Chapter 2

The Chemical Context of Life

Chemistry

• Definition– Study of matter and its

transformations

• Matter– Has mass and

occupies space

• Transformations– Chemical reactions– Break or form

chemical bonds

Units of Matter

• Element– Substance that cannot be

broken down further by chemical means

• Compound– Substance composed of

more than one element

• Molecule– Substance composed of

more than one atom– Includes diatomic elements

Elements found in Living Organisms

• 98 % of living mass composed of 4 elements– Oxygen– Carbon– Hydrogen– Nitrogen

• Minerals• Trace elements

The AtomBasic Unit of Elements

• Most fundamental unit of an element that retains the properties of the element

• Structure determines properties of element

• Composed of sub atomic particles

Subatomic Particles

• Proton• Neutron• Electron

Protons

• Composed of quarks– 2 ups and a down– http://en.wikipedia.org/wiki/

Quark

• Positive charge of +1• Mass of 1 AMU or Dalton• Resides in atomic

nucleus• Confers identity• Atomic number

Neutrons

• Composed of quarks– 2 downs and an up

• Electrically neutral• Mass of 1 AMU• Resides in atomic

nucleus• Stabilizes atomic nucleus• Contributes to atomic

mass• Number varies• Isotopes

Quarks and Leptonsboth Fermions

Isotopes

• Forms of an element that differ in the number of neutrons

• Atomic mass changes but atomic number does not

• May be radioactive if atomic nucleus is over crowded

Isotopes

Common Radioactive Isotopes

Types of Radioactive emissions

• Alpha – Helium nucleus– 2protons, 2 neutrons

• Beta– Electron equivalent– Neutron converts to

proton

• Gamma– EM radiation– Accompanies other

particles

G-

Electrons

• Fundamental particle called a lepton http://en.wikipedia.org/wiki/Lepton

• Electrical charge of -1

• Mass negligible(1⁄1836 of that of the proton)

• Located outside the atomic nucleus in the electron cloud

• Neutralizes the charge of the protons

• Participates in chemical bonding

Electron location within the Electron Cloud

• Distance from atomic nucleus indicates electron energy level

• The farther from the nucleus, the higher the energy

• Levels called shells (1st quantum number,N)

• Named as numbers (1,2,3…)• Maximum number of electrons

in each shell= 2N2

• Outer shell=valence shell• Outer shell electrons most

readily available for chemical bonding

Electron location within the Electron Cloud

• Orbitals (2nd quantum number (l)

• Specific region in which an electron is likely to be found

• Named with letters– s- sharp– p-principal– d- diffuse– f-fundamental– Named after groups of

lines in the spectra of alkali metals

Electron location within the Electron Cloud

• Shapes of orbitals• s spherical- 2 electrons• p party balloons- 6 e-

• d complex 10 e-

• f complex 14 e-

• Energy increases from s-f• Electrons usually fill from

s→f with some exceptions

• 3rd quantum number is the axis or magnetic orientation of the orbital (m)

Electron Fill

• Electron configuration of an element

• Fill from lowest energy to highest energy location

• Opposite spins for electrons in same orbital (Pauli exclusion principle)

• Spin is the 4th quantum number (s)

Sub Atomic Particles

Proton Neutron Electron

Mass (kg) 1.6726231x10-27

1.6749286x10-27

9.1093897x10-31

Charge [C] +1.602 0.0 - 1.602

Radius 8x10-16 8x10-16 less than 10-16

Composition

uud ddu N/A

Periodic table

• Periods– Horizontal rows– Represent electron

filling of a shell– Fill from left to right– s 1st 2 groups– d transition metal– p right block,

metaloids, non metals– f lanthides and

actinides

Electron configurations

• C

• N

• O

• Li

• Cl

• F

• Ne

Bohr Model

• C

• N

• O

• Li

• Cl

• F

• Ne

Chemical bonding

• Forms by interactions between valence shell electrons

• Goal- full valence shell• Two main types

– Ionic– Covalent

• Type determined by electro negativity differential

Ionic Bonds

• Electron transfer• Large electro negativity

differential• Between metals and non

metals• Ions formed

– Cation– Anion

• Attraction between oppositely charged ions

Covalent Bonds

• Electrons shared between atoms

• Low electro negativity differential

• Hybrid orbitals• Electron timeshare• Single• Double• Triple

Polar Covalent Bonds

• Electrons not shared equally

• Creates a dipole• Rotates in electrical

field• Fosters dipole

interactions

Non-Polar Covalent Bonds

• No separation of charges

• Equal electron sharing

• Even distribution of charges

• Fosters induced dipole interactions

Dipole Interactions

• Occur between polar covalent molecules

• Stronger than interactions between non-polar molecules

• Slightly negative region of one molecule is attracted to slightly positive region of a neighboring molecule

• Cohesive

Hydrogen Bonds

• Special type of dipole interaction

• Involves H as the slightly + component

• Very significant in biological systems– 2 strands of DNA– Protein shape– Genetic code

Induced Dipole Interactions

• May be referred to as Van der Waals interactions

• Very weak

• Between non-polar covalent molecules

• Volatility of non-polars compared to polars

Induced Dipole Interaction

Solubility

• Like dissolves like

• Test for polar substance- dissolve in water

• Non polar solvents clean non polar stains

• Polar solvents clean polar stains

• Oil and water