Intro to Chemical Bonding

Why do atoms form chemical bonds? atoms bond to become more stable (lower energy) energy must be released when a bond is formed (exothermic) ex) C(s) + O2(g) → CO2(g) + 393.5 kJ How do they for bonds? atoms tend to lose, gain, or share electrons in a bond Elements will react to have a full valence energy level (8 e-)

Electron Dot Diagrams (Lewis diagrams) Method used to analyze the type of bonds formed between atoms simplified model of the atom Kernal – consists of the nucleus and the inner e-, represented by the element symbol

Li Be B C N How do we write them? Valence electrons are placed as dots around the element symbol place one dot on each of the 4 sides of the symbol after one electron is on each side, double up Li Be B C N

O F Ne each unpaired electron represents one bonding site

Ionic Bonding

Formation of Ionic Bonds Atoms with strong attraction for electrons (nonmetals), remove electrons from atoms with low electronegativity (metals) Result: electrons are TRANSFERRED from a metal to a nonmetal which form: Negatively charged ion Positively charged ion Ionic bond is the result of the attraction between the oppositely charged ions

Electron Dot Diagrams of Ionic compounds Draw the dot diagrams of the following atoms: Ex) K + Br K Br K + Br K Br

DO NOW Draw electron dot diagrams for the compounds made up of: Strontium and chlorine Potassium and oxygen

Electron Dot Diagrams of Ionic compounds Mg + O Mg O Ca + Cl

Electron Dot Diagrams of Ionic compounds Li + O Al + O

Ionic Bond Predictors The greater the difference in electronegativity values between the elements, the more ionic in nature the bond will be. Typically the EN difference is 1.7 or greater (HF is an exception to the rule)

Properties of Ionic Compounds Solids at room temperature Ions held in fixed positions called crystal lattice Not composed of molecules, but formula units (ratio of ions in crystal) High Melting points and boiling points Brittle, break along planes

Properties of Ionic Compounds Poor conductors of heat and electricity in solid state Good conductors of electricity when melted or dissolved in water (electrolyte) Ex) NaCl(l) or NaCl(aq)

Formulas of Binary Ionic Compounds Chemical Formula: combination of symbols and subscripts used to represent the composition of a compound - Binary means it is made up of 2 different elements Subscripts represent the simplest ratio of ions that make the compound neutral

How do we write formulas for ionic compounds? Rules: Write the symbols for the elements that make up the compound. 2. positive ion must be written first 3. sum of charges of ions must equal zero - criss cross charges if they don’t add up to zero

Examples Write for formula for: Calcium oxide Potassium sulfide

Transition Metals Can have more than one possible charge, Roman numerals are used to designate the charge of the metal in the compound Ex) Nickel (II) chloride Iron (III) oxide

What’s in a name? Binary compounds always end in –ide Some compounds end in –ite or –ate This indicates the presence of a polyatomic ion!!

Polyatomic Ions Polyatomic ions (Table E) are groups of elements that are covalently bonded together and carry a charge.

Writing formulas with polyatomic ions. Ex) sodium phosphate copper (II) sulfate calcium hydroxide magnesium nitrate

Naming Ionic Compounds Name the cation and anion - use metal name - change nonmetal name ending to –ide carbon nitrogen oxygen carbide nitride oxide - if there are more than 2 elements, check table E for polyatomic name

Naming Ionic Compounds 2. Figure out if you need a Roman numeral in the name. - only when the metal can have more than one charge 3. Figure out what the Roman numeral should be - reverse criss cross

Practice Problems ZnO NaNO3 FeCl3 Pb(NO3)2

One more, for good luck NH4NO3