Chemistry 10/21/11

Brainteaser 10/24/11 Name these ionic compounds: –NH 4 Br –Fe(SO 4 ) Write the correct formula of these ionic compounds –Manganese (II) perchlorate –Sodium nitrate –Cesium iodide

Answers Name these ionic compounds: –NH 4 Br - ammonium bromide –Fe(SO 4 ) - iron (II) sulfate Write the correct formula of these compounds –Manganese (II) perchlorate - Mn(ClO 4 ) 2 –Sodium nitrate - NaNO 3 –Cesium iodide - CsI

Gaining an Octet There are two ways that elements want to be like the nearest noble gas: 1)Gain or lose electrons to form an ionic compound. 2)Share electrons with other elements to form covalent compounds.

How does this happen? In ionic bonding (as with NaCl, Cl is electronegative and Na isn’t, so Na doesn’t mind giving electrons to Cl). This way, both fill their octets. Whenever two nonmetals bond to each other, electrons don’t get transferred because both elements have similar electronegativities (i.e. they both want to gain electrons to be like the nearest noble gas). –In covalent bonding (as with F 2 ), both elements have similar electronegativities so neither will give electrons to the other. As a result, they’re forced to share electrons.

Definitions Covalent compound: A compound formed when nonmetals bond by sharing two or more valence electrons. –Share valence electrons  co”valent” compound Covalent bond: A chemical bond formed when nonmetal atoms share two valence electrons.

The Octet Rule and Covalent Compounds  Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level.  Covalent compounds involve atoms of nonmetals only.  The term “molecule” is used exclusively for covalent bonding

The Octet Rule: The Diatomic Fluorine Molecule F F 1s 2s 2p seven Each has seven valence electrons FF

The Octet Rule: The Diatomic Oxygen Molecule O O 1s 2s 2p six Each has six valence electrons O O

The Octet Rule: The Diatomic Nitrogen Molecule N N 1s 2s 2p five Each has five valence electrons N N

 Lewis structures show how valence electrons are arranged among atoms in a molecule.  Lewis structures Reflect the central idea that stability of a compound relates to noble gas electron configuration.  Shared electrons pairs are covalent bonds and can be represented by two dots (:) or by a single line ( - ) Lewis Structures

Properties of Compounds All properties of covalent compounds are determined by the fact that covalent compounds form molecules, while ionic compounds form crystals. A good model for thinking of this: –Ionic compounds are like stacks of Legos all locked together into a big block. –Covalent compounds are like rubber balls thrown together into the same bucket.

Properties contd. Low Melting and Boiling Points –In covalent compounds, the molecules only have very weak forces (called Van der Waals forces) holding them to each other. As a result, covalent compounds can be found as solids, liquids, or gases at standard room conditions. BONDS ARE NOT BROKEN WHEN THERE IS A STATE CHANGE!!! How is this different than ionic compounds? –Magnetically stuck together in big crystals

Naming Binary Covalent Compounds Write the name of the first nonmetal Write the name of the second nonmetal with the ending changed to the suffix “–ide.” Insert prefixes into the name to reflect subscripts in the formula. mono =1hexa= 6 di = 2hepta= 7 tri = 3octa = 8 tetra = 4nona= 9 penta = 5deca = 10 Never start a name with the prefix “mono-”

Practice Name the following binary molecular compounds –CCl 4 –CO –SO 2 –NF 3

Practice Write the formula of these binary molecular compounds –Dihydrogen monoxide –Tricarbon tetrachloride –Diphosphorus pentoxide –Sulfur dioxide

Review Rules for naming ionic compounds –Metal (+) and Non-metal (-) –Monatomic (only one atom present) O 2- –Polyatomic (more than one atom present) Example: NO 3 - Rules for naming covalent compounds –Non-metal(-) and Non-metal(-) Hydrogen is a non-metal (if it comes after a non-metal) –CH 4 –Use Prefixes (Never start with Mono in the first non-metal)