have low melting point solids. Most small molecules will dissolve in some solvent to form a solution. Eg. 1 Chlorine- Two chlorine atoms could both achieve stable structures by sharing their single unpaired electron Picture 04: Diagram showing arrangement of chloride. E. g. 2 Hydrogen- The covalent bond holds the two atoms together because the pair of electrons is attracted to both nuclei. Picture 05: Diagram showing arrangement of ydrogen. Simple Molecular crystals Simple Molecular crystal consist of a three dimensional array of discrete molecules held together by weak Van der Waals forces this is why intermolecular forces in such solids are weak; they have lower melting and boiling points. Simple molecular crystals are not good conductors of electricity; they are in neither in a solid or molten state or in solution. This is because of lack of electrons or ions in these solids. E. g. 1Iodine- the three axes of the unit cell are all at right angles to each other and all of different length. Picture 06: Diagram showing arrangement of odine. E. g. 2 In each ice molecule, hydrogen is bonded to 4 other molecules. Picture 07: Diagram showing arrangement of ice. Giant Molecular crystals A giant molecular crystal is a crystalline solid in which the atoms are all linked together by covalent bonds. The crystal is a large molecule which accounts for the hardness and high melting point of substances as well as high heats of fusion and vaporisation. They are usually insoluble in polar and non-polar solvents. E. g. 1 Silicon Dioxide- Each silicon atom is bridged to its neighbours by an oxygen atom. Picture 08: Diagram showing the giant covalent tructure of silicon dioxide. E. g. 2 Graphite- Allotropy Allotropy is the existence of an element in two or more forms, known as allotropes in the same solid, liquid, or gas. In each allotrope, the elements atoms are bonded together in a different manner. The physical properties may differ widely but they are chemically identical. These allotropes are particularly prevalent in groups 14, 15, and 16 of the periodic table. E. g. 1Comparison of diamond and graphite- Diamonds and graphite are two crystalline allotropes of carbon. They are both covalent crystals but they differ considerably in their properties.The atoms in diamond are bonded to four others while the atoms in graphite are arranged in flat six-membered rings. Picture 09: Diagram showing the comparison of diamond and graphite structures. Polar Covalent Compounds Polar covalent compounds are when the shared electron moves towards the atom with greater mass. The Atom towards which the electron pair shift gets a slight negative charge while the other atom has slight positive charge. The polar covalent molecule has two centres of charge known as â€Å"dipole†. There is great difference in the electro negativity of the atoms. E. g.Ammonia- The ammonia molecule has a pyramidal shape which has a central nitrogen atom that has five outer electrons with an additional electron from each hydrogen. Picture 10: Diagram showing the structure of ammonia. Sodium Chloride As shown in the photograph you can see that a crystal of salt has a regular shape, this is because the structure of sodium chloride is regular and has a repeating arrangement of ions. Sodium Chloride has high melting and boiling points. A large amount of energy is required to break the crystal lattice, because the ions are held together by strong electrostatic forces of attraction.Sodium Chloride is soluble in water and other polar solvents since they have high dielectric constants; this is because they have larger electrostatic interactions between polar solvents and ions. However sodium chloride is insoluble in non-polar solvents. In solution or in their molten states, ions become free to move. On applying an electric field, the ions get drifted towards their respective electrodes. Therefore, sodium chloride is a good conductor of electricity in these states. However, it does not conduct in their solid state because of strong electrostatic forces between the ions, which does not make them mobile.Salt split up into oppositely charged ions when dissolved in water. Iodine Iodine is the most electropositive halogen and the least reactive of the halogens even if it can still form compounds with many elements. Iodine can easily sublime on heating to give a purple vapour. It is dissolvable in some solvents, such as carbon tetrachloride and it is only slightly soluble in water. Iodine normally exists as a diatomic molecule with an I-I bond length of 270 pm, one of the longest single bonds known. The I2 molecules tend to interact via van der Waals forces, and this interaction is responsible for the higher melting point. The I-I bond is relatively weak, with a bond dissociation energy of 36 kcal/mol. Diamond Diamond is the hardest natural material. It is also the least compressible and stiffest substance. It is an exceptional thermal conductor and has an extremely low thermal expansion. Diamond has a high melting and boiling point and has a high heats of fusion and vaporisation. Diamonds are not conductors of electricity. They are insoluble in water and organic solvents. There are no possible attractions which could occur between solvent molecules and carbon atoms which could outweigh the attractions between the covalently bound carbon atoms.Graphite Graphite is a dark grey crystalline. In graphite each carbon atom is covalently bonded to three carbon atoms to give trigonal geometry. Bond angle in graphite is 120oC. Three out of four valence electrons of each carbon atom are used in bond formation with three other carbon atoms while the fourth electron is free to move. The layers in graphite can slide over one another and so is very soft and is used as lubricant. Graphite ions are conductors of electricity due to the infinite electron delocalization within the carbon layers but the electricity is only conducted within the plane of the layers. It is used as moderator in nuclear reactors. Due to high melting it is used to prepare crucible for making high grade steel. Conclusion In conclusion we learnt about the types of chemical bonds, their properties and why these properties were observed. We learnt what is a covalent bond, an ionic bond, a dative(co-ordinate)covalent bond and a metallic bond. We also learnt the definitions of ionic crystals, simple covalent molecules, simple molecular crystals, giant molecular crystals, allotropy and polar covalent compounds.