Chapter 8 : Covalent Bonding Section 8.1: Molecular Compounds
What is a molecule? A molecular compound? A molecule is a neutral group of atoms joined together by covalent bonds A molecular compound is a compound that composed of molecules A covalent bond is a bond formed by the sharing of electrons between atoms This is like a tug of war between atoms for electrons, joining the atoms together
What are common examples of molecules? Oxygen gas consists of oxygen molecules Each oxygen molecule is made of two covalently bonded oxygen molecules This is referred to as a diatomic molecule A molecule that contains two atoms Other common examples are Hydrogen, nitrogen and the halogens
How are molecules represented? Molecular formulas are used to show how many atoms of each element a molecule of a substance contains Example - H2O Subscript refers to the number of atoms of that element in the molecule. In water there are two hydrogens bonded to one oxygen atom These formulas are not always the lowest whole number ration - why? C4H5 represents Butane. C2H5 would be a different molecule Molecular formulas do not tell you about a molecule's structure. For this you need a structural formula
What else does the formula not tell you? The chemical formula does not tell you the overall shape of the molecule Carbon dioxide - linear Water - bent
So what are the key differences between ionic and molecular compounds? The formulas of molecular compounds describe a molecule The formulas of ionic compounds describe a formula unit
Characteristics of molecular and ionic compounds continued
The Nature of Covalent Bonding Section 8.2
How do Covalent bonds relate to the Octet rule? Like Ionic bonds, noble gas electron configurations are key Atoms will share electrons so each atom has a noble gas electron configuration This is more common for nonmetals in the periodic table Atoms are able to share electrons until they complete their octet The octet rule still applies!
What is meant be a single covalent bond? A single covalent bond forms when two atoms share a pair of electrons The attraction between the shared electrons and the positive nuclei of the atoms holds the atoms in the molecule together On a structural formula this single bond is represented as a dashed line Halogens are an example of atoms that form diatomic molecules with a single covalent bond. When this occurs, each halogen will have three unshared pairs; a pair of valence electrons that is not shared between atoms
What about more complex molecules? Water: Top Right, two unshared pairs Ammonia : Top Left, one unshared pair Methane: Right: No unshared pairs
What is a double covalent bond? A triple bond? Sometimes atoms must share two or three pairs of electrons to reach a noble gas electron configuration Double bonds involves two shared pairs of electrons - example carbon dioxide (CO2) A triple covalent bond is the sharing of three pairs of electrons - example Nitrogen (N2)
What is a coordinate covalent bond? A coordinate covalent bond is an atom in which one atom provides both bonding electrons Example - Carbon monoxide. How does bonding in carbon monoxide work? Once a coordinate covalent bond forms, it is like any other covalent bond. It is often drawn as an arrow in a structural formula
What are some of the exceptions to the octet rule? Try and draw an electron dot diagram for Nitrogen Dioxide (NO2) You can t draw one! The same applies for all molecules with odd number of valence electrons Some molecules that do have an even number of valence electrons still fail to follow the octet rule - example BF3 Some molecules expand the octet to more than 8 - example PCl5
How does energy relate to bonds? When atoms combine to form a bond energy is released. The product is more stable than the reactants The energy needed to break a bond is the bond dissociation energy, expressed in kj/mol The larger the bond dissociation energy, the stronger the bond, and the shorter the bond length Breaking bonds would require energy Double and tripple bonds have higher bond dissociation energies than their single bond counterparts Bond dissociation energies also contribute to reactivity of molecules - high energy means unreactive molecule.
Bond dissociation energy examples
What does an electron dot structure for Ozone (O3) look like? Ozone has two different types of bonds - one single coordinate covalent bond and one double covalent bond. This would mean that the bonds in Ozone would have different lengths, however measurements suggest that the two bonds are the same length, being the average of the two structures. When you can draw two or more valid electron dot structures for a molecule or polyatomic ion it is referred to as resonance structures
Section 8.3 Bonding Theories
What is a molecular orbitals A molecular orbital is an orbital that applies to the entire molecule Individual atomic orbitals can combine to produce a different shaped molecular orbital There are a number of similarities between atomic and molecular orbitals Both contain two electrons when filled Those that can be filled by two electrons of a covalent bond are referred to as bonding orbitals Covalent bonds are caused by an imbalance between attraction and repulsion of the nuclei and electrons Bonds can form because the attraction between atoms is stronger than repulsions
What is VSEPR Theory? VSEPR theory states that the repulsion between electron pair causes molecular shapes that allow for the greatest distance between valence electron pairs Stands for Valence Shell Electron Pair Repulsion Theory It is used to estimate the 3D shape of molecules Essentially the valence electron pairs want to stay as far apart as possible, which can give rise to a number of different molecular shapes
What is going on in Linear Molecules? Linear is the simplest shape The valence electron pairs are furthest apart when arranged in a straight line Example - Carbon Dioxide In molecules such as methane it is not so straightforward
What is happening in these diagrams?
The relationship between molecular shapes and unshared pairs Unshared pairs of electrons are important when predicting molecular shapes Unshared pairs of electrons will be held closer to the nucleus than bonded pairs - why? Because, there is no atom competing for them Unshared pairs will always repel bonded pairs - dictating the overall shape of a molecule This explains why the angle between bonds is water is less than in ammonia
Can VSEPR theory be used to predict types of bonds in a molecule? No, instead this can be achieved through orbital hybridization Hybridization is the mixing of several atomic orbitals to form the same total number of equivalent hybrid orbitals Orbital hybridization provides information about both molecular bonding and molecular shape In this example, orbital hybridization is the only way that carbon can form 4 bonds
Section 8.4 Polar bonds and molecules
What is the difference between a polar and a nonpolar covalent bond? Nonpolar: Covalent bond in which two atoms share the electrons equally Polar: A covalent bond in which two atoms share the electrons unequally The more electronegative atom will attract the electrons more strongly, giving slight changes to the atoms in the bond
When do polar covalent bonds form? Whenever there is a difference in electronegativity values for the two atoms involved Electronegativity differences are used to assess the strength of the polar bond
Relationship between polar bonds and polar molecules The presence of a polar bond in a molecule often makes the entire molecule polar This means that one end of the molecule is slightly positive, and the other is slightly negative When the molecule has two poles of opposite charge it is called a dipolar molecule, or dipole. Sometimes molecules can have polar bonds but not be polar themselves how? Example - carbon dioxide and water. Which is Polar?
What effects can bond polarity cause? Bond polarity can cause intermolecular attractions; weak attraction between molecules These can be important, and can determine the state of matter of a molecular compound Van der Waals forces: the two weakest intermolecular attractions Dipole interactions: Polar molecules are attracted to one another (oppositely charged parts). Weaker version of ionic bonds Dispersion forces: The weakest of all intermolecular interactions, caused by motion of electrons.
Dispersion forces explained... Dispersion forces are caused by the motion of electrons All molecules experience dispersion forces - not just polar molecules They occur when moving electrons of one molecule happen to move to one side of the molecule Their force then repels electrons in a nearby molecules, causing a slight attraction Halogen diatomic molecules attract each other by dispersion forces Fluorine and chlorine are gases due to weak dispersion forces. Bromine is a liquid due to more electrons and more dispersion forces
What is a hydrogen bond? Hydrogen bonds are a special case of dipole interaction, formed when hydrogen is covalently bonded to a very electronegative atom, and is also weakly bonded to an unshared electron pair Example: water They are the strongest of the intermolecular forces, but only have about 5% the strength of an average covalent bond They always involve hydrogen, but can be found in other molecules besides water Form as a result of a highly polar bond and lack of shielding in the hydrogen
How do intermolecular attractions relate to Molecular properties? The physical properties of compounds relate to the type of bond it displays (largely ionic vs covalent), but the diverse range is largely due to varying intermolecular forces Melting and Boiling points of most molecular compounds are low compared to ionic compounds, why? Some covalent compounds exist where all the atoms are covalently bonded to each other - a network solid Melting would require breaking of all the covalent bonds throughout the solid Example - diamond. Each carbon atom is bonded to four other carbons - all atoms are connected. Explains why diamonds is so hard, does not melt, and vaporizes to gas at 3500 C. Essentially carbo behaves as a single molecule