4.1 The Development of the Atomic Theory Chapter 4 Atoms 4.1 The Development of the Atomic Theory                                               

Atoms are everywhere! Aluminum containers are lightweight because of the properties of the Al atoms that make them up. Nanotechnology (making products that are atom-size) is being used to make micro-submarines which will eventually be able to travel in our bodies to detect health problems.

U decide…Fact or Fiction An atom cannot be broken down into smaller parts. An atom has the same charges throughout. Atoms are made up of several different particles.

Democritus Beginning of “The Atomic Theory” (~2000 years ago) Believed all matter consisted of extremely small particles that could not be divided. Called these particles atoms. Problem  no evidence / data to support his theory                      

Dalton

Dalton’s Atomic Theory Like Democritus, proposed atoms could not be divided. All atoms of a particular element are exactly alike, and atoms of different elements join to form compounds. Substances join together in consistent ways (Law of Definite Proportions). Based his theory on experimental evidence. Example: 100 g of Mg combines with 65.8 g of O. 10 g of Mg combines with 6.58 g of O.

Theories can change… Dalton made some significant contributions to the Atomic Theory, BUT the theory did change over time as more experiments were done and additional data collected.

Thomson’s Model

In a nutshell, Thomson discovered ELECTRONS! Thomson’s Model First to suggest that atoms were not indivisible. Based on an electricity experiment with a cathode ray tube. Particles in the beam must be negative b/c they were attracted to the positive plate. In a nutshell, Thomson discovered ELECTRONS!

Thomson’s Plum Pudding Model Said that negative charges (electrons) are evenly scattered throughout atoms. To bring this into 2009, call it the blueberry muffin model or the chocolate chip ice cream model.

Rutherford’s Atomic Theory Worked with a student to find out what happens to positively charged alpha particles when they pass through a thin sheet of gold foil. He believed the particles would travel straight from their source to the screen that lit up when struck.

What actually happened.

The Results of Rutherford’s Experiment The results of Rutherford’s experiment did not match with his predictions. More particles were deflected than he expected. Some of the particles even acted as if they struck an object and bounced straight back. SO, Rutherford concluded that the positive charge of an atom is NOT evenly spread throughout. The positive charge is concentrated in a small, central area he called the nucleus.

More about Rutherford… So, in essence, Rutherford discovered PROTONS and the nucleus! He also proposed that electrons orbited the nucleus of an atom like planets orbit the sun. Today we know that the nucleus not only contains protons, but also neutrons (which have no charge).

FUN FACT IF AN ATOM was the same size as the Cleveland Browns football stadium, its nucleus would be the size of a marble. Most of an atom’s volume is the space around the nucleus, not the nucleus itself.

Review Questions 1. Democritus coined what word for a tiny piece of matter that cannot be divided? Answer – Atom 2. If 2 g of element X combine with 4 g of element Y to form compound XY, how many grams of element Y would combine with 12 grams of X to form the same compound? Answer – 24 grams

Review Questions 3. In Rutherford’s experiment, what caused some of the alpha particles to bounce straight back from the gold foil? -Answer – The positively charged nucleus