Chapter 2 Atoms, Molecules, and Ions

• Atomic Theory of Matter
• The Discovery of Atomic Structure
• The Modern View of Atomic Structure
• The Periodic Table
• Molecules and Ions
• Naming Inorganic Compounds

Objectives

1. describe the approximate size, relative mass and the charge of an atom proton, neutron, and electron.
2. describe the experiments which characterized the atom and subatomic particles.
3. use the concepts of atomic number and mass number to determine the number of subatomic particles in isotopes and to write symbols for atoms, their isotopes and ions.
4. Use isotope percent abundances and masses to calculate atomic weights of elements.
5. use the periodic table to predict
6. the charges of monoatomic ions.
7. whether an element is a metal, non-metal or a metalloid.
8. distinguish between empirical formulas, molecular formulas and structural formulas.
9. write the formula for a compound given the ions from which it is made.
10. write the name of simple inorganic compounds and acids from the formula and write the formula given the name.

Atomic Theory of Matter

Dalton's Atomic Theory is based on the ideas of the Greek philosopher Democritus and three "Natural Laws" of science. Know these laws and their meaning.

• Democritus-believed that matter could not be subdivided infinitely, but only subdivided to small indivisible particles that he called "atomos" meaning not divisible.
• Law of Conservation of Mass (Lavoisier)- Matter is neither created nor destroyed in a chemical reaction.
• Law of Definite Proportions (Proust) a.k.a. "the law of constant compostion"- The relative number and kind of atoms in a compound is always the same.
• Law of Multiple Proportions (Dalton)- When elements form more than one compound, the atom ratio differs. (Different compounds have different atom ratios)

Dalton's Atomic Theory know these.

1. An element is composed of tiny particles called atoms
2. All atoms of a given element have the same chemical properties.
3. Atoms are neither created nor destroyed nor changed into other atoms in normal chemical reactions.
4. Compounds contain atoms of two or more elements in a fixed ratio.
5. When elements form more than one kind of compound, the atom ratio differs. Different compounds have different atom ratios.

The Discovery of Atomic Structure

(overheads from text)

The Modern View of Atomic Structure

• Rutherford Model of the Atom-a dense nucleus surrounded by a very diffuse cloud of electrons.
• Nucleus- dense central core of an atom that consists of protons and neutrons.
• electron cloud- large volume around the nucleus in which the electrons move about.

Characteristics of the Fundamental Subatomic Particles.

Atomic Weight- the relative mass of an average atom of the element expressed in atomic mass units

Atomic Mass Unit (amu)- A unit used to express the relative mass or atomic weight of an element. The mass of 1 proton or 1 neutron is equal to 1 amu. The mass of an electron is negligible by comparsion (1/1836). It would take 1836 electrons to equal the mass of one amu.

Isotopes

atomic number, Z.

• Represents the number of protons
• Represents the number of electrons in a neutral element

Mass number, A.

• Represents the number of protons plus the number of neutrons in a nucleus.

Number of neutrons= A-Z.

Symbol ^A_zE

• Fill in the blanks in the following table:

isotopes and atomic weights

• isotope percent abundance- the percent abundance represents the number of atoms of that particular isotope out of a sample of 100 atoms.
• average mass- the weighted average of all isotopes of an element.

Calculate the atomic weight of boron, B, from the following data:

The Periodic Table

Insert overhead and define groups, periods, metals, nonmetals etc.

Molecules and Ions

Molecules and Chemical Formulas

• Compound Formulas-A symbol for the molecule of a compound, consisting of the symbols of the atoms found in the molecule. Atoms present in numbers greater than 1 have the numbers indicated by a subscript.

• Example- phosphoric acid consists of 3 atoms of hydrogen chemically bonded to 1 phosphorus atom and 4 oxygen atoms. Write the chemical formula for one molecule of phosphoric acid.

Molecules

• A molecule is a definite group of atoms that are chemically bonded together — that is, tightly connected by attractive forces.
• A molecular substance is a substance that is composed of molecules, all of which are alike.
• A molecular formula gives the exact number of atoms of elements in a molecule.
• Structural formulas show how the atoms are bonded to one another in a molecule.

An important class of molecular substances that contain carbon is the organic compounds.

Organic compounds make up the majority of all known compounds.

The simplest organic compounds are hydrocarbons, or compounds containing only hydrogen and carbon. Common examples include methane, CH₄, ethane, C₂H₆, and propane, C₃H₈.

Molecular , Empirical and Structural Formulas

• Molecular - A symbol for the molecule of a compound, consisting of the symbols of the atoms found in the molecule. Atoms present in numbers greater than 1 have the numbers indicated by a subscript. The TRUE atom ratio in a compound.
  
• Empirical - the SIMPLEST atom ratio in a compound.
  
• Structural - a formula which shows the TRUE atom ratio AND the ARRANGEMENT of atoms in a compound.
  
• Formula Unit - Although many substances are molecular, others are composed of ions and are written as empirical ratios of ions..

Ion - A positive or negative charged particle formed by an atom or group of atoms gaining or losing electrons.

negative ions are called anions. Most non-metal atoms tend to gain electrons to become negatively charged.

example: O + 2e^- ==> O^-2 , the oxide ion

F + e^- ==> F^- , the fluoride ion.

positive ions are called cations. Most metal atoms will lose electrons to become positively charged.

example: Al ==> Al⁺³ + 3e^- , the aluminum ion

Na ==> Na⁺ + e^- , the sodium ion

Polyatomic ions - ions consisting of more than one atom bonded together as in a molecule, but with a net positive or negative charge.

example: PO₄^-3 , phosphate ion

Predicting Ionic Charge

• Most atoms will try to form ions so that they have the same number of electrons as the "Noble Gases". This allows one to predict the charge the ion of an atom will have.

Use the atom’s position on the Periodic Table to predict the charge the ion will take when forming a compound.

Figures in text with charge and rules for assigning oxidation numbers.

Writing chemical formulas and names. Know the ions on the handout in the exercise packet and lab packet (same list in both places)

Handout in lab packet-Nomenclature

Writing chemical equations

reactants ––> products

(s), (l), (g) (aq), Æ, ––>

C₂H₆(g) + O₂(g) ––> H₂O(l) + CO₂ (g)

The law of conservation of mass dictates that the total number of atoms of each element on both sides of a chemical equation must match. The equation is then said to be balanced.

Examples

Ca(s) + H₂O(l) ––> Ca(OH)₂(aq)

Al(s) + O₂(g) –––> Al₂O₃(s)

CaCl₂(aq) + AgNO₃(aq) –> AgCl(s) + Ca(NO₃)₂(aq)

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Updated Jan. 3, 2004Questions or comments on this Web site should go to Robin Terjeson.