Chapter 8 Intro to Chemistry Covalent Bonding

Why do atoms bond It’s all about stability Atoms want to achieve the stable octet of the noble gases Ionic bonds achieve this octet through an exchange of electrons The exchange occur between a metal ion (cation) and a non-metal ion (anion) Covalent bonds are different

What is a covalent bond A covalent bond occurs when atoms share electrons Covalent bonds occur between two non-metals Valence electrons play a role in covalent bonds

Molecules and Molecular Compounds Molecule – a neutral group of atoms joined together by a covalent bond. Diatomic molecule – a molecule consisting 2 of the same element. Example: O2 Molecular compound – a compound composed of molecules. (covalently bonded atoms).

The Diatomic Elements The elements Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, and Bromine are always found as diatomic molecules: HONClFIBr (say HONKLE-fibber) BrINClHOF (say Brinckle-hoff) I Have No Bright Or Clever Friends There are seven such elements. The first one is the first element Hydrogen; the rest form a 7 on the periodic table: N, O, F across, then going down Cl, Br, I.

Characteristics of Molecular Compounds (elements that are bonded covalently) Lower melting points and boiling points than ionic compounds. Most are gases or liquids at room temperature. Are composed of atoms of 2 or more nonmetals.

Molecular Formulas A molecular formula shows how many atoms of each element a molecule contains. Ex: H2O has 2 hydrogen atoms and 1 oxygen atom You can represent a molecule by its molecular formula, structural formula, space-filling molecular model, perspective drawing or by a ball-and-stick molecular model.

Different ways to represent a molecule: Structural formula Perspective drawing Molecular Formula NH3 Ball-and-stick molecular model Space-filling molecular model

How to determine a covalent bond In the case of hydrogen combining to form H2 Each hydrogen has 1 valence electron In order to achieve the noble gas configuration of helium, they each need two valence electrons H2 achieves this by sharing their valence electrons and forming a bond http://www.visionlearning.com/library/flash_viewer.php?oid=1348&mid=55

What happens in a covalent bond In a covalent compound, electrons are shared between atoms to form a covalent bond in order that each atom in the compound has a share in the number of electrons required to provide a stable, Noble Gas, electronic configuration (octet rule). Each atom “thinks” it has a full outer shell containing 8 valence electrons The sharing of electrons is what makes the bond and holds the atoms together

Lewis Structures and Covalent Bonds Electrons in the Lewis Structure (electron dot diagram) are paired to show the bonding pair of electrons. Often the shared pair of electrons forming the covalent bond is circled Sometimes the bond itself is shown with a dash (-) A pair of valence electrons that is not shared between atoms is called an unshared pair, lone pair or non-bonding pair.

Water – Is an example of a covalent compound. Hydrogen needs 2 electrons to fill its outermost shell Oxygen needs two electrons to fill its outermost shell

Lewis Structure of Water (single covalent bonds) http://web.visionlearning.com/custom/chemistry/animations/CHE1.7-an-H2Obond.shtml

Lewis Structure of Carbon Dioxide: CO2 (Double covalent bonds) Carbon has four valence electrons, and oxygen has six. Each pair of shared electrons forms a single bond A bond that involves two shared pairs of electrons is called a double covalent bond.

Lewis Structure of Nitrogen (triple covalent bonds) T A bond formed by sharing 3 pairs of electrons is called a triple covalent bond. + =

Covalent Compounds http://link.brightcove.com/services/player/bcpid9113583001?bctid=1396941780 Covalent Bonding http://www.saskschools.ca/curr_content/chem20/covmolec/covintro.html

You Try It What would the formula be for a compound containing carbon and fluorine What is the formula for a compound containing nitrogen and oxygen Carbon and hydrogen? Antimony and bromine? Chlorine and oxygen?

Bond Dissociation Energies Covalent bonds are extremely strong! It would take 435 KJ of energy to break apart 1 mole (about 2 grams) of a hydrogen molecule (H2). The energy required to break a covalent bond is known as the bond dissociation energy. A single bond has a lower bond dissociation energy than a double bond or a triple bond.

Resonance Structures A resonance structure occurs when there is more than one valid Lewis structure for a molecule or ion Resonance structures differ only in the position of the electron pairs, never in the position of the atoms

Lewis Structure of Ozone (O3) Begin by drawing the basic structure Then add the third bond However, we cannot be sure if the double bond is on the left or the right Draw the resonance structures for ozone

You Try It Draw resonance structures for SO2

Naming Covalent Compounds Simple covalent compounds are generally named by using prefixes to indicate how many atoms of each element are shown in the formula The ending of the last (most negative) element is changed to -ide.

Prefixes The prefixes used are mono-, di-, tri-, tetra-, penta-, hexa-, and so forth The mono- prefix is not used for the first element in the formula The "o" and "a" endings of these prefixes are dropped when they are attached to "oxide." Monoxide Hexoxide Pentoxide

Table of Prefixes 1 Mono- 2 Di- 3 Tri- 4 Tetra- 5 Penta- 6 Hexa-

Which Element is First? Generally, they are in the same left-to-right order that they have on the periodic table Except that you would have to squeeze hydrogen in between nitrogen and oxygen

Name the following compounds PH3 phosphorus trihydride CO carbon monoxide HI hydrogen monoiodide  N2O3 dinitrogen trioxide

What is the Formula Carbon Tetrachloride Dinitrogen Pentoxide CCl4 Dinitrogen Pentoxide N2O5 Silicon Tetrabromide SiBr4