CHEMICAL BONDING

INTRODUCTION Cl Atoms do not tend to exist on their own Most often, they are found joined together, by CHEMICAL BONDS This is true of elements and compounds: H O Cl The only exceptions are the Noble Gas elements

TYPES OF BONDING There are 3 types of chemical bond: Covalent – found in elements and compounds Ionic – found only in compounds Metallic – found only in elements

BONDING IN ELEMENTS He Only the Noble Gases exist as single atoms, e.g. Helium

A molecule is a group of atoms covalently bonded together BONDING IN ELEMENTS 2 All other elements in the Periodic Table have bonds between their atoms In all non-metals, these are COVALENT bonds All these non-metal elements are therefore made up of MOLECULES A molecule is a group of atoms covalently bonded together

MOLECULES O P P P P Cl Cl H O

BONDING IN COMPOUNDS Covalent bonding also occurs in compounds The elements involved must both be NON-METALS Examples: Hydrogen oxide, carbon dioxide, nitrogen hydride So non-metal compounds are made up of molecules

COVALENT BONDS + - + - - + The positive nucleus of one atom attracts the negative electrons of another atom At the same time, the two positive nuclei repel each other The electrons are therefore held between the two atoms in a balance between these attracting and repelling forces The electrons are SHARED between the two atoms

ENERGY LEVELS Electrons are arranged around the nucleus in ENERGY LEVELS The first energy level can hold TWO electrons Once the atom has more than two electrons, the first energy level is full Electrons are then found in the second energy level, which can hold EIGHT electrons Hydrogen Lithium Oxygen Helium Boron Neon

ENERGY LEVELS 2 Once the second energy level is full, the third one starts to fill. The third energy level can hold EIGHT electrons NOTICE: Each time a new PERIOD is started, a new ENERGY LEVEL starts to fill As you move across a period from left to right, a new electron is added to the same energy level The number of electrons an atom needs to fill its OUTER energy level is the same number as its VALENCY Sodium Magnesium Chlorine Argon

COVALENT 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

COVALENT BONDS 3 Here, each fluorine atom SHARES ONE electron Both atoms now have EIGHT OUTER ELECTRONS Both their outer energy levels are FULL Note that the number of electrons an atom shares is equal to its VALENCY

IONIC BONDING Formed in compounds made up of a METAL and a NON-METAL Examples: Lithium chloride, Magnesium sulphide Does not involve sharing electrons like covalent bonding Instead, there is TRANSFER of electrons

IONIC 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.

IONIC BONDING 3 The number of electrons that an atom LOSES OR GAINS is equal to the VALENCY Metals always LOSE electrons Non-metals always GAIN electrons Both atoms then have FULL OUTER ENERGY LEVELS, and are stable

IONS As an atom, lithium has 3 POSITIVE protons and 3 NEGATIVE electrons When forming an ionic bond, the lithium atom LOSES one electron The atom now has 3 POSITIVE protons and 2 NEGATIVE electrons The 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.

IONS 2 The fluorine atom has 7 POSITIVE protons and 7 NEGATIVE electrons When forming an ionic bond, the lithium atom GAINS one electron The fluorine atom now has 7 POSITIVE protons and 8 NEGATIVE electrons The overall charge is now NEGATIVE 1 (1-) Fluorine Note that when bonds form, it is the number of electrons which changes but the number of protons ALWAYS stays the same.

SUMMARY COVALENT BONDING IONIC BONDING In both types of bonding, atoms are joining together in order to achieve a stable electron arrangement This means that the OUTER ENERGY LEVEL must be FULL (like the Noble Gases) COVALENT BONDING IONIC BONDING When found Elements (except Noble Gases) Compounds of non-metals only Compounds of metals with non-metals Compound made up of… Molecules Ions Electrons are… Shared Transferred Relationship to valency number Number of electrons shared Number of electrons transferred