Molecular Compounds (a.k.a. Covalent Compounds)
Ionic Compounds Review • Ionic compounds are the combination of ions. E.g. Na+ + Cl- NaCl. • This formula represents the Ratio of sodium ions and chloride ions not the exact number of them. • They are composed of a Metal and a Non-Metal and form a crystal structure (e.g. NaCl and CaCl2 which are both salts)
Molecular Compounds • Consist of atoms covalently bonded together • The elements involved are all Non-Metals
Some examples • N2O Nitrous Oxide= Gas used at the dentist to relax patients
• NO2 Nitrogen Dioxide= poisonous toxin emitted from car exhaust
More Examples • H2O Water • CO2 Carbon Dioxide= Gas exhaled and created during combustion reactions • CO Carbon Monoxide= Lethal gas created during incomplete combustion
Recall • In ionic compounds, the non-metal rips away the electron(s) from the metal because the metals loosely hold their electrons • This creates a positive and negative ion which attract each other making them stick together (opposites attract)
Molecular Compounds • made up of a non-metal and a non-metal. • also called Covalent Compounds
Molecular Compounds • Share electrons (e-) to form a stable arrangement. • Neg. e- are attracted to the Pos. nuclei of both atoms
Molecular Compounds Molecular compounds are formed when non metals share electrons to fill their outer electron orbit (shell). If we draw the Lewis Structure for Fluorine, We can see that it needs one more electron to become stable
It wants to share one electron with another atom. If it bonds with another fluorine atom We draw the shared pair of electrons between the two atoms.
F
F
F
F
Shared electron pair ďƒ both now have 8 electrons in outer orbit.
Molecular Compounds Many molecular compounds are predictable. How will Oxygen bond with Hydrogen to form a molecule. Draw the Lewis Structure of both Atoms. Oxygen wants to share two electrons and hydrogen wants to share one.
H
H
O
H H O
Clearly we need another Hydrogen atom Putting the shared electrons between the atoms. The molecule is H2O
**The Oxygen atom has 8 valence electrons (full) and the Hydrogen has 2 valence electrons (full)
Molecular Compounds Not all molecular compounds are as predictable. How will Oxygen bond with Carbon? Draw the Lewis Structures of each atom. Carbon wants to share 4. Oxygen wants to share 2
C
O
C
O If we add one more Oxygen, each oxygen can share two. However, if Oxygen will share two and donate one of its other electrons pairs, Oxygen can bond with just on Carbon atom.
O
Carbon Dioxide C
Carbon Monoxide
O
Types of Bonds
H H O
C
Water – Single bond because One pair of electrons are shared O
C
O
Carbon Monoxide – Triple Bond because three pairs of electrons are shared
O
Carbon Dioxide – Double bond because two pairs of electrons are shared
Molecular Compounds – Naming So Oxygen can form CO or CO2 We need a more flexible naming system for covalent compounds to reflect the many different bonding possibilities. To name a covalent compound, 1. Starting with the atom that is to the left in the periodic table, (or lower) 2. Write the name of the atoms with the prefix indicating the number of that atom in the compound. 3. Change the ending of the last atom to – ide.
The prefixes are, 1 Mono 2 Di 3 Tri 4 Tetra 5 Penta 6 Hexa 7 Hepta Note, the mono is omitted on the first atom. CO Carbon Monoxide CO2
Carbon Dioxide
Memorize me! Number
Prefix
Number
Prefix
1
mono
6
Hexa
2
Di
7
hepta
3
tri
8
octa
4
Tetra
9
nona
5
penta
10
deca
Naming Example • P4O10 Phosphorus - use prefix tetra (4) Oxygen – change ending – Oxide Oxide – use prefix deca (10) = Tetraphosphorus Decaoxide
Molecular Compounds - Naming Name the following compounds
Write the chemical formula for,
CF4
Sulphur Dioxide
Carbon tetrafluoride
SO2
H2O
Dihydrogen Monoxide PF5
Phosphorus Pentafluoride
DiCarbon Tetrahydride
C2H4
Molecular Compounds – Diatomic Gases Molecular compounds show an incredible variety in structure, physical and chemical properties.
There are categories that further divide covalent compounds into categories with common properties.
Diatomic Gases – When two atoms of the same type form a covalent molecule, the result is often a colorless, odorless gas.
The Halogens, Oxygen, Nitrogen and Hydrogen form diatomic gases. H2 is called Hydrogen Gas not Dihydrogen.
Writing Formulas left-most element goes 1st Carbon monoxide - Carbon (C) no prefix therefore = C - Monoxide – Mono = 1 therefore, 1 oxygen atom = O = CO
Writing Formulas • Nitrogen dioxide - Nitrogen (N) no prefix therefore = N - Dioxide – Di = 2 therefore, 2 oxygen atoms = O2 = NO2
Writing Formulas DiCarbon Tetrahydride - Dicarbon (C) ďƒ Di = 2 therefore = C2 - Tetrahydride (H) ďƒ Tetra = 4 therefore, 4 Hydrogen atoms = H4 = C2H4
Eg. N2 - Nitrogen Gas O2 - Oxygen Gas
Air
Exceptions that need to be Memorized! CH4 – Methane
H2O2 – Hydrogen peroxide
H2O – Water
These Too! NH3 – Ammonia O3 – Ozone