Chemistry 11 Chemical Bonding Electronegativity – the tendency of an atom to attract electrons from a neighbouring atom. Get lost, loser! Hey! I find your electrons attractive!

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

Valence Electrons – electrons in the outermost occupied energy level Valence Electrons – electrons in the outermost occupied energy level. (s and p electrons outside the core) Valence electrons can be represented by “dots” drawn around the atom.

Gilbert Newton Lewis Invented “Electron-dot” formulas or “Lewis Structures” I’m so tired of writing all those useless inner electrons, in the Bohring models!

When the electronegativities of two atoms are quite different from each other: One atom loses an electron (or electrons) The other atom gains an electron (or electrons) This results in an Ionic Bond. Chemical Bonding (Choose Ionic Bonds) crystal lattice viewer

NaCl Crystal Lattice

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

- - 2+ Be F F An F Atom An F Atom A Be Atom F Be F A Be2+ Ion An F- Ion An F- Ion

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

When Electronegativities of bonding atoms are the same (as they are in diatomic molecules) or close to the same, they SHARE electrons. Bonds formed when atoms share electrons are called Covalent Bonds. In diatomic molecules (like H2 or Cl2), the electronegativities of both atoms are exactly the same so electrons are shared equally! Covalent Bond animation

In Covalent bonds, electrons are Shared

H H

Covalent bonds in large networks (Network Bonding) gives rise to substances with very high melting points.

Diamonds are “forever”! diamond structure Diamonds are “forever”!

Covalent bonds are very strong! Some melting points of Network Solids: Diamond (Carbon) 3550 oC Silicon Carbide (SiC) 2700 oC Boron Nitride (BN) 3000 oC Covalent bonds are very strong!

When electrons are shared unequally between two atoms, the bond is called Polar Covalent. A type of PC bond formed when “H” from one atom attracts “O” or “N” from another atom is called Hydrogen Bonding. polar covalent bonds

Hydrogen Bonding in Water gives rise to the structure of ice when water solidifies.

Hydrogen bonds between the “bases” hold the two strands of DNA together.

Bonds within molecules that hold the atoms of a molecule together are called intramolecular bonds. They are strong covalent bonds. Covalent Bonds

The covalent intramolecular bond in I2 is very strong. There are weaker intermolecular forces which hold covalent molecules together in a molecular solid. I I I I

A dipole is a partial separation of charge which exists when one end of a molecule has a slight positive charge and the other end has a slight negative charge. Eg. A water molecule has two dipoles. The Greek letter “delta” means “partial” d

Just by pure chance, there are some times when both electrons in helium are on the same side. This forms temporary dipoles +2 e- +2 e- He He d - d + d - d + The weak attractive forces between the (+) side of one molecule and the (-) side of another molecule are called London Forces

The covalent intramolecular bond in I2 is very strong. There are weaker intermolecular forces which hold covalent molecules together in a molecular solid. These are called London Forces. Since they are relatively weak, Iodine has a low melting point. I I I I

Lewis Structures (Electron-dot formulas) for Ionic Compounds. Remember, in an ionic compound, the metal loses e-’s and the non-metal gains. There is no sharing. Here is the e-dot formula for sodium chloride (NaCl) Cl Na+

Here is the e-dot formula (Lewis Structure) for the ionic compound MgF2 : Notice, there is no sharing. The F atoms took both valence e-’s from Mg, forming ions which do not share electrons. The + and – charges on the ions cause them to attract each other.

Electron-dot Formulas (Lewis Structures) for Covalent Compounds. When atoms form covalent bonds, they are trying to achieve stable noble gas electron arrangements: Hydrogen will share e-’s until it feels 2 e-’s like Helium. Other elements share e-’s to achieve what is called a “Stable Octet” (8 valence e-’s)

Electron-dot formula for Methane (CH4) Here is a Carbon atom (4 val e-’s) and four Hydrogen atoms (1 val e- each)

Electron-dot formula for Methane (CH4) Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet. H C H Each H atom “feels” like a stable “He” atom with 2e-s H

Electron-dot formula for Ammonia (NH3) Here is a Nitrogen atom (5 val e-’s) and three Hydrogen atoms (1 val e- each) H

Electron-dot formula for Ammonia (NH3) “N” now feels like it has a stable octet Each “H” feels like it has 2 e- like Helium. H N H H

Write the electron-dot formula for CF4 Because “F” is a halogen, it has 7 valence e-s, so you must show all 7 red dots around each “F” atom! C F

Write the electron-dot formula for H2S The two H’s MUST be at right angles to each other!!

Write the Electron-Dot Formula for SeF2 Because “F” is in Group 17, they have 7 valence e-s, so they must have 7 red dots around them. Se F