Unit II: Atoms and The Periodic Table Chemical Bonding

Valence Electrons 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!

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

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

Ionic Bonds 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

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

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

Melting Points 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 - Trends The greater the radius of the ions – the weaker the bond strength

Ionic Bond - Trends The greater the charge on the ions the stronger the ionic bond strength

Covalent Bonds 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.

Covalent Bonds In diatomic molecules (like H 2 or Cl 2 ), the electronegativities of both atoms are exactly the same so electrons are shared equally! Covalent Bond animation

In Covalent bonds, electrons are Shared

HH

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

diamond structure Diamonds are “forever”!

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

Covalent Bond Trends The more electrons shared in a covalent bond, the stronger the bond The larger the atoms involved the weaker the bond The larger the number of shared electrons, the shorter the bond length

Polar Covalent When electrons are shared unequally between two atoms, the bond is called Polar Covalent.

Polar Covalent Bond

Hydrogen Bonding 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.

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

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.

The Greek letter “delta” means “partial”  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.

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

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. These are called London Forces. Since they are relatively weak, Iodine has a low melting point.

Na + Cl 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)

Mg 2+ F F Here is the e-dot formula (Lewis Structure) for the ionic compound MgF 2 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 (CH 4 ) C H H H H Here is a Carbon atom (4 val e - ’s) and four Hydrogen atoms (1 val e - each)

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

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

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

Electron-dot formula for Methane (CH 4 ) C H H H H

C H H H H

C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet.

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet.

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet.

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet.

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet.

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet. Each H atom “feels” like a stable “He” atom with 2e - s

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet. Each H atom “feels” like a stable “He” atom with 2e - s

Electron-dot formula for Methane (CH 4 ) C H H H H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet. Each H atom “feels” like a stable “He” atom with 2e - s

Electron-dot formula for Ammonia (NH 3 ) N H H H Here is a Nitrogen atom (5 val e - ’s) and three Hydrogen atoms (1 val e - each)

Electron-dot formula for Ammonia (NH 3 ) N H H H Here is a Nitrogen atom (5 val e - ’s) and three Hydrogen atoms (1 val e - each)

Electron-dot formula for Ammonia (NH 3 ) NH H H Here is a Nitrogen atom (5 val e - ’s) and three Hydrogen atoms (1 val e - each)

Electron-dot formula for Ammonia (NH 3 ) NH H H Here is a Nitrogen atom (5 val e - ’s) and three Hydrogen atoms (1 val e - each)

Electron-dot formula for Ammonia (NH 3 ) N H H H

N H H H

N H H H “N” now feels like it has a stable octet

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H “N” now feels like it has a stable octet Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H Each “H” feels like it has 2 e - like Helium.

Electron-dot formula for Ammonia (NH 3 ) N H H H

N H H H

N H H H

N H H H

N H H H

N H H H

N H H H

N H H H

N H H H

N H H H

N H H H I’m so HAPPY!

Write the electron-dot formula for CF 4

C F F F F Because “F” is a halogen, it has 7 valence e - s, so you must show all 7 red dots around each “F” atom!

Write the electron-dot formula for H 2 S

SH H The two H’s MUST be at right angles to each other!!

Write the electron-dot formula for H 2 S SH H These are called “lone pairs”. They must also be at right angles to each other when the central atom is in Group 16!

Write the Electron-Dot Formula for SeF 2

Se F F

Write the Electron-Dot Formula for SeF 2 Se F F Because Se is in Group 16, the F atoms MUST be at right angles to each other!

Write the Electron-Dot Formula for SeF 2 Because “F” is in Group 17, they have 7 valence e - s, so they must have 7 red dots around them. Se F F

Write the Electron-Dot Formula for O 2

Write the Electron-Dot Formula for N 2

Shapes of Hydrocarbons CH 3 CH 2 CH 2 CH 3

Shapes of Hydrocarbons CH 3 CH 2 CH 2 Cl

Hebden Do Hand-In Assignment #3