Covalent Bonding L.O. To know what a covalent bond is. To be able to draw representations of a covalent bond and discuss properties.

“ a shared PAIR of electrons” Covalent Bonding: the sharing of PAIRS of electrons between NON-METAL atoms to form molecules or giant structures. i.e. bonding between atoms of SIMILAR and HIGH electronegativities “ a shared PAIR of electrons” a double covalent bond = 2 shared pairs (4) of e- a triple covalent bond = 3 shared pairs (6) of e-

A hydrogen chloride molecule, HCl Hydrogen H : 1s1 Chlorine Cl : 1s22s22p63s23p5 a SINGLE covalent bond LONE PAIR of electrons H x Cl . .. H x Cl . .. A hydrogen chloride molecule, HCl

x x x O O O Oxygen Oxygen O : 1s22s22p4 a DOUBLE covalent bond XXXX An oxygen molecule, O2

.x . . x H H C C Methane Hydrogen H : 1s1 Carbon C : 1s22s22p2 Draw the dot and cross diagram for this compound. N.B. First identify the formula! H x x 4 ! .x . x C H C . A methane molecule, CH4

The number of covalent bonds to any particular non-metal atom is usually it’s group number GROUP IV V VI VII Examples NUMBER OF BONDS PER ATOM C , Si N , P O , S H, F, Cl, Br, I He,Ne,Ar 4 3 2 1

Activity: Draw dot-and-cross diagrams to represent the covalent bonding in each of the following molecules : 1 Hydrogen H2 2 Nitrogen N2 3 Water H2O 4 Carbon dioxide CO2 5 Hydrogen peroxide H2O2 6 Tetrachloromethane CCl4 7 Ethane C2H6 8 Ethene C2H4

.x . . x H H N N Now try Ammonia LONE PAIR of electrons x 3 ! An ammonia molecule, NH3

the electrostatic attraction What holds covalently bonded atoms together? the electrostatic attraction between the positive nuclei and the negative electron pairs shared between those nuclei. +

1. ALL covalent substances are non-conducting What are the characteristics of covalent substances? 1. ALL covalent substances are non-conducting because no mobile ions or electrons present (except graphite!!) 2. SIMPLE covalent molecules are : (a) Gases eg nitrogen ALL have LOW melting & boiling points (b) Liquids eg water or (c) Low melting point solids eg iodine Strong covalent bonds within the molecules DO NOT BREAK during melting or boiling Weaker intermolecular forces between molecules DO BREAK during melting or boiling

3. GIANT covalent structures are ALL high melting point solids because a lot of energy is needed to break the STRONG covalent bonds Strong covalent bonds can exist throughout most, or all of, the structure All Most Weak Van der Waal forces between layers Diamond Graphite

Co-ordinate (Dative Covalent) Bond a covalent bond formed when both shared electrons are provided by only one atom. e- pair comes from a lone pair on the donor atom eg N of NH3, O of H2O the atom sharing the lone pair must have an incomplete outer energy level eg H+, Al in AlCl3, C in CO

Co-ordinate (Dative Covalent) Bond Dative are physically and chemically identical to ordinary covalent bonds - only the method of formation differs. Represent by X: Y instead of X Y

NH3(g) + HCl(g)  NH4+Cl-(s) Ammonium ion : NH4+ NH3(g) + HCl(g)  NH4+Cl-(s) . . . H N + H+ H + . . . N H

H2O(l) + HCl(g)  H3O+(aq) + Cl-(aq) Draw the Hydronium ion : H3O+ H2O(l) + HCl(g)  H3O+(aq) + Cl-(aq) . . . H O + H+  . . . H O + H4O2+ unlikely to form via the second lone pair because the second H+ is repelled by H3O+

Aluminium chloride dimer, Al2Cl6 . . . Cl Al . . . . Cl Al

Can you draw carbon monoxide, CO? Isoelectronic with N2 : N X

Extension: Why does NH3 react readily with BF3 to form a Extension: Why does NH3 react readily with BF3 to form a stable compound ? N of NH3 has a lone pair on the N atom. The B atom of BF3 has an incomplete energy level and can form a dative bond with the N atom. . . . H N . . . F B ALL atoms now have a noble gas configuration