The Chemical Context of Life

Matter - anything that has mass and takes up space

Mass - the amount of matter an object contains

Weight - the measure of how strongly an object is pulled by the earth’s gravity

Element - substance which cannot be broken down into other substances by chemical reactions

* Life elements

--All matter made of elements

--92 naturally occurring

--C, O, H, N

--Ca, P, K, S, Na, Cl, Mg

Trace Elements - required in very small amounts

Compound - a pure substance composed of 2 or more elements combined in a fixed ratio

Atom - smallest unit of matter that retains the physical & chemical properties of an element

Subatomic particles

--Neutrons - neutral (1.009 dalton)

--Protons - + charge (1.007 dalton)

--Electrons - -charge (1/2000 dalton)

Dalton - mass at the atomic level

Atoms are neutral, so protons = electrons

Atomic number - number of protons

--Written as a subscript to the left of the symbol

Mass number - protons + neutrons

--Superscript to left

Subtract atomic # from mass # = neutrons

23 (# of electrons)

Na (# of protons)

11 (# of neutrons)

12 (# of electrons)

C (# of protons)

6 (# of neutrons)

Isotopes - atoms of an element that have the same atomic # but different

--mass number = protons, not neutrons

--Occur as natural mixtures

--Some radioactive

--Isotopes are chemically similar

Radioactive isotope - unstable isotope in which the nucleus spontaneously decays emitting sub-atomic particles or energy as radioactivity

--Transforms 1 element to another

--Fixed half-life - time for 5O% of atoms in a sample to decay

--Dating fossils less than 5O,OOO yrs

--Radioactive tracers

--Chemicals labeled with tracers used to trace path of biochemical reactions

--Isotopes of P, N, H used to determine DNA structure used to diagnose disease - PET scanner

--Radioactive cobalt - cancer

* Energy Levels --Electrons - equal in mass & charge

--Only stable subatomic particles involved directly in chemical reactions

--Have potential energy (energy stored in matter because of its position or location)

--Moves naturally to lowest state of PE

PE of electrons exists in discrete amounts = quanta

Different fixed PE states = energy levels or electron shells

Further from nucleus = more energy

Electrons move from 1 orbit to another

Gain or lose energy = to differences in PE

* Electron Orbitals

Orbital = 3 D space where an electron is most likely found 9O%

--3D probability cloud

--2 electrons max per orbital

--Ist energy level = 1 spherical “s” orbital

--Max. 2 electrons

2nd energy level

--Max. 8 electrons

--2s orbital

--3 dumbbell shaped p orbitals oriented at right angles to other 2 (2px, 2py, 2pz)

Higher S and p orbitals + additional orbitals

An atom’s electron configuration determines its chemical behavior

--1st 18 elements arranged sequentially by atomic # into 3 rows(periods)

--Never more than 8 electrons in outermost shell

--Occupies lower shells before higher

--Added to p orbitals singly, then paired

--Only outermost can have empty spaces

--Valence electrons - those in outermost shell

--Complete = inert (noble gases)

--Incomplete = reactive

Atoms combine by chemical bonding to form molecules

Covalent bond - chemical bond between atoms by sharing Strong chemical bond

--Single, double or triple(structural, molecular, empirical formulas)

Valences of: H (1); O (2); N (3); C (4); P (3); S (2)

Compound - a pure substance composed of 2 or more elements combined in a fixed ratio

Electronegativity - atom’s ability to attract and hold electrons

Scale designed by Linus Pauling

Nonpolar covalent bond - covalent bond formed by an equal sharing of electrons between atoms

electronegativity = Diatomic molecules

Polar Covalent Bonds - unequal sharing with different electronegativities

* Water Ionic Bonds - charged atom or molecule

Anion gains electrons (-)

Cation loses electrons (+)

Ionic bond - formed by electrostatic attraction after transfer of electron

Strong in crystals, weak in water

* Salts

Weak chemical bonds play important roles in the chemistry of life: H bonds, ionic bonds in water, Van der Waals, hydrophobic interactions

Temporary associations - enable chemical signaling Stabilize large molecules

Hydrogen bonds

-- formed by the charge attraction when a H atom covalently bonded to 1

-- atom is attracted to another

A molecule’s biological function is related to its shape

--2 atoms = linear

--2 + = complex shapes

--Chemical reactions change composition of matter

--Reactants changed to products

--Matter conserved

--Many reactions reversible

--concentrations affect rate

Chemical equilibrium - established when rate of forward reaction = rate of reverse reaction

Relative concentrations in parity BUT not necessarily = Chemical conditions on early earth set the stage for the origin of life