Chapter 2 The Chemical Context of Life

Comment u Much of this chapter should be review from a basic chemistry course. Much of the material is unlikely to be brand new.

Matter u Anything that has mass and occupies space.

Atoms and Elements u Atoms- Smallest particle of elements u Element- Matter made up of only one type of atom. u 92 natural elements. u Each element has a symbol. u Life requires about 25 elements

Compound u Elements combined in fixed ratios. u A compound has characteristics beyond those of its combined elements.

Macroelements u Elements needed in large amounts or quantities. u Examples: HONC PKS Ca Fe Mg Na Cl

Microelements u Elements needed in very small quantities. u Also known as Trace Elements. u Examples: Cu, Co, Zn, I, Mn

Atomic Number u The number of protons in the nucleus. u Each element has its own atomic number. If you change the atomic number, you no longer have the same element.

Atomic Mass u The number of protons and neutrons in the nucleus. u The atomic mass can change.

Atomic Model

Isotopes u Atoms of the same (iso-) element with different atomic mass. u Caused by changes in the number of neutrons. u Used as “tracers”.

Types of Isotopes 1. Radioactive - where the nucleus decays spontaneously, giving off particles and energy. 2. Heavy - has a stable nucleus, but masses more than the standard isotope for the element.

Energy u The ability to do work.

Potential Energy u Is the energy that matter stores because of its position or location. u Electrons have potential energy because of their position relative to the nucleus.

Electron Energy Levels u Energy levels around the nucleus of an atom. u 1st level can have 2 electrons and has the lowest potential energy. u Other levels can hold more than 2 electrons and have higher energy levels.

Chemical Behavior Of An Atom u Is determined by its electron configuration in the energy levels and orbitals.

Valence Electrons u The electrons in the outermost energy level. u Electrons available for chemical bonds.

Octet Rule u The most stable condition is to have an outer level of 8 electrons. u Exception - 1st level is stable with only 2 electrons. u When stable - no chemical reactions will take place. Ex: Ne, He, (Noble gases)

Electrons of the first elements

Chemical Bonds u Forces that join atoms together to form molecules. u Usually caused by sharing or transferring valence electrons.

Bond Formation Depends On: u The number of valence electrons that must be gained, lost, or shared to reach the stable condition.

Chemical Bond Types u Nonpolar Covalent u Polar Covalent u Ionic u Hydrogen

Nonpolar Covalent u When electrons are shared equally between atoms. u Very strong bond. u Important in many molecules found in living things. Ex: carbon to hydrogen

Nonpolar Covalent u Can be single, double, or triple between two atoms. u Each nonpolar covalent bond involves a pair of electrons.

Polar Covalent u When electrons are shared unequally between atoms. u Results in “polar” molecules that have charged areas. Ex: Water, H to O bonds

Ionic Bonds u Formed when electrons are transferred from one atom to another and ions are formed.

Types of Ions u Cations - have lost electrons (p+ > e-) giving them a positive charge. u Anions - have gained electrons (p+ < e-) giving them a negative charge.

Ionic Bonds u Formed when cations and anions attract each other. u Weak chemical bond.

Ionic Bonds

Hydrogen Bonds u When a hydrogen atom bonded to one molecule is attracted to the slightly negative area (often N or O) of another molecule. u Very weak individual bond. u Can be a “strong” force if there are many H bonds.

Hydrogen Bonds

Molecular Shape u Determined by the positions of the atom’s orbitals. u Molecular shape is crucial in Biology because it determines how most molecules of life recognize and respond to one another.

Chemical Reactions u The making and breaking of chemical bonds. u Reactions do not destroy matter, they only rearrange it.

Parts of the Equation 2 H 2 + O 2 2 H 2 O u Reactants: - the starting materials. u Products: - the ending materials. u Note - all atoms of the reactants must be accounted for in the products.

Chemical Equilibrium u When the conversion of reactants to products is balanced to the reverse reaction. Ex: 3 H 2 + N 2 2 NH 3

Summary u Element vs compound u Macro and micro elements u Atomic Number and Mass u Isotopes u Valence electrons u Chemical Bonds

Summary u We will now put elements together to form molecules and build the next level in the hierarchy.