Chapter 1Elements and Compounds

Measurements and observations are made in the macroscopic world. We interpret these measurements and observations using the microscopic world. Chemistry is involved in both worlds.

Chemistry: A Definition

● Find patterns● Develop models● Make predictions● Be quantitative● Experimental testing

Elements, Compounds and Mixtures

● Elements● Compounds● Mixtures

Homogeneous

Heterogeneous

Atomic Symbols

● Shorthand notation● Some derived from English names.

Bi for bismuth.● Others derived from non-English names.

Fe for iron.● Upper and lower case important.

CO and Co are different.

Chemical Formula

● Atomic symbols and subscripts● Compounds

Molecular

Ionic● Molecular elements

e. g. O2, H2

Evidence for the Existence of Atoms

● Dalton’s Atomic TheoryIndestructible atoms

Elemental identity

Elemental distinction

Atomic combination

The Role of Measurement in Chemistry

● Scientific KnowledgeProducts

Processes

The Role of Measurement in Chemistry

● MeasurementsNumbers and units

– Scientific notation– SI and other units

Prefixes

Conversion factors

The Structure of Atoms

● Beyond Dalton’s Theory● Three subatomic particles that are

important for chemists:Electron

Proton

Neutron

The Structure of Atoms

● Absolute vs Relative ChargesElectron charge = Proton charge (with the sign reversed).

● Nucleus holds the protons and neutrons.

Atomic Number and Mass Number

● Both positive integers.● Mass Number (A) Atomic Number (Z)

Z = number of protons.

A = Z + number of neutrons.● X is Atomic Symbol.

XAZ

Isotopes

● Same Z● Different A● Identical chemistry

XAZ

1

1XA

Z2

1

Isotopes

● Some elements have only one.● Some elements have a few.● Some elements have many.● % natural abundance is the percentage of

atoms occurring as a given isotope.

Isotopes

● Mass of an atomAbsolute mass

– Uses a mass unit: gram, ounce, … – Rarely used

Relative mass– Relative to specific isotope of carbon

By convention 12C = 12.00000... amu– Ratio called atomic mass.– Used frequently: amu

The Difference Between Atoms and Ions

● Atoms are neutral.● Ions are charged.

Positively charged ions: cations.

Negatively charged ions: anions.

Polyatomic Ions

● Many polyatomic anions.● A few polyatomic cations.

Polyatomic ions

● Spelling● Chemical Formula● Charge

Table 1.6

The Periodic Table

● Atomic number● Groups● Periods

Figure 1.8

The Periodic Table

● Metals● Nonmetals● Semimetals (metalloids)

Important to know where an element is situated in the periodic table.

The Macroscopic, Atomic and Symbolic Worlds of Chemistry

Figure 1.9

The Mass of an Atom

● Average masses reported.● Average masses are weighted averages.● amu used.

Chemical Reactions and the Law of Conservation of Atoms

● Fundamental law of chemistry:Conservation of Mass

● What does it mean?In a chemical reaction, matter is neither created nor destroyed.

Chemical Reactions and the Law of Conservation of Atoms

● Established empirically.● May not be true, but no counter example

has ever been found.● Atomic model based on this law.

Example of scientific method.

Chemical Equations as a Representation of Chemical Reactions● Heart of Chemistry● Chemical formulas used.● An arrow is used to separate reactants and

products.● Phase information is sometimes included.● Equation carries no implication as to how

fast the reaction occurs.

Chemical Equations as a Representation of Chemical Reactions

Reactants → Products

2Mg(s) + O2(g) → 2MgO(s)

Balancing Chemical Equations

Notice the equation did not read

Mg(s) + O2(g) → MgO(s).

Why not?

Balancing Chemical Equations

Notice the equation did not read

Mg(s) + O2(g) → MgO(s).

Why not?

Chemical equations must be balanced.