Atoms

Democritus Democritus believed :- “all matter consists of extremely small particles that could not be divided Particles called “atoms” from the Greek word “atomos”, meaning “indivisible” or uncut

Aristotle Aristotle thought that all substances were made of only four elements — earth, air, fire, and water. He did not think there was a limit to the division of matter. For many centuries, most people accepted Aristotle’s views on the structure of matter. By the 1800s, scientists had enough experimental data to support an atomic model.

Dalton’s Atomic Theory Evidence for Atoms  John Dalton studied the behavior of gases in air.  Concluded gas consists of individual particles.  Measured masses of elements that combine when compounds form.  Observed ratio of the masses of the elements in each compound was always the same.  So, compounds must have a fixed composition.

Dalton’s Model (1803) All elements composed of indivisible particles Atoms of the same element are exactly alike Atoms of different elements are not alike Compounds are formed by joining atoms of two or more elements

When some materials are rubbed, they gain the ability to attract or repel other materials. M aterials are said to have either a positive or a negative electric charge. Objects with like charges repel, or push apart. Objects with opposite charges attract, or pull together. Thomson’s Model of the Atom

Thomson’s Experiments – J. J. Thomson used a sealed tube containing a very small amount of gas. Thomson’s Model of the Atom Sealed tube filled with gas at low pressure Glowing beam Metal disk Source of electric current Metal disk Source of electric current

Thomson’s Experiments – Beam deflected due to applied field from plates, so beam must be negatively charged. Thomson’s Model of the Atom Sealed tube filled with gas at low pressure Glowing beam Metal disk Source of electric current Metal disk Source of electric current Positive plate Negative plate

JJ Thomson (1897) Atom consists of positively charged particles and negatively charged particles Atom could be divided Atom is neutral

Thomson’s model is called the “plum pudding” model. Today, it might be called the “chocolate chip ice cream” model. “Chips” = negatively charged particles, spread evenly “Vanilla ice cream”= positively charged matter Thomson’s Model of the Atom

Rutherford’s Hypothesis Ernest Rutherford designed an experiment to find out what happens to alpha particles when they pass through a thin sheet of gold. Alpha particles are fast-moving, positively charged particles. Based on Thomson’s model, Rutherford hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle. He predicted that most particles would travel in a straight path from their source to a screen that lit up when struck. Rutherford’s Atomic Theory

The Gold Foil Experiment Rutherford’s Atomic Theory Beam of alpha particles Source of alpha particles Slit Deflected particle Undeflected particle Screen Gold atoms Alpha particles Nucleus

Discovery of the Nucleus The alpha particles whose paths were deflected must have come close to another charged object. The closer they came, the greater the deflection. Many alpha particles passed through the gold without being deflected. These particles did not pass close to a charged object. Rutherford’s Atomic Theory

Rutherford Model ( 1907) Atom has a nucleus that contains massive particles A nucleus is very small e.g. size can be compared to a bee in a stadium

Rutherford proposed a new model. The positive charge of an atom is not evenly spread throughout the atom. Positive charge is concentrated in a very small, central area. The nucleus of the atom is a dense, positively charged mass located in the center of the atom. Rutherford’s Atomic Theory

If an atom is blown up to the size of the Houston Astrodome then, a MARBLE would represent the size of the nucleus. Volume of an atom is trillion (10 12 ) times the volume of its nucleus.

Write one paragraph Thompson’s Experiment and Atomic Theory Write one paragraph on Rutherford’s Experiment and Atomic Theory Turn this in before you leave class!!