unit 5 - molecular geometry and forces

day 1

lewis dot structures

electron dot notation

• the first step of Lewis Dot Structure

• Deals with elements valence electorns

• electrons are placed around the element symbol in a specific order

• must know how many valence electrons are in the main group elements !

steps of lewis structures!

1. find the sum of valence electrons of all atoms in the polyatomic ion or molecule.

   1. if it is an anion, add one electron for each negative charge

   2. if it is a cation, subtract one electron for each positive charge

2. the central atom is the least electronegative element that isn’t hydrogen

   1. connect the outer atoms to it by single bonds

3. fill the octets of the outer atoms

4. go back and review and fill in the rest- missed a step

5. if you run out of electrons before the central atom has an octet… form multiple bonds until it does (so they share the electrons)g

1. add valence electrons (divide by 2)

2. find central atom (Written first)

3. make outside atoms happy with

4. Make central atom happy

5. Form double/triple bonds if we run out of electorns

(what the fu-)

multiple covalent bonds

• double and triple bonds are referred to as multiple bonds, or multiple covalent bonds

  • in general, more bonds have greater bond energies and are shorter bonds

• carbon, nitrogen, or oxygen can create multiple bonds something i could catch oops…

ammonium

NH₄⁺¹

expanded octet

some elements cna have expanded octets - allowing the central atom to have more than 8 elements around it. those elements are in period 3 and below - mainly in the p block

VSEPR - molecular geometry and polarity

VSEPR - valence-shell electron-pair repulsion

VSEPR theory is used to predict the shape of the molecule

VSEPR theory states that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible

YOU MUST ALWAYS DRAW A LEWIS STRUCTURE BEFORE IDENTIFYING THE SHAPE

• VSEPR theory also takes into account geometries of molecules with unshared electron pairs

• ex ammonia, NH₃, and water H₂O

• VSEPR theory postulates that the lone pair occupies space around the nitrogen atom just as the bonding pairs do

• Those lone pairs need more space than a bond does due to the concentration of negative charge in one place

• You have to know that lone pairs are there, but ONLY THE BONDED ATOMS account for the shape of the molecule

unshared electron pairs repel other electron pairs more strongly than bonding pairs do

molecular geometry vs electron domain geometry

• there are 2 types of geomtries; molecular and electron domain

• molecular geometry: the true shape of a 3d model

• electron domain geometry: the general shape based on the idea that lone pairs of electrons take up space

  • the arrangement of all the electrons in the 3D space

  • groups of electrons around the central atom

polarity

• a polar bond is an unequal sharing of of electrons between two atoms

  • usually, one atom has a negative charge and the other has a positive charge

• a nonpolar bond is an equal sharing of electrons between two atoms

classification of bonds

• you can determine the type of bond between two atoms by calculating the difference in electronegativity values between the elements

• the bigger the electronegativity difference the more polar the bond.

using molecular geometry to figure out polarity

1. lone pairs on central atom?

   1. y → polar

   2. no → keep going

2. atoms around central atom

   1. same: nonpolar

   2. diffferent : polar

lone pairs on the central atom will always make it polar except for the expanded octets linear

intermolecular forces

• intermolecular forces (IMF) - the forces of attraction between molecules, known as intermolecular forces.

  • the boiling point of a liquid is a good measure of the IMF between its molecules: the higher the boiling point, the stronger the forces between the molecules

• intermolecular forces vary in strength but are generally weaker than intramolecular forces (bonds)

  London Dispersion/Van der Waals Forces

• even noble gas atoms and nonpolar molecules can experience weak intermolecular attraction

• in any atom/molecule, polar or nonpolar, the electrons are in continous motion

• so, as a result, at any instant, the electron distribution may be uneven

• a momentary uneven charge can create a positive pole at a one end of an atom of a molecule and a negative pole at the other

• this temporary dipole can then induce a dipole in an adjacent atom or moelcule - the two are held together for an instant by the weak attraction between temporary dipoles

• the intermolecular attractions resulting from the constant motion of electrons and the creation of instanteous dipoles are called London dispersion forces

• a dipole is created by equal but opposite charges separated by a short distance

• the direction of a dipole is from the dipole’s positive pole to its negative pole

• the negative region in one polar molecule attracts the positive region in adjacent molecules, so the molecules all attract each other from opposite sides

• such forces of attraction between polar molecules are known as dipole-dipole forces

• dipole-dipole forces act at short range, only between nearby molecules

• besides 2 polar molecules having dipoles, ions in a solution can also create a dipole moment

• an ion-dipole is created when an ion is attracted to an oppositely charged polar molecule

• some hydrogen compounds have unusually high boiling points

• this is explained by a very strong type of dipole-dipole force

• in compounds containing H-F, H-O, or H-N bonds, the large electronegativity differences between hydrogen atoms and the atoms they are bonded to make their bonds highly polar

• this gives the hydrogen atom a positive charge that is almost as half as large as that of a bare proton

• the small size of the hydrogen atom allows the atom to come very close to an unshared pair of electrons in an adjacent molecule

• the intermolecular force in which a hydrogen atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule is known as hydrogen bonding

• only with H-F, H-O, HN → between atoms that also have hydrogen bonding

bonds ranked from strongest to weakest : ionic bond, covalent bond, hydrogen bonding, dipole dipole force, london forces

WHAT THE HECK BRO