Exambusters Chemistry Study Cards 9

What are the energy changes when a chemical bond is formed?
Energy is released
What are the energy changes when a chemical bond is broken?
Energy is absorbed
Electronegativity difference
A number found by taking the difference between the electronegativities of two atoms in a bond. Its value determines the type of bond.
Describe an ionic bond in terms of electronegativity difference.
When the electronegativity values, EN differ by 1.7 or more, the atom with the higher EN borrows the bonding electrons from the atom with lower EN. Resulting positive and negative ions attract.
Nonpolar covalent bonds
When the electronegativity difference is very small (less than 0.5), two bonded atoms share the valence e. The resulting molecule has no electrostatic charge.
Polar covalent bonds
When the electronegativity difference is between 0.5 and 1.7, bonding electrons stay closer to the more electronegative atom. Electrons are shared unequally.
Describe a non-polar molecule with polar bonds.
When the polar covalent bonds are arranged symmetrically around the central atom. e.g. CO2, CH4, CCl4
Coordinate covalent bonds
When both electrons in a covalent bond are supplied by one atom.
Metallic bonds
A sea of electrons surrounding positive metal ions.
Van der Waals forces
intermolecular forces (forces between molecules)
List three types of Van der Waals forces
dipole-dipole attraction london dispersion forces hydrogen bonds
Dipole-dipole attraction
Uneven distribution of electronic charge leads to positive and negative poles in a molecule. These poles attract electrostatically.
London dispersion forces
A temporary dipole caused by instantaneous, uneven, electron distribution in a nonpolar molecule.
Hydrogen bonding
Formed when hydrogen is bonded to oxygen, fluorine, or nitrogen. The hydrogen of one molecule becomes attracted to the more electronegative element of the other molecule. These intermolecular attractions cause higher boiling points than predicted.
Octet Rule
Atoms tend to gain or lose outer shell electrons in order to achieve a noble gas configuration of eight electrons.
Double Covalent Bond
Two pairs of electrons are shared.
Triple Covalent Bond
Three pairs of electrons are shared.
Resonance structures
Where there is more than one possible bonding structure in a molecule
Hybrid Orbitals (three types)
Where two or more pure atomic orbitals are mixed to form identical hybrid orbitals: sp, sp2, sp3
List the hybrid orbitals present in BF3 and the bond angles.
sp2 (120° angle)
List the hybrid orbitals present in BeF2 and the bond angles.
sp (180° angle)
Describe hybrid bonding in: water
sp3 bonding resulting in a tetrahedron shape with bond angle less than 109.5 degrees.
Describe hybrid bonding in: ammonia
sp3 bonding resulting in a tetra hedron shape with bond angle less than 109.5 degrees
Describe hybrid bonding in: methane
sp3 bonding resulting in a tetrahedron shape with bond angle of 109.5 degrees
Sigma bonds
Present between any two orbitals except when two p orbitals share electrons.
Pi bonds
When two p orbitals share electrons.
Properties of Ionic substances

solids at 25° C

non-conducting as solids

conducting as aqueous solutions or liquids

high melting point

high boiling point

brittle

low volatilities

Properties of Molecular substances

non-conducting as liquids and solids

volatile liquids and solids

many are gases at 25° C

low melting point

low boiling point

soft and waxy solids