COVALENT BONDING

This occurs when two non-metallic atoms _________ electrons in order to obtain the stable number of eight electrons in their outer shells. The atoms share pairs of electrons in overlapping partially filled atomic orbitals. Each shared pair of electrons forms a ______ bond and the molecule formed is a covalent molecule.   E.g.chlorine gas – Cl 2. p112 LEWIS DIAGRAMS   Lewis diagrams (or Electron Dot Diagrams) – only include the ________ or outer shell electrons to simplify the above drawings.   The nucleus and inner-shell electrons are represented by the element ________ and the valence electrons are shown as dots. Eg. phosphorus Unpaired – bonding electrons Paired – non-bonding electrons or ‘lone pairs’... P..

LEWIS SYMBOLS .  The outer shell electron arrangements of the elements in periods 2 and 3 can be simplified into LEWIS SYMBOLS. ClArSCNOFNe

NOTE: A shared pair of electrons is represented by a ________ _______ in a structural formula : Cl – Cl When more than one pair of electrons are shared a multiple covalent bond is formed. Two pairs = double bond Eg. oxygen O 2 three pairs  triple bond E.g. nitrogen N 2 Covalent bonding also occurs between different elements to form covalently bonded compounds such as water, carbon dioxide and methane. The formation of water: NB. Bonding electrons and non-bonding electrons or (lone pairs.) Now you draw the following molecules – hydrogen H2, hydrogen chloride HCl, ammonia NH3, methane CH4, difluoromethane CH2F2, carbon dioxide CO2, ethane C2H6. Electron dot diagrams and structural formulae.

COVALENT BONDING TO FORM MOLECULAR ELEMENTS/DIATOMIC MOLECULES Single bond Single Bond Triple bond

METHANE AMMONIAWATERHYDROGEN FLUORIDE COVALENT MOLECULES

SHAPES OF COVALENT MOLECULES: Covalent molecules are _______ dimensional in shape. These shapes are extremely important in predicting and understanding the properties of many covalent substances. Diamond (an allotrope of carbon) is very hard and brittle while graphite (another form of carbon) is a good lubricant. These predictions are made using the VSEPR. VALENCE SHELL ELECTRON PAIR REPULSION THEORY (VSEPR): This theory states that the electron pairs in the molecule _______ each other and take up positions as far apart as possible (multiple bonds only count as one pair). Eg. Methane and water NOTE: Table 6.2 p112

SHAPES OF SOME COMMON MOLECULES

NAMING COVALENT COMPOUNDS MOLECULAR ELEMENTS Not all non-metallic elements are molecular. E.g. Helium and carbon. All molecular elements are non-metallic elements. These non-metallic molecules are known by their elemental name. E.g. N 2 is nitrogen MOLECULAR COMPOUNDS Molecular compounds contain discrete __________ units. Binary molecular compounds are those which contain _____ elements. There are four rules for naming binary covalent compounds: → → The first element in the formula is named in ______. → → The second element is named as if it was an anion and given the suffix – __. → → The number of each type of atom in the molecule is indicated by a ______. These prefixes are listed on page119 Note that the prefix mono is never used for the first element. → → If the name of the second element begins with a vowel and the prefix for that name ends with a or o, these letters are dropped to make pronunciation easier.

ELECTRONEGATIVITY Different atoms have different electron _________ powers. The relative attraction that an atom has for shared electrons in a covalent bond is known as ____________. The Pauling scale of electronegativities are shown below:

NON-POLAR AND POLAR MOLECULES   Differences between the electron attracting powers of the atoms in covalent bonding is very important as it leads to ________ SHARING of the pair of electrons forming the bond. When this happens the bond becomes POLARIZED with a small ________ charge at one end and a small negative charge at the other.   Eg. the electron sharing in a H – O bond in water:  -  + O : H   Oxygen gains an unequal share of the electron pair and therefore gains a small negative charge (  -). Bonds of this type are called _______ BONDS.   Molecules which contain polar bonds and which are not symmetrical will be polarized and contain a permanent dipole.   Eg. the ammonia molecule NH3:    -   3.0 N H H H  PERMANENT DIPOLE QUESTIONS P.122

INTERMOLECULAR ATTRACTIONS How do molecules ‘stick’ together? The attractions between molecules are called __________ forces of attractions and these forces of attractions hold molecules to each other. They are ________ than covalent or ionic bonds but they are responsible for whether the compound exists in the solid, liquid or gaseous states. The stronger the intermolecular forces the higher the melting or boiling point. The three types of weaker intermolecular forces are known as van der Waal forces and these include _________ forces, dipole-dipole interactions and ________ bonding.

DISPERSION FORCES These are _______ bonds which exist between all molecules due to the formation of ‘instantaneous dipoles’ within molecules from random unequal distribution of their electrons. These instantaneous dipoles are very ______ lived but may occur thousands of times a second. The resulting bonds are weak but ___________ with the number of electrons in the molecules (more possible instantaneous dipoles) and the similarity of the molecular shapes (fit more closely together).

DISPERSION FORCES

DIPOLE-DIPOLE INTERACTIONS When two dipolar molecules interact, the ______ end of one molecule attracts the negative end of the other molecule (and visa versa) and the molecules become bonded together by a dipole – dipole bond.

HYDROGEN BONDING This is a special case of dipole – dipole interactions where _______ is bonded to a highly electronegative element such as F, O and N, which also has non- bonding electron pairs available. Quite a ________ bond forms between the  + charge on the H atom and the non –bonding electron pair on the F, O or N atoms.  These bonds are quite strong and result in H-bonded substances having ______ melting points and boiling points than we might expect. Eg. HF and H 2 O

PROPERTIES OF MOLECULAR COVALENT SUBSTANCES: Strong bonding within molecules – weak bonding between molecules. The physical properties of a molecular substance is due largely to the strength of the ____________ attractions. A few generalisations can be made: (summarise dot points on P 128)

GIANT COVALENT LATTICES   TWO TYPES: (i) THREE-DIMENSIONAL (ii) TWO-DIMENSIONAL   THREE DIMENSIONAL GIANT COVALENT NETWORK LATTICES − − These consist of millions of atoms bonded to each other in a three dimensional (3-D) array in all directions such that no individual molecules exist and the array forms one giant ‘molecule’. Eg. a diamond p129, silicon dioxide (quartz or sand), tungsten carbide (cutting tools). − − COPY P 129 PROPERTIES OF COVALENT NETWORK LATTICES

TWO DIMENSIONAL COVALENT LAYER LATTICES   These consist of atoms held strongly together in ____ dimensional layers by covalent bonds, while between the layers there are only weak Van der Waals bonds (dispersion forces). This allows the layers to slide ‘freely’ over each other.   GRAPHITE (an allotrope of carbon) is a classical layer lattice: graphite is made of _____ sheets of C atoms bonded into hexagonal rings to three other C atoms.   This leaves one unused valence electron which is _________ among the other atoms in the layer and gives the graphite many of it’s properties.   The unused electrons are ____________ but travel only along the layer occupied by their parent by their ‘parent’ atom. The arrangement around each carbon atom in graphite. Models of part of the sheet of hexagonal rings found in graphite.

THE LAYERS OF GRAPHITE Copy the properties of graphite from P 131. Do the multiple questions and chapter review questions.