Chemical Bonding

Chemical Bond The forces that hold groups of atoms together and make them function as a unit Bonding involves only the valence electrons There are 2 types of bonds: Ionic: Transfer of electrons from a metal and to a nonmetal Covalent: Sharing of electrons between 2 nonmetals Note: When 2 metals bond an alloy is formed Electrons are transferred or shared to give each atom a noble gas configuration (stable octet) This is known as the octet rule

Lewis Diagrams Valence electrons involved in bonding can be represented by Lewis dot diagrams A chemical symbol represents the nucleus and the core electrons (not involved in bonding). Dots around the symbol represent valence electrons.

Drawing Lewis Diagrams Cl Write the element symbol. Draw dots, one for each valence electron Dots should be spread over 4 sides It does not matter what side the dots are placed, but do not start to pair dots until there is one on each side The number of valence electrons is equal to the group number. With one exception.

Lewis diagrams for the first 20 elements

Ionic Bonding

Ionic Bonding Involves the transfer of valence electrons Attraction between a cation and an anion Cation Positively charged ion Typically metals But could be a polyatomic ion Anion Negatively charged ion Typically nonmetals But could be a polyatomic ion

Ionic Bonding

Ionic Bonding The oppositely charged ions are attracted to each other by a force called an ionic bond

Properties of Ionic Compound Most ionic compounds: Have high melting points Are hard and brittle Are soluble in water Form electrolytes (conduct electricity) when dissolved in water

Crystal Lattice Structure Ionic compounds form crystal lattices This explains why they have high melting points and are hard but brittle Na+ Cl-

Ionic Compounds in Water When ionic compounds dissolve, they separate into their ions The free ions can now move and conduct electricity

Representing Ionic Compounds Lewis Diagrams Formation of sodium chloride: Cl · · · ® Na+ [ ] Cl · · Na · +

Lewis Structures for Ionic Compounds Ba • O •• •• O Ba 2+ 2- Ba and O BaO Mg • Cl •• •• Cl Mg 2+ - 2 Mg and Cl Binary ionic compounds. Note the types of arrows used to move electrons – fishhooks for single e-. Write the Lewis symbol for each atom Determine how many e- each atom must gain or lose. Use multiples of one or both ions to balance the number of electrons. MgCl2

Representing Ionic Compounds Criss-Cross Method For monatomic ions: Take the absolute value of the ionic charge for the cation and make it the subscript for the anion and vice versa. Example: Al3+ and Cl- The 3 becomes the subscript for the chloride ion and the 1 becomes understood for aluminum. Forming aluminum chloride: AlCl3

Representing Ionic Compounds Criss-Cross Method For polyatomic ions: Additional step of including brackets around the polyatomic ion if it has a subscript other than one. Example: Mg2+ and OH- The 2 becomes the subscript for the hydroxide ion, but brackets are needed to indicate 2 of each the O and the H. The 1 becomes the understood subscript for Mg. Forming magnesium hydroxide: Mg(OH)2

Polyatomic Ions NICK the CAMEL ate a CLAM for SUPPER in PHOENIX Underlined letter represents the symbol of the element. The consonants represent the number of oxygen The vowels represent the negative charge. Eg. Underlined letter= N Number of consanants= 3 represents oxygens Number of vowels= 1 represents charge NO3- Nitrate

Covalent Bonding

Covalent Bonding Electrons are shared between two nonmetals Weaker attractive force than ionic bonding

Properties of Molecular Compounds State at SATP Typically gases & liquids, but some solids Structure Variable (waxy to brittle) Melting & Boiling Points Low Electrical Conductivity Nonelectrolyte Solubility in Water Variable (low to high) Confused?? Don’t worry…more answers to come

Covalent Bonding Formation of hydrogen chloride: Cl ® Cl H H + Lone pairs, valence electrons not involved in covalent bond Formation of hydrogen chloride: · · Cl · · · ® Cl · · H H · + ® H - Cl · · · · Covalent bond, shared electrons Structural Formula: H-Cl (lone pairs are not drawn)

+ Lewis Structures H H2: ® H H or H H Cl + ® Cl Cl Cl2: or Cl Cl · + H2: ® H H or H H Cl · · · + ® Cl Cl · · Cl2: or Cl Cl · · Structural Formula: Cl-Cl

Double and Triple Bonds Atoms can share 4 electrons to form a double bond or 6 electrons to form a triple bond. = O O · · O2: · · N N  N2: E.g. H, C, N, O, F The number of shared electron pairs (covalent bonds) that an atom can form is the bonding capacity.

Multiple Covalent Bonds • • N • •• •• N • • N •• • • N • •• N ••

Multiple Covalent Bonds • • • C O • •• • • O • C • • • O • • • • • • • • C O •• C O ••

Drawing Lewis Structures Arrange the element symbols. Central atoms are generally those with the highest bonding capacity. Carbon atoms are always central atoms Hydrogen atoms are always peripheral atoms Add up the number of valence electrons from all atoms. For polyatomic ions, add one electron for each negative charge and subtract one for each positive charge. Draw a skeleton structure with atoms attached by single bonds. Complete the octets of peripheral atoms. Place extra electrons on the central atom. If the central atom doesn’t have an octet, try forming multiple bonds by moving lone pairs.