Atomic Structure Section 4-1

Democritus Greek philosopher 4 th Century BC First to come up with “atom”. Matter is composed of tiny particles called atoms. These atoms are invisible, indestructible fundamental units of matter.

John Dalton English ( ) School teacher. Studied the ratios in which elements combine in chemical reactions. Formulated first modern Atomic Theory.

Dalton’s Atomic Theory 1.All elements are composed of indivisible particles called atoms which cannot be divided. 2.Atoms of the same element have the same mass. Atoms of different elements have different masses.

Dalton’s Atomic Theory 3.Compounds contain atoms of more than one element. 4.In a particular compound, atoms of different elements always combine in the same way.

Dalton’s Atomic Model Based on Dalton’s theory, each type of atom is represented by a tiny, solid, sphere with a different mass. Wooden spheres made by Dalton

Atomic Theory Much of Dalton’s theory still holds true today However, we now know that atoms are not indivisible (atoms CAN be divided)

Joseph John (JJ) Thomson ( ) English 1897 discovered the electron while experimenting with cathode rays. Thomson proved that the rays consisted of charged particles. This was the first evidence that atoms are made of smaller particles. This discovery revised Dalton’s model of the atom proving it was divisible.

JJ Thomson with the CRT

Thomson’s Discovery Thomson found that cathode rays were attracted to metal plates with a positive electrical charge. Plates with a negative charge repel the ray. Knowing that like charges repel and opposite charges attract, he determined that the ray was made of negatively charged particles – electrons.

Deflection of Cathode Ray

CATHODE RAY Thompson showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes. He concluded that these particles were part of every atom.

Applications of the Cathode Ray Tube Televisions Computer Monitors

Thomson’s Atomic Model An atom was believed to be a sphere of uniform positive charge into which the negatively charged electrons were embedded. This model was like a plum pudding dotted with raisins and was thus called the 'plum pudding' model of the atom. The evenly spread out “negative charges” balance out the “positive charges”.

Ernest Rutherford ( )England Tested Thomson’s theory of atomic structure with the “gold foil” experiment in 1910.

The Gold Foil Experiment Bombarded thin gold foil with a beam of ‘alpha’ particles. If the positive charge was evenly spread out, the beam should have easily passed through.

Rutherford's Experiment Expected Found

Rutherford's Experiment Rutherford and coworkers aimed a beam of alpha particles at a sheet of gold foil surrounded by a florescent screen.

Rutherford's Experiment Most particles passed through with no deflection, while some were highly deflected. Rutherford concluded that most particles passed through because the atom is mostly empty space.

Rutherford’s Conclusions All of the positive charge, and most of the mass of an atom are concentrated in a small core. This region would have a positive charge capable of causing the deflection observed. This region is called the nucleus.

How big is the nucleus? If the Houston Astrodome were a model for an atom, a marble at its center could represent the nucleus.

Review what you learned … What was Dalton’s theory about the atom? What did Dalton’s model of the atom look like? Which part of Dalton's theory was later revised? What subatomic particle did J.J. Thomson discover?

Review what you learned … What was the name of the device J.J, Thomson used in his experiment? What was the charge of the beam in Thomson’s experiment? What was Thomson’s model called?

Review what you learned … What was the name of Rutherford’s experiment? What was the difference in the expected results versus what actually occurred? During Rutherford’s experiment why were some particles deflected and bounce back?

Review what you learned … What subatomic particle did Rutherford discover? According to Rutherford where is all of the positive charge of an atom located?