LECTURE 1 A&B - CHE214

Carbon

• Central to the structure of living organism

• Can form strong bonds to other -- atoms

• Form strong bonds to elements such as H, N, O, and S

mid-1700s

Foundation of Org. Chemistry

Torben Bergman (1770)

• Made a distinction between org. and inorg. chemistry.

Chervut (1816)

Found that organic compounds can be synthesized like inorganic compounds.

Wohler (1828)

Found that it was possible to convert inorganic salt ammonium cyanate into organic urea.

Positively charged nucleus

• very dense, protons, and neutrons

• 10^-15 m

Negatively charged electrons

• In a cloud around the nucleus

• 10^-10m

Atomic Number (Z)

• No. of protons

• #p = #e

Mass No. (A)

• No. of protons + neutrons

Atomic weight

average mass of an atom in an element

Isotopes

Atoms of the same element but having different masses.

Electron arrangement

• Electrons fill layers around nucleus from low to high.

• Shells = Energy Levels

Valence electrons

Where most chemical reactions occur

Octet Rule

• Atoms are most stable if they have a filled/empty outer layer of electrons.

• Except H and He, a filled layer which contains 8 electrons.

• Atoms gain, lose, or share electrons to make a filled or empty outer layer.

Quantum mechanics

• describes electron energies and locations by a wave equation

• each wave function is an orbital

Atomic Orbitals

Electron cloud has no specific boundary so we show most probable area.

Orbitals and shells

— are grouped in — of increasing size and energy

1st shell

1 s orbital

2nd orbital

1 s orbital

3 p orbitals (2p)

3rd shell

1 s orbital

3 p orbitals

5 d orbitals

Aufbau “build-up" principle

• Lowest energy orbitals fill up first

• 1s → 2s → 2p → 3s → 3p → 4s → 3d

Pauli Exclusion Principle

• Electron spin can have only two orientations, up and down.

Hund's Rule

• If two or more empty orbitals of equal energy are available, electrons occupy each with spins parallel until all orbitals have one electron.

Electronegativity

• tendency of an atom to draw electrons to itself

• increases from left to right, and bottom to top

• F is the most - element

Ionic Bonds

e- moves from metal to nometal

+ and - ions attract to form an —

Covalent Bonds

• sharing of electrons

Lewis Structures

• electron dot

• show valence electrons of an atom as dots

Kekule structures

• line-bond structures

• line drawn between two atoms indicating a 2 electron covalent bond

• stable molecule results at completed shell

Nonpolar bond

• covalent bond w/ electrons shared equally

Polar bond

• electrons are shared unequally

• electrons are more strongly attracted to Cl than to C because Cl is more electronegative than C

Non-bonding electrons

• valence electrons not used in bonding

• also called lone-pair electrons

Formal Charge

• the charge calculated for an atom in a lewis structure on the basis of an equal sharing of bonded electron pairs

group no. in periodic table (valence) - no. of bonds - no. of unshared electrons

Formal Charge Formula

Valence bond theory

• Electrons are paired in the overlapping orbitals and are attracted to nuclei of both atoms

• Bonds are an overlap of atomic hybrid orbitals.

Bond length

• Distance betweennuclei that leads to maximum stability

• If too close, they repel because both are positively charged

• If too far apart, bonding is weak

Tetrahedral angle

109.5 degrees

sp3

• 4 sigma bonds

• no. of bonds + lone pair = 4

sp2

• 3 sigma, 1 pi bond

• no. of bonds + lone pair = 3

sp

• 2 sigma, 2 pi bonds

• no. of bonds + lone pair = 2

sp3 orbitals

• C overlap with 1s orbitals on 4 H atoms to form four identical C-H bonds

• Each C–H bond has a strength of 436(438) kJ/mol and length of 109 pm

• Bond angle: each H–C–H is 109.5°, thetetrahedral angle.

sp2 hybrid orbitals

• 2s orbital combines with two 2p orbitals, giving 3 orbitals (spp = sp2). This results in a double bond.

• are in a plane with 120° angles

• Remaining p orbital is perpendicular to the plane

• Geometry is called trigonal planar

Restricted Rotation and Double Bond

• there is a large energy barrier to rotation (about 264kJ/mol) around the double bond

• rotational barrier is around 13-26kJ/mol

Isomerism

• restrcited rotation of groups joined by a double bond causs a new type of --

• the two compounds are called isomers

sp orbitals

• are linear, 180° apart onx-axis