AP Chem unit two: Molecular and Ionic Compound Structure & Properties

unit two of ap chem curriculum, goes over molecular shapes and bonding, .

Lewis Structure

a way to visually represent an elements atom and the number of valence electrons

The octet rule

atoms tend to gain, lose, or share electrons until they are surrounded by 8 valence electrons

(there are exceptions - hydrogen)

Ionization Energy

the amount of energy needed to remove an electron from an atom

Electron Affinity

the energy change that occurs upon when an electron is added to a neutral atom to create an anion

usually, exothermic reaction and essentially measuring how much an atom “wants” an extra electron.

Ionic structures

non-metal and metal, brittle, crystalline structure, high melting point, & cleavable.

lattice energy

the energy required to completely separate one mole of a solid ionic compound into its gaseous ions. 

• always exothermic

• measures the strength of the ionic bond in a crystal lattice

losing electrons is what kind or reaction

endothermic

lattice energy going down a group

decreases - since ionic radius increases

covalent structures

non-metal and non-metal, low melting point, sharing electrons (Lewis structures)

number of bonds means what?

the more bonds, the shorter the bond lengths, and the stronger the attraction between the two atoms.

breaking bonds is a…

endothermic reaction (energy is required to break bond -lose electrons-)

forming a bond

is an exothermic reaction (energy being released -adding electrons-)

bond polarity

the measure of how equally or unequally the electrons in any covalent bond are shared (electronegativity is high = high bond polarity)

nonpolar covalent

symmetrical; when electrons in a bond are shared equally 

ex) N₂ or Cl₂

polar covalent

non-symmetrical; when one of the atoms exerts greater attraction forces for the bonding electrons than the other atom. (where partially positive and partially negative come into play). 

ex) H₂O with Oxygen as the partially negative and Hydrogen as partially positive

Electronegativity

the ability of an atom in a molecule to attract electrons to itself (an atom’s attraction for another atoms electrons)

Electronegativity trends

down a group: decreases because atomic radius grows larger as well as there is more shielding. (already its hard to keep the atoms own electrons, now its harder to attract other electrons too)

across a period: increases because with more protons and less shielding, so there is a stronger pill to pull in the own electrons as well as pull in other electrons. . 

the greater the difference in electronegativity=

the more polar the bond.

a dipole

is established when two electrical charges of equal magnitude but opposite sign are separated by a distance (represented by dipole moment (μ) = the product of 2 equal and opposite charges (Q+ & Q-) and distance (r) so: μ= Qr

the larger the dipole moment

the more polar the bond

dipole moment =

the quantified magnitude of a dipole

Lewis Structures

1) NASP (needed, available, shared [subtracted from NA], pairs)
OR: 
1) sum all valence electrons in whole compounds
2) write the symbol and connect with single bonds
3) complete the octet
4) place any remaining electrons on the central atom
5)if there isn’t enough electrons to give the central atom an octet, try multiple bonds

Formal charge

the charge the atom would have if each bonding electron pair in the molecule were shared equally between its two atoms.

Formal charge equation

[valence electrons] - [1/2 bonding electrons] - [nonbonding electrons] = formal charge

• the formal charges on a NEUTRAL molecule must add up to zero but, charges on an ION must add up to the charge of that ion.

Lewis Structure Dominance

1) the dominant Lewis structure is generally the one in which the atoms bear a formal charge closest to zero

2) the Lewis structure which any negative charges reside on the more electronegative atoms is generally more dominant

Resonance

describing a molecule as a blend of different resonance structures (lewis structure)

exceptions to the octet rule:

1)molecules and ions contain an odd number of electrons

2) molecules and ions contain less than an octet of valence electrons

3) molecules and ions contain more than an octet of valence electrons (hypervalent)

Relationship between bond length + bond enthalpy

as the number of bonds between atoms increases, the bond lengths decreases and the bond enthalpy increases. 

Bond enthalpy

the amount of energy required to break one mole of a bond in a gaseous molecule, and it represents the strength of that bond. 

• stronger bonds= higher bond enthalpy

Bond angles

the angle between two adjacent bonds that originate from the same central angle.

Shape and size of a molecule is determined by:

the bond angles + bond lengths