Chapter 4 Atomic Structure

Studying Atoms Discovery of Subatomic Particle Democritus was a Greek philosopher who believed that all matter consisted of small particles that could not be divided. He called these particles atoms. He thought that there were different types of atoms with specific sets of properties. Example: atoms in solids were rough and prickly.

Aristotle described matter differently Aristotle described matter differently. His model was that all substances were built from four elements- earth, air, fire, and water. He thought these elements were combinations of hot, cold, wet, and dry. He also did not think there was a limit to the number of times matter could be divided.

More people accepted Aristotle’s model of the atom over Democritus. However, in the 1800’s scientists had enough data from experiments to support what is known as the atomic model of matter. An English teacher named John Dalton developed a theory that all matter is made up of individual particles called atoms, which cannot be divided. There are 4 points to his theory (page 101).

However, scientists found that not all of Dalton’s ideas were correct so his theory was revised as new discoveries were made. J.J. Thomson used an electric current to learn more about atoms. The tubes he used to perform his experiments are known as cathode ray tubes (similar to picture tubes in old televisions). He thought a glowing beam in the tube was charged particles.

He noticed that the beam of charged particles was repelled by the negatively charged plate and attracted to the positively charged plate. His experiments provided the first evidence that atoms are made of even smaller particles and knew that an atom was not charged. He is given credit for discovering the electron. His model of the atom is known as the plum pudding model. A big positive ball with negative electrons placed in the ball.

Rutherford’s Atomic Theory Ernest Rutherford discovered that J.J. Thomson’s model of the atom was not correct. He discovered that Uranium emits fast-moving particles with a positive charge (alpha particles). He asked a student (Ernest Marsden) to find out what happens when alpha particles pass through gold foil. This is known as the Gold Foil Experiment.

He discovered that not all of the alpha particles passed through the gold foil. Some bounced back at an angle greater than 90°. This made him realize that the positive charge of an atom cannot be spread out. The positive charge was concentrated to a small, central area. He called this area the nucleus. He came up with a new model of the atom which stated that an atom’s positive charge is concentrated in its nucleus.

Section 4.2 The Structure of an Atom There are 3 subatomic particles. Protons (p+)- positively charged (1+) relative mass of 1 actual mass in grams is 1.674 × 10-24. 2. Electrons (e-)- negatively charged (1-) relative mass is 1/1836 that of a hydrogen atom actual mass is 9.11 × 10-28g.

Neutrons (n)- no charge (neutral) Relative mass of 1 Actual mass in grams is 1.675 × 10-24 Subatomic particles can be distinguished by mass, charge, and location in an atom. Atomic number- is the number of protons in an atom of that element. Mass number- is the sum of the protons and neutrons in the nucleus of that atom. Number of neutrons = Mass # - Atomic #

Atoms are neutral so the protons and electrons in an atom are equal Atoms are neutral so the protons and electrons in an atom are equal. Isotopes- are atoms of the same elements that have different numbers of neutrons and different mass numbers. Ex: Oxygen-16, Oxygen-17, Oxygen-18 (show on board) The 16, 17, and 18 represent the mass number for that isotope of oxygen.