Unit: The Chemistry of Living Things

I. Composition of MatterMatter—anything that occupies

space and has mass Elements—fundamental units of

matter◦About 96% of an organism is made

from four elements Carbon (C) Oxygen (O) Hydrogen (H) Nitrogen (N)

Composition of MatterAtom: smallest unit in which an

element may be divided while retaining all of the characteristic properties of that element.

Atomic StructureNucleus

◦ Protons (p+, +1)◦ Neutrons (n0)

Outside of nucleus

Electrons (e-, -1)Atomic number=

# number of protons

*Each element has a unique atomic number.

2. Neutrons and Protons have significant mass.A proton has a mass of 1 AMUA neutron has a mass of 1 AMUAn electron has negligible mass Mass #= protons + neutrons

3. LocationNeutrons & protons -nucleus.Electrons orbit nucleus in patterns

within electron shells.# of shells an atom has is determined

by # of electronsElectron shells AKA energy levels since

shells closet to the nucleus have the least energy; those farthest away have the most.

4. Energy LevelsEach energy level (shell) holds:

◦Level 1 can hold a maximum of 2 electrons

◦Level 2 can hold a maximum of 8 electrons

◦Level 3 can hold a maximum of 18 electrons

Octet Rule

◦Electrons will follow the Octet rule or rule of eights: Energy levels > shell 2 can contain

more than eight electrons BUT # of electrons that can

participate in bonding (outer shell) is still limited to a total of 8.

Electrical charge of an atomIons – atoms with an electrical

charge◦Charged particles

Anions are negative Cations are positive Either donate or accept electrons Ions in the body are known as

electrolytes

ElementsElement: a substance that cannot

be ordinarily broken down chemically to form simpler kinds of matter.

1. Atomic Number (# of protons)Nucleus contains a fixed # of protons.# of protons in nucleus is the primary

determining factor for the general characteristics of an atom.

Atoms with the = # of protons in the nucleus are the same element, regardless of whether they have the same #s of neutrons or electrons.

The atomic # is written as a subscript to the left of its atomic symbol, i.e. Helium has 2p+, (2He).

2. Chemical Symbol

Rules:a. 1 or 2 letters are used to

represent an element.b. If 1 letter is used to represent an

element it is capitalized.c. If 2 letters are used to represent

an element, the first letter is uppercase and the second letter is always lowercase

Isotopes and Atomic Weight

Elements w/2+ variants are Isotopes Isotopes of an element have the same atomic number but vary in their mass numbers; thus, all isotopes of an element have the same number of protons, but differ in the number of neutrons

I.e.: The most abundant isotope of hydrogen is 1H. However, some hydrogen atoms have a mass of 2 or 3, which means they have 1 proton and 1 or 2 neutrons respectively.

Isotopes and Atomic Weight

Uses of Isotopes

- Carbon dating – fossils, organic remains

- Medical imaging

Atomic weightAtomic mass is the average of the relative masses of all the isotopes of an element

The atomic mass is written as a superscript to the left of its atomic symbol.

The atomic mass of an element is =P + NThe atomic weight of hydrogen is 1.008. Thus, the lightest isotope 1H is present in much greater amounts in our world than its 2H or 3H forms.

DefinitionsMolecule: the simplest

structural unit of a single element (O2) or compound (CO2)

◦Compound: a pure substance made up of atoms of 2 or more elements. (Literally means “to put together”)

1. Atoms in a Compound

In a compound, the proportions of each kind of atom are fixed.◦Chemical formula: gives the kind and

proportion of atoms of each element in the compound

◦EXAMPLE: H2O (there are 2 hydrogen atoms for every 1 oxygen atom in the compound water).

Structural formula: an expanded molecular formula showing the arrangement of atoms within the molecule

2. Properties of Compounds

The physical and chemical properties of compounds differ from those of the individual elements that compose it◦Example: Oxygen and hydrogen are

usually found as gases, however when they combine, they form a liquid.

D. Chemical Bond: a chemical attachment between atoms

Breaking and Forming Chemical Bonds◦During chemical reactions, bonds

are broken, atoms are rearranged, and new chemical bonds are formed

Role of Electrons◦Elements combine and form bonds

based on the number of electrons in their outermost shell.

◦Valence is the number of single bonds an atom will usually form (generally = to the number of electrons required to fill outer shell).

◦Most atoms are not naturally stable, so they undergo chemical bonding with other atoms to become more stable.

Chemical EquationReactants are generally shown

on the left side of the equation Products are shown on the rightThe arrow means “yields”

2. Bonding for Stability

 ◦Atom is stable when its outer energy

level (valence shell) is “filled”. ◦Incomplete valence shell are

unstable and therefore chemically reactive.

◦To fill their valence shells, they form bonds by sharing or transferring e-

with other atoms.

Examples of elements that do not form bonds.

3. Covalent Bond: a bond that forms when two atoms share one or more pairs of electrons.Nonmetals form covalent

bonds.

Covalent BondsA line drawn between chemical

symbols represents a covalent bond (-).

Single, double, and triple covalent bonds

A molecule = 2+ atoms held together by covalent bonds.

Oxygen needs 2 electrons to be stable.

 So, it will share 4 VEs or 2 pair= double bond

Exercise:Finish the electron shell diagrams

for N2, H2O, and CO2.

Note the two pairs (4 electrons) shared between each of the atoms.

Note the 2 pairs (4 electrons) shared between the atoms.

4. Ionic Bond: a bond formed by electrical attraction between two oppositely charged ions.

Remember, an ion is an atom or molecule with an electrical charge.

Opposites attractElectron(s) are transferred from

one atom to another

Use the practice sheet to form ionic bonds.Animation: Bonding in an Erlenm

eyer Flask.