Lecture 2

Ionic Bonds

Recap: transfer electrons to another atom

- Usually between metal (left, light blue) and nonmetal (right, yellow)

- high melting and boiling points

Charges

Positive ion: equal to group number

ex. group 1(A) = 1+, group 2(A) = 2+

Negative ion: group number - 8

ex. group 5(A) = 3-, group 6(A) = 2-

Metals: become positive ions by loss of valence e-, have fewer electrons than protons (cations)

Nonmetals: become negative ions by gaining valence e-, have more electrons than protons (anions)

Both achieve octet (full outer shell)!

Writing Formula from Charge

Formula

Positive charge = negative charge

Symbol of metal followed by symbol of nonmetal (ex. NaCl)

- Na loses one electron, Cl gains an electron

Depending on the charge, there may need to be multiple metals or nonmetals used to fill valence shells

Ex. Ca2+ and Cl-

2+ charge vs 1- charge
so, we need two Cl- to balance the charges

Formula: CaCl2

Transition Metals

- Most transition metals can form 2 or more positive ions

- Use roman numeral to identify charge (ex. copper (I) or copper (II)

Note: both forms of iron are important in human metabolism

Ionic Bonds and Nutrition

Main electrolytes: Na+, K+, Cl-

Major buffer: HCO3-

- PO4 3- (phosphate) is important to energy metabolism

- NH4 (ammonium)

- NH3 (ammonia) is removed from amino acids during protein catabolism, would be toxic if it wasn't, converted to a non-toxic substance in the liver called urea

Covalent Bonds

Recap: share electrons with another atom

- Usually between transition metals

The number of covalent bonds can be determined based on the number of electrons needed to complete a full octet

Single and Multiple Bonds

Single Bond - one pair of electrons is shared

Double Bond - two pairs of electrons are shared

Triple Bond - three pairs of electrons are shared (ex. nitrogen)

The Octet Rule: Lewis Structures

Naming Covalent Bonds

Step 1: Name the first nonmetal as an element

Step 2: Name the second nonmetal with an "-ide" at the end

Step 3: Use prefixes to indicate number of atoms of each element

Ex. SO3

1. first nonmetal is sulfur

2. second nonmetal is O named oxide

3. Subscript of 3 of O is shown as prefix "tri"

Sulfur trioxide

Polarity

Non-Polar Covalent Bonds

- occurs between nonmetals

- equal or almost equal sharing of electrons

- has almost no electronegativity difference (0.0-0.4)

Polar Covalent Bonds

- occurs between nonmetal atoms

- unequal sharing of electrons

- has moderate electronegativity difference (0.5-1.7)

Ionic Bonds

- occurs between a metal and a non metal

- result of electron transfer

- has a large electronegativity difference (>1.8)

Polarity and Shapes

- molecules are 3D in shape

- shape of a molecule determines its functions

- shape can be predicted based on geometry of central atoms and electron groups

- electron groups stay as far apart as possible from each other to avoid repulsion

- use VSEPR

Polarity and Molecules

- Molecules can be polar or non polar depending on their shape

- molecules with two or more polar bonds can be non polar if the polar bonds are arranged symmetrically

Polar Molecules

- Contains polar bonds

- Has separation of positive and negative charge called a dipole indicated with δ+ and δ-

- has dipoles that do not cancel

Non- Polar Molecules

- Contains non-polar bonds or symmetrical arrangement of polar bonds

VSEPR

Valence Shell Electron Pair Repulsion Theory

Guide to Predicting Molecular Shape

- Electron groups around a central atom

- Electrons as far away from each other as possible

- valence electrons have the least amount of repulsion of the negatively charged electrons

- have a geometry around the central atom that determines molecular shape

1. Write the electron dot structure

2. Determine the # of electron pairs bonded to atoms and the number of lone electrons (non bonded)

3. Arrange electron groups around the central atom to minimize repulsion

4. Use the atoms bonded to the central atom to determine shape of the molecule

Examples:

Methane (CH4), 4 shared pairs, no unbonded pairs

Four electron groups around C, repulsion is minimized by placing four electron groups at angles of 109.5°, which is tetrahedral (shape with 4 bonded atoms)

Water (H2O), 2 shared pairs, 2 unbonded pairs

Two electron groups are bonded to H atoms and two are lone pairs

Makes this a bent shape

Ammonia (CH4), 3 shared pairs, 1 unbonded pair

Three electron groups bond to H atoms and the fourth is a lone (nonbonding pair)

With three bonding atoms, the shape is pyramidal