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Atomic Structure. Image courtesy of B.C. 400 B.C. Demokritos and Leucippos use the term "atomos” . Chemistry Timeline #1.  2000 years of Alchemy . 1500's Georg Bauer: systematic metallurgy Paracelsus: medicinal application of minerals. 1600's

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Atomic structure

Atomic Structure

Image courtesy of

Chemistry timeline 1


400 B.C. Demokritos and Leucipposuse the term "atomos”

Chemistry Timeline #1

2000 years of Alchemy

  • 1500's

  • Georg Bauer: systematic metallurgy

  • Paracelsus: medicinal application of minerals


Robert Boyle:The Skeptical Chemist.Quantitative experimentation, identification of


  • 1700s'

  • Georg Stahl: Phlogiston Theory

  • Joseph Priestly: Discovery of oxygen

  • Antoine Lavoisier: The role of oxygen in combustion, law of conservation of

    mass, first modern chemistry textbook

Chemistry timeline 2
Chemistry Timeline #2

  • 1800's

  • Joseph Proust: The law of definite proportion (composition)

  • John Dalton: The Atomic Theory, The law of multiple proportions

  • Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic molecules

  • Amadeo Avogadro: Molar volumes of gases

  • Jons Jakob Berzelius: Relative atomic masses,modern symbols for the elements

  • Dmitri Mendeleyev: The periodic table

  • J.J. Thomson: discovery of the electron

  • Henri Becquerel: Discovery of radioactivity

  • 1900's

  • Robert Millikan: Charge and mass of the electron

  • Ernest Rutherford: Existence of the nucleus, and its relative size

  • Meitner & Fermi: Sustained nuclear fission

  • Ernest Lawrence: The cyclotron and trans-uranium elements

Dalton s atomic theory 1808
Dalton’s Atomic Theory (1808)

  • All matter is composed of extremely small particles called atoms

  • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

John Dalton

  • Atoms cannot be subdivided, created, or destroyed

  • Atoms of different elements combine in simple whole-number ratios to form chemical compounds

  • In chemical reactions, atoms are combined, separated, or rearranged

Modern atomic theory
Modern Atomic Theory

Several changes have been made to Dalton’s theory.

Dalton said:

Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

Modern theory states:

Atoms of an element have a characteristic average mass which is unique to that element.

Modern atomic theory 2
Modern Atomic Theory #2

Dalton said:

Atoms cannot be subdivided, created, or destroyed

Modern theory states:

Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions

Discovery of the electron
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.

Thomson s atomic model
Thomson’s Atomic Model

J.J. Thomson

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Mass of the electron
Mass of the Electron

1909 – Robert Millikan determines the mass of the electron.

Mass of the electron is

9.109 x 10-31 kg

The oil drop apparatus

Conclusions from the study of the electron
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

Rutherford s gold foil experiment
Rutherford’s Gold Foil Experiment

  • Alpha particles are helium nuclei

  • Particles were fired at a thin sheet of gold foil

  • Particle hits on the detecting screen (film) are recorded

Try it yourself
Try it Yourself!

In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The answers
The Answers

Target #1

Target #2

Rutherford s findings
Rutherford’s Findings

  • Most of the particles passed right through

  • A few particles were deflected

  • VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”


  • The nucleus is small

  • The nucleus is dense

  • The nucleus is positively charged

The atomic scale
The Atomic Scale


  • Most of the mass of the atom is in the nucleus (protons and neutrons)

  • Electrons are found outside of the nucleus (the electron cloud)

  • Most of the volume of the atom is empty space

Image: User YzmoWikimedia Commons.

About quarks
About Quarks…

Protons and neutrons are NOT fundamental particles.

Protons are made of two “up” quarks and one “down” quark.

Neutrons are made of one “up” quark and two “down” quarks.

Quarks are held together by “gluons”

Images: Arpad Horvath, Wikimedia Commons.


Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

Atomic masses
Atomic Masses

Atomic mass is the weighted averageof all the naturally isotopes of that element.

Carbon = 12.011

Atomic number
Atomic Number

Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Mass number
Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0