The Scientific Method

Scientific Method Scientific Method A series of logical steps to follow, including experimentation, in order to solve problems or answer questions. The Process is NOT linear but circular.

Observe Question Hypothesis Evaluate Data Results partially support hypothesis Reflect/generate new questions Repeat experiment Repeat expt. Revise hypothesis. Publish results Results don’t support hypothesis Design/Conduct Experiment OR Results support hypothesis

Hypothesis Preceded by a question Takes the general form: If “question”, then “prediction” or “outcome”, because “tentative explanation”. Example: QUESTION: why doesn’t my flashlight light? HYPOTHESIS: If a flashlight doesn’t light then the batteries are exhausted, because the power for the flashlight comes from the batteries. A scientific hypothesis MUST be testable!!

In Science, a Hypothesis is an attempt at an explanation for the events that have been observed. A hypothesis has to be testable. If a lot of evidence (data) is collected through experiments to support the hypothesis, then scientists accept the hypothesis as a good explanation. An accepted hypothesis may, given even more testing and agreement, become an accepted theory. Usually takes many years. THEORYS – Explain

Scientific Laws DESCRIBE observations, they DO NOT EXPLAIN them. Experiments are not usually associated with Laws. Remember this very important idea: Isaac Newton knew what gravity did, but he could not explain why gravity did it. Even today the topic of “what is gravity” is an active topic for scientific discussion using Einstein’s general theory of relativity. LAWS – Describe

You Can Never Turn A Theory Into A Law!!

Scientific Method is Circular Science is Social (scientists talk to scientists) A Law Describes A Theory explains Can’t change a law into a theory or vice versa. Summary

The Modern Atomic Model of the Atom Dalton, Thomson and Rutherford

What do We Know About the Atom? and… How do we know what we know?

Outline Early ideas about the atom Discovery of the electron Discovery of the nucleus and protons Atomic Number, Mass Number Atomic Mass

Greek Model (Democrites) Everything is composed of earth, fire, wind and water Atoms are the smallest building block of matter Atoms are indivisible All atoms are made of the same thing. Atoms of different materials are different shapes. (tree atoms look like little trees, rock atoms look like little rocks)

Dalton’s View of the Atom (1803) Elements are made of tiny indivisible particles called atoms. All atoms of one element are unique from atoms of other elements All atoms of a given element are identical and have the same properties. Atoms of one element can combine with atoms of other elements in whole number ratios to form compounds. (Law of constant composition) Atoms are not created or destroyed in a chemical reaction they are just rearranged into new compounds. (Law of conservation of matter)

Crooke’s Tube Cathode (-) MAGNET DC Power Supply Anode (+) Cathode ray

Thomson’s Cathode Ray Tube (1897) + - Cathode (-) Anode (+) Second DC Power Supply

CRT Tube Video (Click to start)

Thomson’s experiment results Beam was made of a particle 2000 times smaller than the smallest atom (hydrogen) Beam was negatively charged – attracted to the positive electrode of a second power supply. (electron) Couldn’t find the corresponding positive particle. Assumed atom had a “fog” of positive charge in which the electrons were dispersed.

“Plum Pudding” Model

Some Information about Radiation Alpha particles are large positively charged particles; they are helium nuclei Beta particles are smaller negatively charged particles; they are electrons Gamma rays are electrically neutral, high energy electromagnetic radiation.

Rutherford’s Experiment Box Radioactive α

Animation

Rutherford’s Gold Foil Experiment Trying to Prove Plum Pudding Model Shot pos. (+) charged alpha particles at thin gold foil. Expected all to go straight through. Most did go straight through but a few (1 in 20,000) bounced off. Gold Foil

Rutherford’s Astonishing Conclusion Atoms are mostly empty space ! There is a very dense, positively charged nucleus that contains most of the mass.

Summary Thomson and Rutherford Thomson – the atom is made up of smaller particles Thomson – one of the smaller particles is negatively charged Rutherford - The atom has a dense positive center. Rutherford - The atom is mostly empty space.

Rutherford Model Center – positive nucleus with neg. electrons dispersed around nucleusCenter – positive nucleus with neg. electrons dispersed around nucleus Atom is mostly empty space.Atom is mostly empty space. Large Positive Nucleus Electrons (randomly dispersed) Empty space

Along Comes Neils Bohr

Bohr Model Electrons are in discreet energy levels orbiting around the nucleus.

… And Then de Broglie - The Wave Model  If electrons are particles and they orbit the nucleus, why don’t they eventually spiral into the nucleus?  Electrons are waves not particles!  Electrons are everywhere around The nucleus all at once. The nucleus all at once.  The Double Slit Experiment

Wave Model (de Broglie) Electron are traveling at near the speed of light. Electrons are everywhere all at once in their energy level or orbital around the nucleus Electrons are in standing waves of potential around the nucleus at discreet distances from the nucleus (Bohr)

Electrons as Waves – The Double Slit Experiment. (Click to start)

The Atom and its Parts Nucleus Protons and Neutrons Protons and Neutrons Protons and neutrons are made up of quarks Around the Nucleus Electrons – # electrons equals # protons when the atom has no charge. Electrons – # electrons equals # protons when the atom has no charge. In Between Electrons and Nucleus – EMPTY SPACE !!!

Atomic number: Z, the number of protons in the atom (Unique to an element, if the atomic number is different the element is different) Mass number: M, the number of protons and neutrons in the atom. Isotopes are atoms of the same element, ie the have the same atomic number, but they have a different mass number. They differ in the number of neutrons. The chemical properties of the isotopes are the same, but they have different masses.

Notation Mass Number # of protons + neutrons Atomic Number # of protons

Practice Find the number of protons, neutrons and electrons in the following elements

Answers 17 protons 17 electrons = 18 neutrons 95 protons 95 electrons = 148 neutrons

The Atom in Summary ParticleMassChargeLocation in Atom Electron9.1x g ≈ 1/1000 amu x C (-1) Outside the nucleus Proton1.67 x g ≈ 1 amu x C (+1) Inside the nucleus Neutron1.67 x g ≈ 1 amu 0Inside the nucleus (1amu=1.66x g)

Atomic Mass Definition: Mass of an atom of a particular element in amu (atomic mass unit) Definition: Mass of an atom of a particular element in amu (atomic mass unit) 1 amu = 1.67 x g (mass of a proton) 1 amu = 1.67 x g (mass of a proton) Average Atomic Mass: is the weighted average of all of the atomic masses of the isotopes in a naturally occurring sample of the element ON THE PERIODIC TABLE Average Atomic Mass: is the weighted average of all of the atomic masses of the isotopes in a naturally occurring sample of the element ON THE PERIODIC TABLE

Some Useful Internet Sites Cathode Ray Tube: video/Cath.avi video/Cath.avi video/Cath.avi Gold Foil: ava/rutherford/ ava/rutherford/ ava/rutherford/ Millikan Oil Drop, ectricity/Millikan.htm ectricity/Millikan.htm ectricity/Millikan.htm

Law of Constant Composition (or Definite Proportions): Dalton different samples of the same compound always contain its constituent elements in the same proportion by mass Example: Water always contains hydrogen and oxygen in a mass ratio of 1:8 no matter how it is formed or where it is found.

Law of Multiple proportions: If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. Example: Water and Hydrogen peroxide both contain hydrogen and oxygen: Water H:O by mass is 1:8 Hydrogen peroxide is 1:16

Cathode Ray Tube Experiment Thomson