Nivaldo J. Tro

http://academic.cengage.com/chemistry/tro

Deborah Koeck • Texas State University, San Marcos

Chapter 3Atoms and Elements

• The connection between the microscopic atom and the macroscopic element is the key to understanding the chemical world.

Understanding the Macroscopic World

Atoms• An atom is the smallest

identifiable unit of an element.

• Similarities and differences on the atomic scale correlate with similarities and differences on the macroscopic scale.

Atoms

Protons Determine the Element• The atomic number, Z, represents the number of protons in

the nucleus of an atom.• The charge of a proton is assigned numerical value of +1.• Protons constitute a significant part of the mass of an atom.

– Mass of a proton is 1.0 amu

Chemical Symbols• Symbols are an abbreviation of the

chemical name which can be based on:

– The English name for the element: hydrogen H

– The Greek or Latin name: ferrum (iron) Fe

– Their place of discovery: europium Eu

– The scientist honored by the naming: curium Cm

Periodic Table of Elements

Electrons• A neutral atom has as many

electrons outside its nucleus as protons within its nucleus.

• The opposing charges of protons and electrons hold electrons within a spherical region surrounding the nucleus.

• Atoms can lose and gain one or more electrons.

Determining Protons and Electrons

Neutrons• Neutrons are almost as

massive as protons but carry no electrical charge.

• The number of neutrons in the atoms of an element can vary resulting in ISOTOPES.– Can be naturally occurring or

man-made• The sum of neutrons and

protons in an atom is called the mass number of the atom.

Atomic Mass• Atomic masses listed on the periodic table are weighted

averages of the masses of each naturally occurring isotope for that element.

• Silver has two naturally occurring isotopes. One has a mass of about 107 amu and the other has a mass of about 109 amu.

• Silver -107 has an abundance in nature of 51.84%.• Silver -109 has an abundance in nature of 48.16%.

Atomic MassCalculate the average atomic mass of silver.

Silver has two naturally occurring isotopes. • One has a mass of 106.90509 amu (abundance in nature of

51.84%.)• The other has a mass of 108.90476 amu (abundance in

nature of 48.16%.)

106.90509 amu ( 0.5184) = 55.42 amu

108.90476 amu (0.4816) = 52.45 amu

107.87 amu

Molar MassCalculate the Molar Mass of Calcium carbonate

The mass of one mole of a compound. • Calcium carbonate

• Chemical Formula CaCO3

Calcium 40.08 grams X 1 mole = 40.08 grams

Carbon 12.01 grams X 1 mole = 12.01 grams

Oxygen 16.00 grams X 3 moles = 48.00 grams

100.09 grams

Periodic Law• Mendeleev listed the

known elements in order of increasing atomic mass, grouping those with similar properties.

• He noticed that certain similar properties would recur in a periodic fashion.

• His tabulation is the precursor to our modern periodic table.

Mendeleev and the Periodic Table• Proposed that elements would be discovered to fill in gaps in

the table.• Proposed that some measured atomic masses were in error.• Summarized a large number of observations

– The underlying reasons for periodic behavior were as yet unknown

– The Bohr model and the quantum mechanical model were needed to explain the observations.

The Bohr Model• This is a MODEL of the atom that links electron behavior

(microscopic) to the periodic law (macroscopic).

Bohr Diagrams/Electron Configurations

• Based on the behavior of electrons as particles

• Electron orbits– Specified with n, the orbit’s

quantum number• Fixed energies• Fixed radii• Maximum number of electrons

based on n• This model is an

oversimplification (as are most models).

• Atoms with full outer orbits are extremely stable.

• Atoms with outer orbits that are not full are unstable and will undergo chemical reactions attempting to fill the outer orbit.

The Quantum Mechanical Model• Based on the behavior of electrons as waves• Replace Bohr’s orbits with orbitals, a representation of

electron location as the probability of finding it in a certain region of space.

• Orbitals are grouped into shells and fill similarly to Bohr’s orbits.

Determinism and Quantum Mechanics

• Specifics of orbital filling are beyond the scope of this text…BUT– The statistical nature of the quantum mechanical model

rocked the scientific establishment.• An indeterminate universe• Both models are useful even though Bohr’s, by experiment,

has been shown to be invalid.

Families of Elements• Also called groups• Based on outer electron configurations• Vertical columns

– Alkali metals– Alkaline earth metals– Chalcogens– Halogens– Noble gases

Element Families

Metals, Nonmetals, and Metalloids

Molecular Elements• Some elements occur as diatomic molecules.

The Mole Concept• Counting particles as small as atoms is impractical.• The mole concept allows us to relate the mass of a sample

of an element to the number of atoms within it. • The unit of comparison is called a mole and corresponds to

6.022 x 1023.

Avogadro’s Number• Determined such that the

numerical value of the atomic mass of an element in amu is equal to the molar mass of that element in grams per mole.

• These relationships of grams, moles, and atoms can be used as conversion factors.

Chapter SummaryMolecular Concept• Atomic Number and Mass

Number• Isotopes• Molar Mass• Bohr Model• Quantum Mechanical

Model• Orbitals

Societal Impact• Chemical processes that

cause change are caused by changes in atoms or molecules.

• Pollution results from “misplaced” atoms. Atoms that are not where they are supposed to be located.

• Models are used to describe nature.