ATOM MODEL

RESHMA UNNITHAN

PHYSICAL SCIENCE

John Dalton (Early 1800’s)

Proposed an atomic theory that included the following:

• Each element is composed of extremely small particles called atoms.

• All atoms of a given element are identical.• Atoms of one element are different from atoms of

another element.• Atoms of an element are not changed into different

types of atoms by chemical reactions.• Compounds are formed when atoms of more than

one kind combine.

Developing an Atomic Theory

• Many scientists have modified and elaborated on Dalton’s Atomic Theory.

• The first major advances were possible with the development of gas discharge tubes.

• Sir William Crookes was a leader in experiments with gas discharge tubes.

Sir William Crookes (mid to late 1800’s)

Evidence:

• When a high voltage supply was attached to the ends of the tube, there was a glow at the positive end of the tube.

• When the negative terminal was moved the glow moved with it, but not when the positive terminal was moved.

• If a piece of metal was placed between the negative and positive terminals, a shadow was produced in the glow. The shadow had an identical shape to the metal.

Sir William Crookes (mid to late 1800’s)

Evidence:

• When a high voltage supply was attached to the ends of the tube, there was a glow at the positive end of the tube.

• When the negative terminal was moved the glow moved with it, but not when the positive terminal was moved.

• If a piece of metal was placed between the negative and positive terminals, a shadow was produced in the glow. The shadow had an identical shape to the metal.

Cathode Rays

• Something was produced at the negative terminal (cathode) and traveled towards the positive terminal (anode).

• Whatever was being produced at the cathode traveled in straight lines towards the anode and was not able to pass through the metal.

Claim:

Cathode Rays

Reasoning:• The cathode was the source of the rays because the

rays moved every time the cathode was moved.• The rays must travel in straight lines because the

shape of the shadow was identical to the shape of the object that produced it.

Because it was coming from the cathode and it traveled in straight lines, this phenomenon was called a cathode ray.

Sir J.J. Thomson

Sir J.J. Thomson (Late 1800’s)

Evidence:

• When cathode rays traveled through an electric field they were repelled from the negative side of the field and attracted to the positive side.

• From measuring the deflection of the cathode rays in combinations of electric and magnetic fields, Thomson was able to determine that cathode rays had charge and mass.

• The same results were obtained no matter what the cathode was made from.

Electrons

• Cathode rays were composed of a stream of negatively charged particles.

• These particles were fundamental to all atoms.

Claim:

Electrons

Reasoning: • The particles were negative because they

were repelled by negative charges and attracted towards positive charges.

• The particles were fundamental to all atoms because the same results were found no matter what material was used to produce the electrons.

Electrons

These particles were given the name electrons.

Later Robert Millikan, in the USA, determined the charge on an electron.

His results, when combined with Thomson’s, allowed scientists to calculate the mass of an electron.

Thomson’s Model of the Atom

Evidence:• Thomson had determined that atoms

contained negative particles called electrons.

• However, atoms were electrically neutral.

Thomson’s “Plum Pudding” Model

Thomson suggested that the electrons in an atom were embedded in a positively charged, diffuse sphere.

(Early 20th century)Claim:

Atoms contain an equal amount of positive charge as negative charge

Thomson’s “Plum Pudding” Model

Reasoning:

• Since atoms were electrically neutral but contained negatively charged electrons, they must also contain an equal amount of positive charge.

• Thomson had no experimental evidence for his model but built on Lord Kelvin’s idea that positive charge in an atom was spread evenly and diffusely throughout a spherical shape.

(Early 20th century)

Rutherford, Geiger, and Marsden(Early 20th Century)

Positively charged alpha-particles were directed at a piece of thin gold foil.

Rutherford

• Most of the alpha particles passed through the gold foil, but some were deflected at varying angles.

Evidence:

The Nuclear Model of the Atom

• The positive charges in an atom are not diffuse and instead must be concentrated into a small space.•Most of an atom is empty space.

Claim:

The Nuclear Model of the Atom

Reasoning:• Most of the alpha particles were able to pass through the atoms

with little or no deflection, therefore the atom was mainly empty space.

• There must be a concentration of positive charge in order to cause the alpha particles to be deflected by large angles.

• This concentration of positive charge must be very small or more alpha particles would have been deflected by large angles.

The region of concentration of positive charge in an atom was called the nucleus.

The positively charged particles in the nucleus were called protons.

Expanding the Nuclear Model of the Atom

• Atoms are electrically neutral.

• The mass of an atom is greater than the sum of the mass of its protons and electrons.

Evidence:

Sir James Chadwick (1932)

• There must be other particles in the atom.

• These particles must have mass but no charge.

Claim:

Sir James Chadwick (1932)

Reasoning:• The mass of an atom is greater than the mass of

its protons and electrons.Therefore there must be another particle in the atom that has mass.

• This additional particle must not have any charge, because if it had charge then the atom would no longer be electrically neutral.

In 1932, James Chadwick confirmed the existence of these particles, called neutrons.

Summary of the Nuclear Model of the Atom

• Atoms contain protons, neutrons, and electrons.• Protons are positive, electrons are negative, neutrons

have no charge.• Protons and neutrons contain most of the mass of an

atom.• Protons and neutrons are located in the nucleus,

which is very small.• Electrons are located outside the nucleus.