II. The Structure of Atoms

Atom – smallest particle of an element that retains the chemical properties of that element. All atoms consist of 2 regions – the nucleus (p+ & n) and surrounding the nucleus is the electron cloud – a region occupied by the negatively charged particles called electrons. How do we know this?!

Thomson’s Plum Pudding Model

• In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

Thomson Model

• Thomson studied the passage of an electric current through a gas.

• As the current passed through the gas, it gave off rays of negatively charged particles.

Source ofElectricalPotential

Metal Plate

Gas-filledglass tube Metal plate

Stream of negativeparticles (electrons)

A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

J. J. Thomson - English physicist. 1897

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.

But he could never find them.

• Plum Pudding Model – Thompson proposed that the atom had negative electrons scattered throughout a positively charged area (proton area).

Rutherford’s Gold Foil Experiment

• In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure.

• Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

Rutherford’s Gold Foil Experiment

beam of alpha particles

radioactive substance

gold foil

circular ZnS - coated

fluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.

What he expected…

What he got…richochetingalpha particles

Interpreting the Observed Deflections

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

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gold foil

deflected particle

undeflected particles

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Explanation of Alpha-Scattering Results

Plum-pudding atom

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Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Rutherford

• Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

4. Neutrons 1932 (Proved by Chadwick)

Knew something else existed in an atom because of the mass of the atom. Neutron is an electrically neutral particle, symbol n, mass equal that of protons.

Bohr Model• In 1913, the Danish

scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.

Bohr Model• According to Bohr’s

atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

Neils Bohr- 1913

•   This planetary model allowed for the idea that electrons can become "excited" and move to higher energy levels for brief periods of time. 

Bohr model of theAtom.

Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

The Electron Cloud Model• Louis de Broglie suggested that particles might

have properties of waves.  The result of this investigation is sometimes called the wave-particle duality of nature.  This duality, which states that particles act like waves and waves like particles, applies to all waves and all particles.  However, the more massive the particles, the less obvious the wave properties.  Electrons, having very little mass, exhibit significant wave-like properties.

Werner Heisenburg

• Pointed out that it is impossible to know both the exact position and the exact momentum of an object at the same time.  Applying this concept to the electron we realize that in order to get a fix on an electron's position at any time, we would alter its momentum.  Any attempt to study the velocity of an electron will alter its position.  This concept, called the Heisenberg Uncertainty principle, effectively destroys the idea of electrons traveling around in neat orbits. 

Erwin Schrödinger• He developed the probability function for the

Hydrogen atom (and a few others).  The probability function (Schrodinger’s Wave Function) basically describes a cloud-like region where the electron is likely to be found.  It can not say with any certainty, where the electron actually is at any point in time, yet can describe where it ought to be.   Clarity through fuzziness

Schrodinger’s Cat

• Here's Schrödinger's (theoretical) experiment: We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of hydrocyanic acid, a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat. 

• The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, according to quantum law, the cat is both dead and alive, in what is called a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.)

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).

Niels Bohr &Albert Einstein