Transcript Chapter 3 Atoms and Elements Nivaldo J. Tro
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.