Atomic Theory Development

What is Today’s Model?Dense,

Positively Charged Nucleus

MostlyEmptySpace

Negatively Charged

Electron Cloud

Most Probable Location of the

Electrons

Composed of Protons, Neutrons, and

Electrons

Timeline of Development of Current Atomic Model

1913450 BC

Democritus proposed the idea of atomos.

1802

Beginning of

Modern Atomic Theory

1897

Discovery of the Electron

1911

Discovery of the Nucleus

The Idea of Energy Levels for Electrons

was Proposed

.

1930

Introduction of the wave mechanical

model

Discovery of the Proton

Discovery of the Neutron

Early Greeks

Matter is made of indestructible particles called “atomos”

Democritus (400 BC)

Summary for Dalton’s Atomic Theory

(Father of the Modern Atomic Theory)

All atoms of a single element have the same mass

Atoms of different elements are different.

Atoms can’t be divided, created or destroyed.

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

Discovery of the ElectronIn 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.Cathode Ray

Crookes Tube

J.J. Thomson• He proved that atoms of

any element can be made to emit tiny negative particles.

• From this he concluded that ALL atoms must contain these negative particles.

• He knew that atoms did not have a net negative charge and so there must be something positive that balances the negative charge.

J.J. Thomson

William Thomson’s (Sir Kelvin) Atomic Model (1910)

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

Ernest Rutherford’s (1871-1937)

electrons embedded in a positive pudding.

Where exactly are those electrons?

Thomson’s Theory: “Plum Pudding”

Shoot something at them to see where they are.

Rutherford’s idea:

Rutherford’s has an idea…

What if I shoot alpha radiation at gold atoms in gold foil?

Discovery of the nucleus

Lead block

Uranium

Gold Foil

Flourescent Screen

He Expected

• The alpha particles to pass through without changing direction very much.

• Because…• The positive charges were spread out evenly.

Alone they were not enough to stop the alpha particles.

What he expected

Because

Because, he thought the mass was evenly distributed in the atom

Because, he thought the mass was evenly distributed in the atom

What he got

How he explained it

+

• Atom is mostly empty.• Small dense,

positive piece at center.

• Alpha particles are deflected by it if they get close enough.

+

Rutherford’s Conclusion (1911)…

Small, dense, positive nucleus.

Equal amounts of (-) electrons at large distances outside the nucleus.

Neils Bohr’s Atomic model (1913)

Small, dense, positive nucleus.Equal amounts of (-) electrons at specific orbits around the nucleus.

This incorrect version of the atom is often used to represented atoms because it shows energy levels for electrons.

And now we know of many other subatomic particles:

Chadwick

** James Chadwick discovered neutrons in 1932.

--

-- n0 have no chargeand are hard to detect purpose of n0 = stability of nucleus

quarks,muons,positrons,neutrinos, pions, etc.

photo from liquid H2 bubble chamber

Quantum Mechanical Model-electron cloud model--charge cloud model-

Schroedinger, Pauli, Heisenberg, Dirac (up to 1940):According to the QMM, we never know for certain where the e– are in an atom, but the equations of the QMM tell us the probability that we will find an electron at a certain distance from the nucleus.

Quantum Mechanical Model

Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

Modern Atomic Theory

Atoms of the same element are chemically alike with a characteristic average mass which is unique to that element.

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

All matter is composed of atoms.

Atoms of any one element differ in properties from atoms of another element

The exact path of electrons are unknown and e-’s are found in the electron cloud.

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

“q” is a particle called a “quark”

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 togetherby “gluons”

Size of an atom

• Atoms are incredibly tiny.• Measured in picometers (10-12 meters)

– Hydrogen atom, 32 pm radius• Nucleus tiny compared to atom

– Radius of the nucleus near 10-15 m.– Density near 1014 g/cm3

• IF the atom was the size of a stadium, the nucleus would be the size of a marble.

California WEB

Models of the Atom

Dalton’s model (1803) Thomson’s plum-pudding

model (1897)Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

Greek model(400 B.C.)

1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945

1803 John Dalton pictures atoms astiny, indestructible particles, with no internal structure.

1897 J.J. Thomson, a Britishscientist, discovers the electron,leading to his "plum-pudding" model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 Hantaro Nagaoka, aJapanese physicist, suggests that an atom has a centralnucleus. Electrons move in orbits like the rings around Saturn.

1911 New Zealander Ernest Rutherford statesthat an atom has a dense,positively charged nucleus. Electrons move randomly in the space around the nucleus.

1913 In Niels Bohr'smodel, the electrons move in spherical orbits at fixed distances from the nucleus.

1924 Frenchman Louis de Broglie proposes thatmoving particles like electronshave some properties of waves. Within a few years evidence is collected to support his idea.

1926 Erwin Schrödinger develops mathematicalequations to describe the motion of electrons in atoms. His work leads to the electron cloud model.

1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

Match The Models

Billiard Ball

Plum Pudding Nucleus NeutronsEnergy

Levels