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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

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Page 1: 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

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

Page 2: 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

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.

Page 3: 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

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.

Page 4: 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

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

Dalton's Postulates

Each element is composed of extremely small particles called atoms.

Page 5: 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

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.

Page 6: 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

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.

Page 7: 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

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.

Page 8: 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

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.http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#

Chem Talk

Page 9: 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

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).

Page 10: 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

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.

Page 11: 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

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.

Page 12: 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

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

Page 13: 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

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.

Page 14: 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

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.

Page 15: 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

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

Other Subatomic Particles

• Protons were discovered by Rutherford in 1919.

• Neutrons were discovered by James Chadwick in 1932.

Page 16: 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

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.

Page 17: 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

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

Radioactivity

• Three types of radiation were discovered by Ernest Rutherford: particles particles rays

Page 18: 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

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.

Page 19: 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

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.

Page 20: 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

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.

Page 21: 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

Atoms,Molecules,and Ions

Law vs. TheoryLaw vs. Theory

A A lawlaw summarizes what summarizes what happens happens

A A theorytheory (model) is an attempt (model) is an attempt to explain to explain whywhy it happens it happens

Chem Talk

Page 22: 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

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.

Page 23: 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

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

Symbols of Elements

Elements are symbolized by one or two letters.

Page 24: 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

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

Page 25: 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

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.

ElementElement # of protons# of protons Atomic # (Z)Atomic # (Z)

CarbonCarbon 66 66

PhosphorusPhosphorus 1515 1515

GoldGold 7979 7979

Page 26: 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

Atoms,Molecules,and Ions

Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0

NuclideNuclide pp++ nn00 ee-- MassMass ##

Oxygen Oxygen - - 1010

- - 3333 4242

- - 3131 1515

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

Page 27: 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

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.

Page 28: 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

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.

Page 29: 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

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

Atomic Mass

Atomic and molecular masses can be measured with great accuracy using a mass spectrometer.

Page 30: 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

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.

Page 31: 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

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

Page 32: 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

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.

Page 33: 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

Atoms,Molecules,and Ions

Atomic Masses

IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus

% in nature% in nature

Carbon-12Carbon-12 1212CC 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

Carbon-13Carbon-13 1313CC 6 protons6 protons

7 neutrons7 neutrons

1.11%1.11%

Carbon-14Carbon-14 1414CC 6 protons6 protons

8 neutrons8 neutrons

<0.01%<0.01%

Atomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011

Page 34: 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

Atoms,Molecules,and Ions

Average Atomic mass

Av. Atomic Mass = (mass of 1st isostope x % abundance)+ (mass of 2nd isotope x % abundance ) + (mass of 3rd isotope x % abundance) + ….

(depending on how many isotopes )

% abundance to be used as decimal percent.

© 2012 Pearson Education, Inc.

Page 35: 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

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.

Page 36: 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

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.

Page 37: 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

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.

Page 38: 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

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

Groups

These five groups are known by their names.

Page 39: 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

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).

Page 40: 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

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

Periodic Table

Metalloids border the stair-step line (with the exception of Al, Po, and At).

Page 41: 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

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

Periodic Table

Metals are on the left side of the chart.

Page 42: 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

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.

Page 43: 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

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

Chemical Formulas

Molecular compounds are composed of molecules and almost always contain only nonmetals.

Page 44: 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

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


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