Atoms, Molecules, and Ions Defining the Atom The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) –He believed that atoms were indivisible and indestructible –His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early nineteenth century, championed by John Dalton.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Dalton's Postulates All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Dalton's Postulates Each element is composed of extremely small particles called atoms.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Dalton's Postulates Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Dalton's Postulates Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Law of Conservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

Atoms, Molecules, and Ions Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. hill.com/sites/ /student_view0/chapter2/animations_ce nter.html# Chem Talk

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. The Electron Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence. J. J. Thomson is credited with their discovery (1897).

Atoms, Molecules, and Ions Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. The Atom, circa 1900 The prevailing theory was that of the “plum pudding” model, put forward by Thomson. It featured a positive sphere of matter with negative electrons imbedded in it.

Atoms, Molecules, and Ions Conclusions from the Study of the Electron  Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.  Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons  Electrons have so little mass that atoms must contain other particles that account for most of the mass  Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.  Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons  Electrons have so little mass that atoms must contain other particles that account for most of the mass

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Millikan Oil-Drop Experiment Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Millikan Oil-Drop Experiment Robert Millikan (University of Chicago) determined the charge on the electron in 1909.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Other Subatomic Particles Protons were discovered by Rutherford in Neutrons were discovered by James Chadwick in 1932.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Radioactivity Radioactivity is the spontaneous emission of radiation by an atom. It was first observed by Henri Becquerel. Marie and Pierre Curie also studied it.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Radioactivity Three types of radiation were discovered by Ernest Rutherford: –  particles –  particles –  rays

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Discovery of the Nucleus Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. The Nuclear Atom Since some particles were deflected at large angles, Thomson’s model could not be correct.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. The Nuclear Atom Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume of the atom is empty space.

Atoms, Molecules, and Ions Law vs. Theory A law summarizes what happens  A law summarizes what happens  A theory (model) is an attempt to explain why it happens Chem Talk

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Subatomic Particles Protons and electrons are the only particles that have a charge. Protons and neutrons have essentially the same mass. The mass of an electron is so small we ignore it.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Symbols of Elements Elements are symbolized by one or two letters.

Atoms, Molecules, and Ions Atomic Number Atoms are composed of identical protons, neutrons, and electrons –How then are atoms of one element different from another element? Elements are different because they contain different numbers of PROTONS The “atomic number” of an element is the number of protons in the nucleus # protons in an atom = # electrons

Atoms, Molecules, and Ions Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon66 Phosphorus1515 Gold7979

Atoms, Molecules, and Ions Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p + + n 0 Nuclide p+p+p+p+ n0n0n0n0 e-e-e-e- Mass # Oxygen Arsenic Phosphorus

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Symbols of Elements All atoms of the same element have the same number of protons, which is called the atomic number, Z.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Isotopes Isotopes are atoms of the same element with different masses. Isotopes have different numbers of neutrons.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Atomic Mass Atomic and molecular masses can be measured with great accuracy using a mass spectrometer.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Average Mass Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations. Average mass is calculated from the isotopes of an element weighted by their relative abundances.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc.

Atoms, Molecules, and Ions Measuring Atomic Mass Instead of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom. –Carbon-12 chosen because of its isotope purity. Each isotope has its own atomic mass, thus we determine the average from percent abundance.

Atoms, Molecules, and Ions Atomic MassesIsotopeSymbol Composition of the nucleus % in nature Carbon C 6 protons 6 neutrons 98.89% Carbon C 6 protons 7 neutrons 1.11% Carbon C 6 protons 8 neutrons <0.01% Atomic mass is the average of all the naturally occurring isotopes of that element. Carbon =

Atoms, Molecules, and Ions Average Atomic mass Av. Atomic Mass = (mass of 1 st isostope x % abundance)+ (mass of 2 nd isotope x % abundance ) + (mass of 3 rd isotope x % abundance) + …. (depending on how many isotopes ) % abundance to be used as decimal percent. © 2012 Pearson Education, Inc.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Periodic Table The periodic table is a systematic catalog of the elements. Elements are arranged in order of atomic number.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Periodic Table The rows on the periodic chart are periods. Columns are groups. Elements in the same group have similar chemical properties.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Periodicity When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Groups These five groups are known by their names.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Periodic Table Nonmetals are on the right side of the periodic table (with the exception of H).

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Periodic Table Metalloids border the stair-step line (with the exception of Al, Po, and At).

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Periodic Table Metals are on the left side of the chart.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Chemical Formulas The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Chemical Formulas Molecular compounds are composed of molecules and almost always contain only nonmetals.

Atoms, Molecules, and Ions © 2012 Pearson Education, Inc. Diatomic Molecules These seven elements occur naturally as molecules containing two atoms: –Hydrogen –Nitrogen –Oxygen –Fluorine –Chlorine –Bromine –Iodine