BONDING AND GEOMETRY Unit 8 Chemistry

ATOMS AND IONS REVIEW Atoms are neutral Ions have a charge They have the same number of protons and electrons Number of positives = number of negatives Example: Na 11 protons, 11 electrons  11 – 11 = 0 Ions have a charge They have a different number of protons and electrons Example: Na+111 protons, 10 electrons 11 – 10 = +1 If an atom GAINS an electron  becomes negatively charged  ANION If an atom LOSES an electron  becomes positively charged  CATION

TYPES OF BONDS Bonding occurs because every element is either trying to get to 0 electrons in the valence or 8 electrons in the valence (zero and 8 are both stable) Valence is the outer electron shell—place where bonding occurs Ionic – Bonding between a metal and a nonmetal Metallic – Bonding between two metals Covalent – Bonding between two nonmetals

IONIC BONDING Very stable and strong Strongest possible bond Requires a large amount of energy to break an ionic bond Forms compounds known as “ionic compounds” All ionic compounds will dissolve in water and carry a current (electrolyte) Generally have high melting and boiling points Compounds are generally hard and brittle

METALLIC BONDING Metal atoms are pieces of metal that consist of closely packed cations (positively ions) Cations are surrounded by mobile valence electrons that are free to drift from one part of the metal to another Metal atoms are arranged in very compact and orderly (crystalline) patterns Metallic bonding is the electrostatic attraction between conduction electrons, and the metallic ions within the metals, because it involves the sharing of free electrons among a lattice of positively-charged metal ions Occurs between 2 or more metals Result of the attraction of free floating valence electrons for the positive ion These bonds hold metals together

COVALENT BONDING Covalent: A covalent compound is called a molecule Covalent bonds are when atoms SHARE VALENCE electrons A covalent compound is called a molecule Covalent bond ALWAYS occurs between 2 nonmetals

TYPES OF COVALENT BONDS Single Bond Covalent bond where one pair of electrons (2 electrons total) are shared between 2 atoms Atoms share electrons so that each has a full octet (8 valence) Electrons that are shared count as valence electrons for both atoms Examples Cl2

COVALENT BONDING Double Bonds Bond in which two pairs of electrons (4 electrons total) are shared between 2 atoms Examples O2 Triple Bonds Bond in which 3 pairs of electrons (6 total electrons) are shared between atoms N2

COVALENT BONDING Electron Pairs Electron pairs involved in the actual bond are called BONDING PAIR or SHARED PAIR electrons Electrons not involved in the actual bond, those surrounding the rest of each element are called LONE PAIR electrons

POLAR BONDS AND MOLECULES Covalent bonds are formed by sharing electrons between two atoms The bonding pair of electrons is shared between both elements, but each atom is tugging on the bonding pair When atoms in a molecule are the same (diatomic) the bonding pair is shared equallythis bond is called non polar covalent When atoms in a molecule are different, the bonding pair of electrons are not shared equallythis is called a polar covalent bond

POLAR BONDS AND MOLECULES Why is the bonding pair not shared equally? The answer lies within electronegativity One of the elements is more electronegative than the other and therefore has a greater desire for the shared pair The MORE electronegative element tends to pull the electrons closer and thus has a slightly negative charge The LESS electronegative element has a slightly positive charge since the shared pair is being pulled away

POLAR BONDS AND MOLECULES Polar Molecules Molecule in which one end of the molecule is slightly negative and the other end is slightly positive Just because a molecule contains a polar bond DOES NOT mean the entire molecule is polar The effect of polar bonds on the polarity of an entire molecule depends on the shape of the molecule and the orientation of the polar bonds

POLAR BONDS AND MOLECULES Example: CO2 O = C = O Carbon and Oxygen lie along the same axis. Bond polarities are going to cancel out because they are in opposite directions Carbon dioxide is a nonpolar molecule even though there are two polar bonds present Would cancel out if the polarities moved towards each other as well When polarities cancel out, the molecule is non-polar

POLAR BONDS AND MOLECULES Example: H2O

INTRAMOLECULAR FORCES Intramolecular forces- (attraction is within the molecule) Types of forces: covalent, Ionic, metallic

INTERMOLECULAR FORCES Intermolecular Forces- (attraction is between molecules) Hydrogen bonding Dipole-Dipole Bonding Dispersion (London/van der Waals)

INTRA/INTERMOLECULAR FORCES Which is a greater force: Intermolecular or intramolecular? Rank the strength of each kind of inter and intra molecular force.

FORCES IN A MOLECULE Dipole-Dipole Forces Dipoles are created when equal but opposite charges are separated by a short distance Have to have a positive and a negative end so that one of the elements is pulling on the electron Only happens in polar molecules Dipole forces are extremely strong and lead to high melting and boiling points

FORCES IN A MOLECULE Hydrogen Bonding Very strong type of dipole force Only occurs when hydrogen is covalently bonded to a highly electronegative atom Always involves hydrogen Example: HF, H2O, NH3

FORCES IN A MOLECULE H – bonding and boiling point   Why does boiling point increase as you go down a group? The increase in boiling point happens because the molecules are getting larger with more electrons, and so dispersion forces become greater

FORCES IN A MOLECULE London Dispersion Forces Electrons are in constant motion around a nucleus At any given time there might be more electrons on one side of an atom than on the other For a split second, the side with more electrons is negative, and the side with less electrons is positive

FORCES IN A MOLECULE London Dispersion Forces Recall that Noble Gases have a full outer shell and you have been told they are unreactive BUT due to London Dispersion Forces, they COULD bond for an instant Example: Ar2 London Forces are very weak The smaller the mass of the atom, the smaller the London Force

BOND DETAILS Terminology Bond strength-energy required to break a bond Bond axis-imaginary line joining two bonded atoms (example: C-C) Bond length-the distance between two bonded atoms at their minimum potential enery; the average distance between two bonded atoms Bond energy-energy required to break a chemical bond and form neutral isolated atoms Chemical compound tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level

BOND DETAILS Comparison of Bond Length/Strength for Covalent Bond Types: Longer bond = less bond strength Rating 1-3 (with 3 as the largest and 1 as the smallest) Bond Length Strength Single 3 1 Double 2 Triple

BOND DETAILS Coordinate Covalent Bonds Very rare Tend to form harmful molecules Occurs when both of the bonding pair of electrons in a covalent bond come from only ONE of the atoms Example: CO

BOND DETAILS Resonance Occurs when there are more than one possible structures for a molecule Refers to bonding in molecules or ions that cannot be correctly represented by a single Lewis structure Example: CO2 To indicate resonance, a double-headed arrow is placed between a molecule’s resonance structures Even though all of the structures are different, the number of bonding pair of electrons and lone pair of electrons stay the same in each structure

VSEPR THEORY Valence Shell Electron Pair Repulsion Theory Allows us to picture molecules in 3 dimensions Centers around the fact that electrons have negative charges and repel one another So electron pairs within a structure try to arrange themselves to be as far away from other pairs as possible

VSEPR THEORY Tetrahedral Central atom bonds to 4 atoms and has zero lone pairs CH4

VSEPR THEORY Pyramidal The central atom bonds to 3 atoms and has 1 lone pair of electons NH3

VSEPR THEORY Trigonal Planar The central atom bonds to 3 atoms and has zero lone pairs BF3

VSEPR THEORY Bent Triatomic The central atom bonds to 2 atoms and has 2 lone pair of electrons H2O

VSEPR THEORY Linear Triatomic The central atom bonds to 2 atoms and has zero lone pair of electrons CO2

VSEPR THEORY Linear One bond between 2 atoms HCl