Atomic Theory

Atomic Structure & the Periodic Table

ObjectivesObjectives

Identify important developments in the history of atomic theory.Identify important developments in the history of atomic theory. Summarize Dalton’s atomic theory.Summarize Dalton’s atomic theory. Describe the size of an atom.Describe the size of an atom. Distinguish among protons, electrons, and neutrons in terms of relative Distinguish among protons, electrons, and neutrons in terms of relative

mass and change.mass and change. Describe the structure of an atom, including the location of the protons, Describe the structure of an atom, including the location of the protons,

electrons, and neutrons with respect to the nucleus.electrons, and neutrons with respect to the nucleus. Explain how the atomic number identifies an element.Explain how the atomic number identifies an element. Use the atomic number and mass number of an element to find the Use the atomic number and mass number of an element to find the

number of protons, electrons, and neutrons.number of protons, electrons, and neutrons. Explain how isotopes differ and why the atomic masses of elements are Explain how isotopes differ and why the atomic masses of elements are

not whole numbers.not whole numbers. Calculate the average atomic mass of an element from isotope data.Calculate the average atomic mass of an element from isotope data.

TEKS: 2A, 2B, 2C, 2D, 2E, 3A, 3C, 3E, 4A, 4C,4D, 5A, 6A, 6B, 6C, 8A, TEKS: 2A, 2B, 2C, 2D, 2E, 3A, 3C, 3E, 4A, 4C,4D, 5A, 6A, 6B, 6C, 8A, 9B, 10A, 11A9B, 10A, 11A

Early Models of the AtomEarly Models of the Atom

400 B.C. – Democritus proposed the existence of 400 B.C. – Democritus proposed the existence of fundamental particles of matter that were indivisible and fundamental particles of matter that were indivisible and indestructible - “atomos”.indestructible - “atomos”.

Aristotle thought all matter was continuous; he did not believe in atoms.

Neither idea was supported by any experimental evidence – speculation only.

Foundations of Atomic TheoryFoundations of Atomic Theory

The late 1700’s –definitions and basic laws had The late 1700’s –definitions and basic laws had been discovered and accepted by chemists.been discovered and accepted by chemists.

Element – substance that cannot be broken down by Element – substance that cannot be broken down by ordinary chemical means.ordinary chemical means.

Chemical Reaction – transformation of substance or Chemical Reaction – transformation of substance or substances into one or more new substances.substances into one or more new substances.

Law of Conservation of MassLaw of Conservation of Mass – mass cannot be created or – mass cannot be created or destroyed just changed from one form to another. (destroyed just changed from one form to another. (Antoine Antoine Lavosier)Lavosier)

Law of Definite ProportionsLaw of Definite Proportions – a chemical compound contains – a chemical compound contains exactly the same elements in the same proportion regardless of exactly the same elements in the same proportion regardless of sample size. sample size. (Joseph Proust from work of Gay-Lussac & (Joseph Proust from work of Gay-Lussac & Amadeo Avogadro – 1802/1804)Amadeo Avogadro – 1802/1804)

Law of Multiple ProportionsLaw of Multiple Proportions – If two or more different – If two or more different compounds are composed of the same two elements, then the compounds are composed of the same two elements, then the ratio of the masses of those elements will always exist as a ratio ratio of the masses of those elements will always exist as a ratio of small whole numbers. of small whole numbers. (John Dalton - 1808)(John Dalton - 1808)

Dalton’s Atomic TheoryDalton’s Atomic Theory

All elements are composed of tiny indivisible particles All elements are composed of tiny indivisible particles called atoms.called atoms.

Atoms of the same element are identical. The atoms of Atoms of the same element are identical. The atoms of one element are different from the atoms of another one element are different from the atoms of another element.element.

Atoms combine in simple whole-number ratios.Atoms combine in simple whole-number ratios. Atoms are separated, joined or rearranged in chemical Atoms are separated, joined or rearranged in chemical

reactions. Atoms of one element are never changed into reactions. Atoms of one element are never changed into atoms of another element as a result of a chemical atoms of another element as a result of a chemical reaction.reaction.

Discovery of ElectronsDiscovery of Electrons

1897 – J.J. Thomson – “Cathode Ray Tube Experiment”1897 – J.J. Thomson – “Cathode Ray Tube Experiment”

Showed existence of first know sub-atomic particleShowed existence of first know sub-atomic particle

Determined charge to mass ratio of the electronDetermined charge to mass ratio of the electron

1909 – Robert Millikan found the charge of the electron – 1909 – Robert Millikan found the charge of the electron – “Millikan’s Oil Drop Experiment” “Millikan’s Oil Drop Experiment”

Cathode Ray Tube

High Voltage

Cathode Ray (electrons)

Metal disk (cathode)

Metal disk (anode)

Gas at very low pressure

Cathode Ray Tube

High Voltage

Cathode Ray (electrons)

Metal disk (cathode)

Metal disk (anode)

Gas at very low pressure

Negative plate

Positive plate

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment

Rutherford, Geiger & Marsden (1912) -showed that Rutherford, Geiger & Marsden (1912) -showed that most of the atom was empty space, but that atoms most of the atom was empty space, but that atoms had a solid, positive core.had a solid, positive core.

Alpha Particles

Radioactive source

Lead shield

Discovery of ProtonsDiscovery of Protons

1919 -J.J. Thomson & James Chadwick– 1919 -J.J. Thomson & James Chadwick– discovered particles traveling opposite of the discovered particles traveling opposite of the cathode rays.cathode rays.

Determined existence, mass and charge of Determined existence, mass and charge of protonsprotons

Idea had actually been previously proposed by Idea had actually been previously proposed by Goldstein in 1886.Goldstein in 1886.

Cathode Ray Tube

High Voltage

Cathode Ray (electrons)

Metal disk (cathode)

Metal disk (anode)

Gas at very low pressure Negative

plate

Positive plate

protons

NeutronsNeutrons

James Chadwick 1932 - confirmed the existence of the neutron. Neutrons are subatomic James Chadwick 1932 - confirmed the existence of the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.particles with no charge but with a mass nearly equal to that of a proton.

Walter Bothe had first reasoned the existence of a third subatomic particle in 1930.Walter Bothe had first reasoned the existence of a third subatomic particle in 1930.

Bothe’s work was based in part on that of Henry Mosely who showed by X-ray analysis that not Bothe’s work was based in part on that of Henry Mosely who showed by X-ray analysis that not all atoms of the same element were identical. (Isotopes – 1907)all atoms of the same element were identical. (Isotopes – 1907)

Counting ParticlesCounting Particles

Atomic Number = number of protonsAtomic Number = number of protons

Mass Number = number of protons and Mass Number = number of protons and neutronsneutrons

Atomic Mass = average mass of the Atomic Mass = average mass of the isotopesisotopes

(also known as atomic weight)(also known as atomic weight)

Periodic TablePeriodic Table

8

15.999

atomic number # of protons

round to 16 - mass number ( # protons & neutrons)unrounded –mass number (average mass of the isotopes)

mass number

-atomic number

# of neutrons

O

Masses of AtomsMasses of Atoms

A scale designed for atoms gives their small atomic A scale designed for atoms gives their small atomic

masses in atomic mass units (amu)masses in atomic mass units (amu)

An atom of An atom of 1212C was assigned an exact mass of 12.00 C was assigned an exact mass of 12.00

amuamu

Relative masses of all other atoms was determined Relative masses of all other atoms was determined

by comparing each to the mass of by comparing each to the mass of 1212C C

An atom twice as heavy has a mass of 24.00 amu. An atom twice as heavy has a mass of 24.00 amu.

An atom half as heavy is 6.00 amu.An atom half as heavy is 6.00 amu.

Atomic MassAtomic Mass

Listed on the periodic tableListed on the periodic table

Gives the mass of “average” atom of each element Gives the mass of “average” atom of each element compared to compared to 1212C C

Average atom based on all the Average atom based on all the isotopes and their abundance %.isotopes and their abundance %.

Atomic mass is not a whole number Atomic mass is not a whole number duedue toto isotopesisotopes..

Na22.99

IsotopesIsotopes

Isotopes – atoms of the same element with different Isotopes – atoms of the same element with different numbers of neutrons.numbers of neutrons.

Oxygen-16Oxygen-16 Oxygen-17Oxygen-17 Oxygen-18Oxygen-18

1616 17 17 18 18

88 8 8 8 8

pp+ ‗‗‗‗+ ‗‗‗‗ ‗‗‗‗ ‗‗‗‗ ‗‗‗‗ ‗‗‗‗

ee- ‗‗‗‗ ‗‗‗‗- ‗‗‗‗ ‗‗‗‗ ‗‗‗‗ ‗‗‗‗

nnºº ‗‗‗‗‗‗‗‗ ‗‗‗‗ ‗‗‗‗ ‗‗‗‗ ‗‗‗‗

Calculating Average Atomic MassCalculating Average Atomic Mass

Percent(%) abundance of isotopesPercent(%) abundance of isotopes

Mass of each isotope of that elementMass of each isotope of that element

Weighted average =Weighted average =

mass isotopemass isotope11(%)(%) + + mass isotopemass isotope22(%)(%) + … + …

100 100 100 100

Atomic Mass of MagnesiumAtomic Mass of Magnesium

Isotopes Isotopes Mass of Isotope Mass of Isotope Abundance Abundance 2424Mg Mg == 24.0 amu 24.0 amu 78.70% 78.70%

2525Mg Mg = = 25.0 amu 25.0 amu 10.13% 10.13%

2626Mg Mg = = 26.0 amu26.0 amu 11.17% 11.17%

Atomic mass (average mass) Mg = 24.3 amuAtomic mass (average mass) Mg = 24.3 amu

Mg24.3

#16 The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for mass = 63.0 amu, and 30.8% for mass = 65.0 amu. Calculate the average atomic mass of copper.

Finding An Isotopic MassFinding An Isotopic Mass

Naturally occurring boron is 80.20% boron-11 (atomic mass 11.0 amu) and 19.80% of a different isotope of boron. What must the mass of this isotope be if the average atomic mass of boron is 10.81 amu?

RadioactivityRadioactivity

Mosely’s X-ray analysis of atoms was an attempt to Mosely’s X-ray analysis of atoms was an attempt to explain radioactivity.explain radioactivity.

1896 – Henri Becquerel – Uranium spontaneously emits 1896 – Henri Becquerel – Uranium spontaneously emits energy.energy.

1898 – Marie & Pierre Curie – first isolated a radioactive 1898 – Marie & Pierre Curie – first isolated a radioactive element - Radiumelement - Radium

Properties of Subatomic ParticlesProperties of Subatomic Particles

Particles Symbol Charge Relative Particles Symbol Charge Relative Mass Mass MassMassElectronElectron ee-- 1- 1- 1/1840 amu1/1840 amu 9.11 x 109.11 x 10-28-28 g g

Proton pProton p++ 1+ 1 amu 1+ 1 amu 1.67 x 101.67 x 10-24-24 g g

NeutronNeutron nnºº 0 1 amu 0 1 amu 1.67 x 10 1.67 x 10-24-24 g g

““Planetary” Model of the AtomPlanetary” Model of the Atom

Niels Bohr (1913) – developed the “planetary” model of Niels Bohr (1913) – developed the “planetary” model of the atom based upon the following: the atom based upon the following:

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment

E = mcE = mc2 2 – Albert Einstein (1905)– Albert Einstein (1905)

Quantum Theory – Max Planck (1910)Quantum Theory – Max Planck (1910)

AtomAtom

1010-13 -13 cmcm electronselectrons

protonsprotons neutronsneutrons

1010-8 -8 cmcm

nucleus

Size of the AtomSize of the Atom

Texas Memorial Stadium @ UT

e-

Aluminum Atom

e-

e-

e-

e-

e- e-

e-

e-

nucleus - size of a marble

1 mm

Outside edge of Al atom

150 me-

goal post

stands