VSEPR

• • Valence electrons are electrons that occupy the outermost energy level of an atom
• • Atoms will always gain, lose, or share electrons in such a way that they create a full outer energy level (known as the octet rule)
• Electronegativity

  The ability of an an atom to “hold on” to its electrons is referred to as . 
• • Metallic elements have a lower electronegativity compared to non-metallic elements.
• • As a result, metals have a tendency to donate or get rid of electrons in a chemical reaction, whereas non-metals will usually gain or accept electrons.
•  

  IONIC

  metal & non-metal

  electrons are transferred from metal to non-metal

   
•  

  MOLECULAR

  two non-metals

  electrons are shared between atoms
• In order to achieve a stable octet, atoms involved in molecular bonding will often share multiple pairs of electrons. As a result, a double bond is formed between each atom: 
• Valence-shell-electron-pair-repulsion theory, or VSEPR, allows us to predict the stereochemistry of a molecular compound based on its Lewis structure. 
• • Stereochemistry refers to the spatial arrangement  or 3D distribution of molecules.
• • VSEPR is based upon the principle that because electrons are negatively charged, they tend to repel one another, causing electron pairs in the valence shell of an atom to stay as far apart as possible.
• • According to the VSEPR model, the shape of a molecule is always determined around a central atom.
• ive possible geometric shapes of molecules based on the number of atoms bonded to the central atom as well as the number of lone pairs...

  1. Linear
  2. Trigonal Planar
  3. Tetrahedral
  4. Pyramidal
  5. Bent
• A linear molecule involves a central atom bonded to two other atoms with no lone pairs OR a molecule composed of only two atoms.
• A trigonal planar molecule involves a central atom bonded to three other atoms and has zero lone pairs.
• A tetrahedral molecule involves a central atom bonded to four other atoms and has no lone pairs
• A pyramidal molecule involves a central atom bonded to three other atoms with one lone pair
• A bent molecule involves a central atom bonded to two other atoms and has either one or two lone pairs
• A polar molecule is one in which the negative charge (electrons) is not distributed symmetrically among the atoms making up the molecule. Thus, it will have partial positive and negative charges on opposite sides of the molecule. 
• A molecule that contains polar covalent bonds can be nonpolar 

  • If individual bond dipoles or vectors are equal in strength and in the opposite direction, they cancel one another out, resulting in a nonpolar molecule
  • This cancelling out happens in symmetrical molecules
  • If the bond vectors do not cancel out, the entire molecule will have a slightly positive and slightly negative end, known as dipoles
• Nonpolar covalent bond- if the bonded atoms have the same (or very similar) electronegativity, they will attract any shared electrons equally
• • Polar covalent bondif the atoms have different electronegativities
• The greater the difference in electronegativities, the more polar the bonds will be
• Dipole- a partial separation of positive and negative charges within a molecule, 

               due to electronegativity differences.
• • Tetrahedral &Trigonal pyramid: Nonpolar if all atoms attached have the same electronegativity, polar if different atoms are attached
• • Pyramidal: Polar as long as there is a difference in electronegativity between the atoms
• • Linear: Nonpolar if atoms have the same electronegativity
• • Bent: Polar
• • Van der Waals forces explain why individual molecules held together by covalent bonds are so difficult to break down, yet it is relatively easy to break the bonds that exist between molecules 
  • For example, it takes much less energy to boil water (overcome intermolecular forces) than it does to break it down into its individual elements (overcome intramolecular forces)
• Van der Waals Forces are actually composed of two separate types of forces:

  1.  DIPOLE-DIPOLE FORCE
  2.  LONDON FORCE
• Attraction between the dipoles of different molecules is referred to as dipole-dipole force. 
• diople

  These forces help determine the ability of solvents to dissolve solutes

  • E.g. In a solution of hydrogen chloride, the H and Cl atoms of different molecules will be attracted to one another due to the polarity that exists within each molecule
  • Thus, a higher degree of polarity results in stronger dipole-dipole forces
• • The more electrons a molecule has, the more easily momentary dipoles will form, resulting in a greater London force
  • Thus, larger molecules are held together by a greater force than small molecules
• More stuff in compound = stronger compond because there are more forces
• • H-bonds are a special type of polarity that results in an unusually strong intermolecular force
  • Occurs when a hydrogen atom is covalently bonded to another very electronegative atom. There must also be at least one lone pair of electrons on the atom bonded to the hydrogen atom
  • Any molecule with an -OH or -NH bond will show hydrogen bonding        
• H bonding is the strongest