Formation of chemical bonds The role of electrons

Learning objectives Describe the octet rule Predict number of valence electrons on atom Write Lewis dot structures for atoms and ions Predict ionic charges Predict composition of simple ionic compounds Describe difference between ionic and covalent bonds Describe differences between ionic and covalent compounds Predict when ionic or covalent compounds are formed Describe a polar bond

Driving force for bonds Chemical bonds make atoms more stable than they are if non-bonded Bond formation involves changes in the electrons on two atoms Electron transfer Electron sharing

Noble gases provide clues

The Octet Rule: Happiness is a filled shell All elements strive to become a noble gas, at least as far as the electrons are concerned. Filling the outer shell – 8 electrons Achieve this by adding electrons Or taking them away

The valence shell Valence electrons: Core electrons: Only the electrons in the outer shell matter Core electrons: Filled inner shells don’t participate Filled valence shells means no bond formation – noble gases Unfilled shells mean reactivity

Lewis dot model The nucleus and all of the core electrons are represented by the element symbol The valence electrons are represented by dots – one for each electron Number of dots in Lewis model is equal to group number (in 1 – 8 system)

Remember this simple rule!! Number of dots in Lewis model is equal to group number (in 1 – 8 numbering system)

Filled shells by gain or loss of electrons Gain electron – outer shell full Lose electron – inner shell full

Rules to predict ion charge Cation Charge = group number Anion Charge = - (8 - group number) Practice with a few examples

Cation loses electrons Ionic bonding Transfer of electrons Create one positive ion – cation One negative ion – anion Cation loses electrons Anion gains electrons

Sodium chloride provides example Na loses electron Cl gains electron

Composition depends on ionic charge (group number) Charges must balance: compounds are neutral – ions are charged Total cation charges = total anion charges One Ca2+ two F-

A B Determining formulae Overall charge must be neutral Metal ion first, nonmetal ion second Coefficient of metal = charge on nonmetal Coefficient of nonmetal = charge on metal x and y are shown with lowest common denominator in most cases. Calcium oxide is CaO not Ca2O2 y+ x- A B x y

Properties of ionic compounds Hard, rigid solids at room temperature High melting point Dissolve in polar solvents (if soluble) Solutions conduct electricity Melts conduct electricity Closely packed dense structures

The ionic model works well for metals and non-metals What about compounds between non-metals CO, PCl3 and diatomic elements like H2, N2, O2, F2 Formation of negative ion is favourable Formation of a positive ion will be very unfavourable (remember Lewis dot structure)

Covalent bonding involves electron sharing Covalent bond is net result of attractive and repulsive electrostatic forces. Nucleus – electron attractions (blue arrows) are greater than nucleus – nucleus and electron – electron repulsions (red arrows).

Sharing two electrons effectively doubles the count Each atom wants 8 Alone each has seven Together each one has eight 14 electrons appear to become 16 Single covalent bond

Covalent bonds between unlike elements Oxygen requires eight – shares two with H atoms Hydrogen requires two – each shares one with O

Lewis dot structures In going from G4 – G7, a H atom is replaced by a lone pair of electrons The total number of electrons is equal to the sum of all the valence electrons The total number of electrons remains the same – 8 Each atom has a complete octet

Multiple bonds are a feature O2 and N2 do not achieve octets by sharing two Must share more electrons O2 has double bond N2 has triple bond – one of the strongest in chemistry N2 is very stable and unreactive – also the major product from explosives

Bond order increases as electron total decreases Molecule Group number Total number of electrons Bond order F-F 7 14 1 O=O 6 12 2 N≡N 5 10 3

Properties of covalent compounds Gases, liquids and solids at room temperature May be hard or soft (diamond is a covalent solid) Dissolve in polar and non-polar solvents, depending on molecule’s polarity Solutions and melts do not conduct electricity Most covalent compounds are molecular

What is this polarity? The ionic bond and the equally shared covalent bond are the two extremes of chemical bonding Ionic - complete transfer of charge Covalent - equal sharing of charge Many bonds are somewhere between Atoms of different elements have different attraction for electrons

Enter electronegativity The degree to which an atom attracts electrons towards itself in a bond with another atom Highly electronegative atoms attract electrons; weakly electronegative atoms do not

Table of electronegativity Most electronegative Least electronegative

Polar bonds and polar molecules Any bond containing different elements will be polar to some degree Molecules contain several bonds Molecular polarity depends on how the bonds are arranged A molecule may contain polar bonds and be itself non-polar We need to understand the molecular structure…

Diatomic molecules are easy

Beyond diatomics More complicated molecules demand knowledge of the molecular shape The next frontier…