Nivaldo J. Tro http://academic.cengage.com/chemistry/tro

Chapter 3 Atoms and Elements

Deborah Koeck • Texas State University, San Marcos

Understanding the Macroscopic World

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

Calculate 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 Mass

Calculate the Molar Mass of Calcium carbonate The mass of one mole of a compound. • Calcium carbonate • Chemical Formula  CaCO 3 Calcium 40.08 grams X 1 mole = 40.08 grams Carbon Oxygen 12.01 grams X 1 mole = 12.01 grams 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

periodic table.

fashion.

• His tabulation is the precursor to our modern

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 6.022 x 10 23 .

impractical

of an element to the number of atoms within it. .

• The mole concept allows us to relate the mass of a sample • The unit of comparison is called a mole and corresponds to

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 Summary

Molecular 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.