Chapter 2 Of Atoms and Molecules: Chemistry Basics

Chapter 2

Of Atoms and Molecules: Chemistry Basics

All organisms consist of matter and energy.

LIFE- based on chemical principles.

MATTER – The basis for everything

A. MatterMaterial that takes up space. Matter

broken down to pure substances- Elements

1. ElementsPure chemical substances

composed of atoms.• Examples?

• Oxygen, sodium, copper, potassium• How many elements exist? (92)

• How many of these elements are essential to life? (25)• The essential elements are divided

into:• Bulk elements: needed in

relatively large amounts• Examples: C, O, H, N

• Trace elements: needed in relatively small amounts• Examples: Fe, K, Na, Zn, Ca

Periodic Table of Elements

2. AtomThe smallest “piece” of an element

that retains the characteristics of that element.

Composed of 3 subatomic particles:• Protons• Neutrons• Electrons

Atoms of each element – characteristic number of protons# of protons – determines size and character of each atom.Charge- attraction between opposite types of particles.Atom – electrically neutralElectron – small compared to proton and neutronOrbital- location for an electron relative to its nucleus.

Element – has a symbol.He- HeliumNa- SodiumAtomic number- shows the # protons in the atom- identity of the atom.Elements are arranged sequentially in the periodic table by atomic number.

Characteristics of Subatomic Particles

Atomic number• # protons in nucleus of an atom

(establishes identity of the atom)• Since most atoms are electrically

neutral, atomic number indicates # of electrons as well.

Atomic mass• # protons plus # neutrons in

nucleus of an atom

Periodic table information on carbon:

Atomic mass given in table is average mass of all the element’s isotopes.

How can we determine the number of neutrons in an atom?• # neutrons = atomic mass - atomic #

• Determine # neutrons in a carbon atom (atomic mass = 12; atomic # = 6).• # neutrons = 12 - 6 = 6

Do all carbon atoms have the same number of protons?

Do all carbon atoms have the same number of neutrons?

Isotopes• Atoms having the same number of

protons, but differing numbers of neutrons.

• Same charge & characteristics but different masses.

• Often one isotope is very abundant and others are rare.

• If an isotope is unstable, it is termed radioactive

Ex. Carbon isotopes• carbon 12 (12C) 6 neutrons• carbon 13 (13C) 7 neutrons• carbon 14 (14C) 8 neutrons

3. CompoundA pure substance formed when atoms of

different elements bond.The number of atoms of each element is

written as a subscript.Examples:• CO2 carbon dioxide• H2O water• CH4 methane• C6H12O6 glucose

Atoms bound together – molecules

4. MoleculeSmallest piece of a compound that

retains characteristics of that compound.

The number of molecules is written as a coefficient.

Examples:• 4CO2 4 molecules of carbon dioxide• 2C6H12O6 2 molecules of glucose• 6O2 6 molecules of oxygen

Chemical Reactions• The reactants of a reaction are

written to the left of the arrow.• The products of a reaction are

written to the right of the arrow.

Example:6O2 + C6H12O6 6H2O + 6CO2 + energy

5. Chemical Bonds• Type of bond formed is determined by the number of valence electrons in the interacting atoms [octet rule].

• The goal of chemical bonding is for all atoms involved to complete or eliminate their outer electron shells.

a) Covalent bonds - form when atoms share electron pairs.• strongest type of bond• tend to form when atoms have 3,

4 or 5 valence electrons• the number of electron pairs

shared determines whether the covalent bond is single, double or triple

• can be nonpolar or polar

Nonpolar covalent bonds - electrons are shared equally between atoms.

Ex. methane

Types of diagrams to represent molecules

Polar covalent bonds - electrons are not shared equally. They are drawn more strongly to 1 atom’s nucleus than the other.

Form when less electronegative atoms bond with more highly electronegative atoms.

Ex. water

b) Ionic bonds - form when oppositely charged ions are attracted to each other. • weaker than covalent bonds • atoms with 1, 2 or 3 valence electrons

give up electrons to atoms with 7, 6 or 5 valence electrons which forms ions

• form salts which break apart in waterEx. NaCl

c) Hydrogen bonds - form when opposite charges on two molecules are attracted to each other.• weakest type of bond*

Ex. DNA H2O

6. Attractive forces• van der Waals attraction• Attraction between molecules

or within molecules that occur when oppositely charged regions approach each other

• Often used to shape molecules

• Interactions with waterhydrophobic: parts of a

molecule that do not have charges cannot interact with water

hydrophilic: parts of a molecule that have charges can interact with water

B. The Importance of Water1. Properties• Cohesion - the attraction of water

molecules for each other.• Adhesion - the attraction of water

molecules for other compounds.• Imbibition – the tendency to absorb

water and swell

• High heat capacity – takes a great deal of heat to raise the temperature of water.

• High heat of vaporization - a lot of heat is required to evaporate water.

• Exists as solid, liquid or gas - solid (ice) is less dense than liquid.

2. SolutionsA solution is a mixture of one or more solutes dissolved in a solvent.

If solvent is water, then it is an aqueous solution.

Water is a strong solvent because it separates charged atoms or molecules.

3. Acids & Bases• Acids - substances

that add H+ to a solution.

• Bases - substances that remove H+ from solution.

• Buffer systems: help stabilize pH

pH scale is measure of acidity/alkalinity based on H+ concentration.