AP Chem: Structure of Matter

Define: Molecule
The combination of two or more atoms. A molecule of a substance is the smallest unit that retains the chemical properties of the substance. 
What is the geometry of a molecule that has two bonding pairs of electrons and two lone pairs of electrons?
The shape of a molecule that has two bonding pairs of electrons and two lone pairs of electrons is bent. 
Define: compound
A substance that is made up of two or more elements joined by chemical bonds in a fixed proportion. 
Of the following molecules, which is the most polar: CH4, NH3, or O2?
Ammonia, NH3, is the most polar molecule of all of the molecules listed here. The electronegativities of nitrogen and hydrogen differ more significantly than do those of the atoms involved in the other molecules. 
What is the geometry of a molecule of silane (SiH4)?

A molecule of silane is tetrahedral.

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What is the geometry of a molecule of nitrogen pentachloride, NCl5?

A molecule of NCl5 is trigional bipyramidal.

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Similar to the PCl5 (phosphorus pentachloride) 

What is the geometry of a molecule of calcium chloride, CaCl2?

A molecule of calcium chloride, CaCl2, is linear. 

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  1. Electronegativity Difference
  2. Describe an ionic bond in terms of electronegativity difference.

 

  1. A number found by taking the difference between the electronegativities of two atoms in a bond. Its value determines the type of bond.
  2. When the electronegativity (EN) values differ by 1.7 or more. The atom with higher EN borrows the bonding electrons form the atom with the lower EN. The resulting pos. and neg. ions attract.

 

What are the energy changes when a chemical bond is formed? Broken?

When a bond forms; energy is released

When a bond breaks; energy is absorbed. 

  1. Nonpolar covalent bond
  2. Polar covalent bond

  1. When the electronegativity difference os very small (less than 0.5). Two bonded atoms share the valence e. The resuting molecule has no electrostatic charge.
  2. When the electronegativity difference is between 0.5 and 1.7 the bonding electrons stay closer to the more electronegative atom. Electrons are shared unequally.

 

Describe a non-polar molecule with polar bonds.

Give 5 examples 

When the polar covalent bonds are arranged symmetrically around the central atom.

 

(CO2, CH4, CCI4)

  1. Coordinate covalent bond
  2. Metallic bonds

  1. When both electrons in a covelent bond are supplied by one atom.
  2. A sea of electrons surrounding positive metal ions

List properties of:

Metalic copounds

Strong bonds, high melting points, great strenght, good conductors of electricity

Van Der Waals Forces

List 3 types

intermolecular forces (forces between molecules)

  1. Dipole-dipole attraction
  2. London dispersion forces
  3. Hydrogen bonds

 

 

  1. Dipole-dipole attraction
  2. London dispersion forces

  1. Unsymmetrical distribution of electronic charge leads to positive and negative poles in a molecule. These poles attract electrostatically.
  2. A temporary dipole caused by instantaneous uneven electron distribution in a nonpolar molecule.

Hydrogen Bonding

Formed when hydorgen is bonded to oxygen, fluorine or nitrogen.

 

The hydrogen of one molecule becomes attracted to the electronegative element of the other molecule. These intermolecular attractions cause higher boiling points than predicted.

  1. Octet rule
  2. Double and triple covalent bond

  1. Atoms tend to gain or lose outer shell electrons in order to achieve a noble gas configuration of 8 electrons.
  2. In a double bond two pairs of electrons are share. In a triple bond three pairs of electrons are shared.

  1. Resonance structues
  2. Hybrid orbitals

(List 3 types)

  1. Where there is more than one possible bonding structure in a molecule
  2. Where 2 or more pure atomic orbitals are mixed to form identical hybrid orbitals. (sp, sp2, sp3)

List the hybrid orbitals present in BF3 and BeF2 and the bond angles in each

BF3 – sp2

(120° angle)

 

BeF2 – sp

(180° angle)

  1. Describe Hybrid Bonding in: Water, Ammonia, Methane
  2. Sigma bonds and pi bonds

  1. sp3 bonding results in a tetrahedron shape with bond angles of 109.5° in methane and slightly less in water and ammonia
  2. A sigma bond is present between any 2 orbitlas except when 2p orbitals share electrons; then, this is a pi bond.

LIst propertied of:

Ionic substances

  • Solids at 25°V
  • Non-conducting as solids but cinducting as aqueous solutions or liquids
  • High melting and boiling points
  • Brittle
  • Low volatilities

List properties of:

Molecular substances

 

  • Non-conducting as liquids and solids
  • Volatile liquids and solids
  • Many are gases at 25°C
  • Low melting and boiling points
  • Soft and waxy solids

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