UNIT VIII The Periodic Table and Chemical Bonding Lesson 4

Review Going DOWN a FAMILY size of atom increases and distance between valence electrons and nuclei increase Going ACROSS a PERIOD properties are affected by differing valence, nuclear charge and charge on the species One more piece of info on trends…Electronegativity!

Electronegativity Electronegativity – the tendency of an atom to attract electrons from a neighbouring atom. Hey! I find your electrons attractive! Get lost, loser!

VIII.3 CHEMICAL BONDING ELECTRONEGATIVITY: tendency of the atom to attract electrons from a neighbouring atom Going from left to right across the periodic table the electronegativity of the atoms will __________________ Going down a family of the periodic table the electronegativity of the atoms will _____________________

Electronegativity increases as you move from left to right. Electronegativity decreases as you move down each column.

Ex: Which is more electronegative : K or Br? Pb or S?

Electronegativity & Ionization Energy(IE) If an atom has high electronegativity, it strongly attracts e- from a neighbouring atom ( also difficult to remove its own e- so it has high IE ) If an atom has low electronegativity, it has little attraction to the electrons of neighbouring atoms ( easy to remove e- so low IE ) Electronegativity and IE have the same trend on PT!

SUMMARY OF PERIODIC TABLE TRENDS:

S IZE OF IONS VS SIZE OF A NEUTRAL ATOM

SIZE OF IONS vs NEUTRAL atoms Li→e- + Li + 1 valence e-closed shell Positive ions are smaller than the corresponding neutral atom Why?? (fewer electrons, so less repulsion between them).

SIZE OF IONS vs NEUTRAL atoms Cl+ e- →Cl - 7 valence e- closed shell Negative ions are larger than the corresponding neutral atom Why?? (more electrons, so more repulsion between them).

Types of chemical bonds Ionic and covalent

Types of Chemical Bonds A. Ionic Bonding: Ionic Bond – Ionic bonds usually form between atoms that have high differences in electronegativity, (metals and non-metals) any difference higher that 1.7 is considered ionic

VIII.3 CHEMICAL BONDING Ex: Na + Cl Ex: Ca + O The smaller the distance between the charges, the greater the attraction !

LiF F A Li Atom An F Atom A Li + Ion - + An F - Ion

NaCl Crystal Lattice

The melting points of some Ionic Compounds are as follows: NaF 993 o C KCl 770 o C LiCl 605 o C These high melting points are experimental evidence that Ionic Bonds are VERY STRONG. (Hard to break just by heating).

Ionic Bonds IONIC BONDS are VERY STRONG, so compounds held together by ionic bonds have HIGH MELTING TEMPERATURES Think about it… 1) when there is a smaller radius, there is a stronger/weaker bond 2) when there is a larger charge, there is a stronger/weaker bond

Covalent Bonds

C OVALENT C OMPOUNDS Covalent Bonding is the most common and, in my opinion, most important kind of bonding. It is a kind of bonding between atoms within a molecule and forms some of the strongest bonds anywhere The human body is made up almost entirely of covalent bonds, as it is 70% water (a covalent compound) and also a lot of organic carbon compounds (mostly covalent).

Covalent Bonds Covalent Bonding: Covalent Bond: Octet Rule:

In Covalent bonds, electrons are Shared

2 Types of Covalent Bonds Polar Covalent HCl Non-polar Covalent Cl 2

Covalent compounds CH o C O o C F o C Covalent compounds have low melting points and low boiling points because… Simple covalent compounds are only held together by weak intermolecular forces…so it requires little energy to break them.

A simple covalent compound, Cl 2 Ex: Cl – Cl Non-mental bonds to a nonmetal (SHARE e-)  Covalent bonds are STRONG, but the bonds between molecules are WEAK Cl 2 molecules bonded together

Covalent compounds exceptions Covalent compounds are usually simple molecules, unless a giant covalent structure is formed, which is rare…but happens! Giant covalent structures: Ex: silicon dioxide, silicon carbide, boron nitride and diamonds! They have extremely high melting points

Network solids Network solids are compounds containing covalent bonds that violate some of the normal "rules". Diamond, for example, consists of carbon atoms held together by covalent bonds in a crystalline structure.of carbon atoms Network solidsNetwork solids typically are transparent, hard, good insulators and have high melting points.

diamond structure Diamonds are “forever”!

Some melting points of Network Solids : Diamond (Carbon) 3550 o C Silicon Carbide (SiC) 2700 o C Boron Nitride (BN) 3000 o C These giant compounds are held by pure covalent bonds! (not weak intermolecular forces, so they are very hard to break)

So what are these intermolecular forces ????

Intermolecular bonding “Van der waals Forces” The sum of the attractive or repulsive forces between moleculesmolecules other than those due to covalent bonds or the electrostatic interaction of ions with one another.covalent bondselectrostatic interactionions

INTRA molecular INTER molecular (within/inside the molecule) (between molecules) Covalent + Ionic Van Der Waals* *some covalent bonds like giant solid networks, and ionic bonding)

I I The covalent intramolecular bond in I 2 is very strong. I I I I I I I I I I There are weaker intermolecular forces which hold covalent molecules together in a molecular solid.

Intra Vs Inter molecular forces

Forces between molecules… Van der Waals Forces (intermolecular) THREE main types: A. London Forces/ London dispersion B. Dipole-dipole Forces (permanent dipole) C. HYDROGEN BONDING All are caused by dipoles!

T HE 3 TYPES OF INTERMOLECULAR BONDS 1) London forces (London dispersion) London Forces are weak attractive forces which exist between atoms as a result of temporary dipoles 2) Dipole-dipole forces A permanent dipole 3) Hydrogen bonding Strong dipole–dipole attraction between molecules containing a H – N, H – O, or H – F bond So what are dipoles??????

What are dipoles? Dipole –

Polarization causes dipoles! Polarization –

LONDON FORCES London Forces –

London Forces London Forces: The WEAKEST TYPE of bonding force ALWAYS present IMPORTANT??? when they are the only force of attraction....  adjacent noble gas atoms  adjacent covalently-bonded molecules The greater the atomic number, the stronger the London forces it experiences. Which of these would have a stronger london force? Xe----Xe or Ne---Ne ???

HOMEWORK P172 #57 P173 #59,60 P 175 #62,63,64 P177 #68, 69 P 179 #72 from a to k P 181 #74,76,78 Read summary on page 182 and try to solve the summary exercises if you have time.