2INTRODUCTION Cl Atoms do not tend to exist on their own Most often, they are found joined together, by CHEMICAL BONDSThis is true of elements and compounds:HOClThe only exceptions are the Noble Gas elements
3TYPES OF BONDING There are 3 types of chemical bond: Covalent – found in elements and compoundsIonic – found only in compoundsMetallic – found only in elements
4BONDING IN ELEMENTS He Only the Noble Gases exist as single atoms, e.g. Helium
5A molecule is a group of atoms covalently bonded together BONDING IN ELEMENTS 2All other elements in the Periodic Table have bonds between their atomsIn all non-metals, these are COVALENT bondsAll these non-metal elements are therefore made up of MOLECULESA molecule is a group of atoms covalently bonded together
7BONDING IN COMPOUNDS Covalent bonding also occurs in compounds The elements involved must both be NON-METALSExamples:Hydrogen oxide, carbon dioxide, nitrogen hydrideSo non-metal compounds are made up of molecules
8COVALENT BONDS+-+--+The positive nucleus of one atom attracts the negative electrons of another atomAt the same time, the two positive nuclei repel each otherThe electrons are therefore held between the two atoms in a balance between these attracting and repelling forcesThe electrons are SHARED between the two atoms
9ENERGY LEVELSElectrons are arranged around the nucleus in ENERGY LEVELSThe first energy level can hold TWO electronsOnce the atom has more than two electrons, the first energy level is fullElectrons are then found in the second energy level, which can hold EIGHT electronsHydrogenLithiumOxygenHeliumBoronNeon
10ENERGY LEVELS 2Once the second energy level is full, the third one starts to fill.The third energy level can hold EIGHT electronsNOTICE:Each time a new PERIOD is started, a new ENERGY LEVEL starts to fillAs you move across a period from left to right, a new electron is added to the same energy levelThe number of electrons an atom needs to fill its OUTER energy level is the same number as its VALENCYSodiumMagnesiumChlorineArgon
11COVALENT BONDS 2 Atoms form bonds in order to become more stable This stability is associated with having 8 electrons in their outer energy level (like the noble gases)In covalent bonding, atoms SHARE OUTER ELECTRONS so that all atoms involved have FULL OUTER ENERGY LEVELS
12COVALENT BONDS 3 Here, each fluorine atom SHARES ONE electron Both atoms now have EIGHT OUTER ELECTRONSBoth their outer energy levels are FULLNote that the number of electrons an atom shares is equal to its VALENCY
13IONIC BONDING Formed in compounds made up of a METAL and a NON-METAL Examples:Lithium chloride, Magnesium sulphideDoes not involve sharing electrons like covalent bondingInstead, there is TRANSFER of electrons
14IONIC BONDING 2 Sodium Fluorine The atom LOSES ONE electron to leave a FULL ENERGY LEVEL. This is easier than gaining seven electrons.The atom GAINS ONE electron to leave a FULL ENERGY LEVEL.
15IONIC BONDING 3The number of electrons that an atom LOSES OR GAINS is equal to the VALENCYMetals always LOSE electronsNon-metals always GAIN electronsBoth atoms then have FULL OUTER ENERGY LEVELS, and are stable
16IONSAs an atom, lithium has 3 POSITIVE protons and 3 NEGATIVE electronsWhen forming an ionic bond, the lithium atom LOSES one electronThe atom now has 3 POSITIVE protons and 2 NEGATIVE electronsThe overall charge is now 1 POSITIVE (1+)An atom which has a charge like this, either positive or negative, is called an ION.Ionic compounds are therefore NOT made up of atoms or molecules, but are made up of IONS.
17IONS 2The fluorine atom has 7 POSITIVE protons and 7 NEGATIVE electronsWhen forming an ionic bond, the lithium atom GAINS one electronThe fluorine atom now has 7 POSITIVE protons and 8 NEGATIVE electronsThe overall charge is now NEGATIVE 1 (1-)FluorineNote that when bonds form, it is the number of electrons which changes but the number of protons ALWAYS stays the same.
18SUMMARY COVALENT BONDING IONIC BONDING In both types of bonding, atoms are joining together in order to achieve a stable electron arrangementThis means that the OUTER ENERGY LEVEL must be FULL (like the Noble Gases)COVALENT BONDINGIONIC BONDINGWhen foundElements (except Noble Gases)Compounds of non-metals onlyCompounds of metals with non-metalsCompound made up of…MoleculesIonsElectrons are…SharedTransferredRelationship to valency numberNumber of electrons sharedNumber of electrons transferred