Topic 6 Bonding

Topic 6-Bonding Energy and Chemical Bonds a. Make bonds-exo (release energy) b. Break bonds-endo (absorb energy) BARF Lewis Dot Revisited for Compounds a. Ionic Substances b. Molecular Substances Octet Rule Covalent Bonds-sharing of electrons a. Nonpolar bonds (EN difference = 0) b. Polar bonds (EN difference is .1-1.6) Molecular Substances a. Polar Molecules-asymmetric b. Nonpolar Molecules-symmetric SNAP 6. Ionic Bonding (EN difference > 1.7) Metallic Bonding Polyatomic Ions Intermolecular Forces between molecules a. Hydrogen Bonds b. Van der Waal Forces aka London Dispersion Forces c. Molecule-Ion attractions

Energy and Chemical Bonds Break bonds-absorb energy-endothermic Form bonds-release energy-exothermic Remember: BARF B = break bonds A = absorb energy R = release energy F = form bonds Break bonds-absorb energy =decrease stability Form bonds-release energy = increase stability The energy stored in a bond is potential energy

Lewis Dot for Molecular Substances Molecular Substances are covalently bonded Covalent bonds are found when nonmetals bond to nonmetals Show the Lewis dot structure for ammonia (NH3) Element # of atoms Valence electrons Total 5 N 1 5 H 3 1 3 ______ 8 You only have 8 electrons to work with. Each element must have 8 electrons around it except H and He which only have 2 electrons around them . . Now each atom has a filled octet x x H . N . H . x N has 8 valence electrons H Each H has 2 valence electrons

You do whatever it takes to ensure that all atoms have 8 valence electrons except H and He which only need 2 valence electrons Lets try: H2 I2 CH4 H2O

Lets try some that are slightly tricky! CO2 O2 N2

Lewis-Dot for Ionic Compounds An ionic compound occurs when a metal bonds to a nonmetal This is a much simpler method The metal has no valence electrons around it and The nonmetal has 8 valence electrons around it (unless it is H) NaCl Na = metal Cl = nonmetal .. [Na]+ [ :Cl:]- ..

Calcium Chloride CaCl2 Ca = metal Cl = nonmetal .. .. [:Cl:]- [Ca]2+ [:Cl:]- .. .. Sometimes, there will be no brackets around the metal .. .. [:Cl:]- Ca2+[:Cl:]- .. ..

Atoms react by gaining (nonmetals) or losing electrons (metals) Octet Rule Atoms react by gaining (nonmetals) or losing electrons (metals) so as to acquire the stable electron configuration of a noble gas, usually 8 valence electrons except H and He which has 2 valence electrons The element with the lower EN (metal) value will always lose electrons The element with the higher EN value (nonmetal) will always gain electrons

Covalent Bond When you hear the word covalent, think molecule and vice-versa Always between NM-NM 2 types of covalent bonds 1. Nonpolar Covalent- Equal SHARING of electrons EN difference is 0 so it only occurs when atoms of same element (nonmetals) are bonded to each other or the seven diatomic elements EN values are on Table S The seven diatomic elements: Br2 I2 N2 Cl2 H2 O2 F2 Or anytime the same element (nonmetal) is bonded to each other C-C

2. Polar Covalent Bond- Unequal SHARING of electrons EN difference is between .1-1.6 The element with the higher EN value will have the electrons most of the time EN values are on Table S The greater the EN difference, the more polar the bond is The smaller the EN difference, the less polar the bond is Covalent substances NEVER conduct a current

when in solution-mixed with water (aq) Ionic Bonding The TRANSFER of electrons from an atom with low EN to an atom with high EN Always occurs between a metal and a nonmetal EN difference is 1.7 or greater Ionic substances conduct a current either when melted (fused) or when in solution-mixed with water (aq) NaCl (s) – does not conduct NaCl (l) – does conduct NaCl (aq) – does conduct

Polyatomic Ions (PAIs) Table E Compounds with PAIs have both ionic and covalent bonds Within the PAI, there are covalent bonds and whatever the PAI bonds to is an ionic bond NH4Cl Look on Table E for the PAI It is NH4 So between N and H there are covalent bonds Between NH4 and Cl are ionic bonds

Polar vs. Nonpolar Molecules When they are talking about molecules (not bonds) they are asking you if the molecule shape has symmetry or not. If it is a nonpolar molecule, it has symmetry If it is a polar molecule, it is asymmetric (no symmetry) Think SNAP symmetry S N A P nonpolar asymmetric polar

+ Polar Molecules are referred to as dipoles Dipole- A molecule that has two poles, or regions with opposite charges The electrons move to the end with the higher EN value, giving it a negative side. The side with the lower EN value is now positive ____ EN = 3.4 EN = 2.2 + Nonpolar molecules cannot be dipoles

You must KNOW the following structures Nonpolar bonds (EN diff. = 0) Nonpolar molecule (symmetry) Linear shape Nonpolar bonds (EN diff. = 0) Nonpolar molecule (symmetry) Linear shape

CO2 N2 Polar bonds (EN diff. =.8) Nonpolar molecule (symmetry) Linear shape Nonpolar bonds (EN diff. = 0) Nonpolar molecule (symmetry) Linear shape

NH3 H2O Polar bonds (EN diff = .8) Polar molecules (asymmetric) Pyramidal shape Polar bonds (EN diff. = 1.4) Polar molecule (asymmetric) Bent shape

CH4 Polar bonds (EN diff. = .4) Nonpolar molecule (symmetry) Tetrahedral shape

If a molecule is: 1. Nonpolar, it has symmetry and equal charge distribution 2. Polar, it has no symmetry and unequal charge distribution Nonpolar molecule = symmetry = equal charge distribution Polar molecule = asymmetric = unequal charge distribution Again….DO NOT confuse polar/nonpolar molecules with polar/nonpolar bonds.

Metallic Bonds Between metal atoms The force of attraction that holds metals together; it consists of the attraction of free-floating valence electrons for positively charged metal ions “Sea of mobile electrons” Ex.: Cu, Au, Ag, Sn, Pb, Fe, Ni

A covalent bond in which one atom contributes both bonding Coordinate Covalent Bond A covalent bond in which one atom contributes both bonding electrons

All of the atoms are covalently bonded to each other Network Solid All of the atoms are covalently bonded to each other All the properties of covalent substances except they are very hard and have extremely high bp and mp Examples of network solids are Diamond (C) Quartz (SiO2) SiC

Diamond encrusted car diamonds quartz Silicon carbide

Intermolecular Forces Forces between molecules not atoms Hydrogen bonds- Very weak forces except when hydrogen bonds to F, N or O When H bonds to F, N or O, those compounds have unusually high boiling points What is so unusual about F, O and N? They have a small atomic radius and high electronegativty

Van der Waals forces Between nonpolar molecules (symmetry) Explains why gases can be liquefied Increase in strength as molecules get bigger Increase in strength as distance between molecules decreases Aka London Dispersion Forces Dipole-Dipole Attraction Occurs between polar molecules (asymmetric) The – end of one dipole is attracted to the + end of the other dipole

Molecule-Ion Attraction Occurs between a covalent substance (H2O) and an ionic compound NaCl (aq) NaCl is the ionic compound aq = water (covalent substance Water is a dipole. The – end (O) is attracted to the + ion (Na) The + end (H) is attracted to the –ion (Cl)

Summary Covalent Bonds-sharing of electrons Nonpolar covalent bond-electronegativity difference of 0 Polar covalent bond-electronegativity difference .1-1.6 Ionic Bonds-transfer of electrons Electronegativity difference of 1.7 or greater Metallic Bonds-sea of mobile electrons

Metallic-sea of mobile e (M-M) Boiling pt High Low high Summary of Ionic/Covalent/Metallic Substances Ionic-transfer (M-NM) Covalent-share (NM-NM) Metallic-sea of mobile e (M-M) Boiling pt High Low Except for network solids high Melting Conductivity Only as a (L) or (aq) Never Only as a (s) or (L)

Why do some substances conduct heat or electricity while others do not? Why do some compounds only conduct in one phase and not other phases? The answer: Why do some substances conduct? Free moving charged particles Why do some substances not conduct? No free moving charged particles

I do not care what type of substance it is: Ionic, Covalent or Metalic I do not care what phase of matter it is in: Solid, Liquid or Gas If it conducts-free moving charged particles If it does not conduct-no free moving charged particles

Why are some solid substances soft and others hard? Hard-strong attractive forces between particles Soft-weak attractive forces between particles