Get out your packet and open to the next notes page Write down the Essential Question: What scientists contributed to the structure of the atom?

Atomic History and Theories

These electron microscopes were invented in the mid- 1900’s. How did scientists figure out atomic structure without being able to look at any atoms?

The History of Atomic Theory They were WRONG….ALOT!

But first….THEORY What is a theory in science?What is a theory in science? A theory is a well-tested explanation of what happens in nature.A theory is a well-tested explanation of what happens in nature. In layman’s terms, if something is said to be “just a theory,” it usually means that it is a mere guess, or is unproven. But in scientific terms, a theory implies that something has been well-tested and verified many times by multiple groups of scientists.In layman’s terms, if something is said to be “just a theory,” it usually means that it is a mere guess, or is unproven. But in scientific terms, a theory implies that something has been well-tested and verified many times by multiple groups of scientists.

But first….Law A scientific law is a statement of something that seems to be true in the natural world, but doesn’t explain why.A scientific law is a statement of something that seems to be true in the natural world, but doesn’t explain why. Example: The Law of Gravity. Newton could use this law to predict the behavior of a dropped object, but he couldn't explain why it happened.Example: The Law of Gravity. Newton could use this law to predict the behavior of a dropped object, but he couldn't explain why it happened.

Remember! A LAW describes what happens in nature, but doesn’t explain why.A LAW describes what happens in nature, but doesn’t explain why. A THEORY is an explanation of what happens in nature based on repeated observations or experiments.A THEORY is an explanation of what happens in nature based on repeated observations or experiments.

The History of Atomic Theory

Democritus Greek philosopher, not scientistGreek philosopher, not scientist 1 st to suggest matter is made of tiny particles called atoms “atomos”1 st to suggest matter is made of tiny particles called atoms “atomos” “atomos” = Greek for “indivisible”“atomos” = Greek for “indivisible” Atomic Model: atoms were small, hard particles that were different shapes and sizesAtomic Model: atoms were small, hard particles that were different shapes and sizes

John Dalton First to propose an ATOMIC THEORY based on science: 1. Each element is composed atoms. 2. All atoms of a given element are identical*; the atoms of different elements are different. 3. Atoms are not created or destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine.First to propose an ATOMIC THEORY based on science: 1. Each element is composed atoms. 2. All atoms of a given element are identical*; the atoms of different elements are different. 3. Atoms are not created or destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine. 5. In a chemical rxns elements are combined, separated or rearranged.

*Asterisk The Asterisk by number 2 in Dalton’s Atomic Theory reminds us that it was later found to be incorrect. The Asterisk by number 2 in Dalton’s Atomic Theory reminds us that it was later found to be incorrect. We now know that all atoms of a given element are NOT identical due to the discovery of isotopes. We now know that all atoms of a given element are NOT identical due to the discovery of isotopes. Remember ISOTOPES are an atom with the same number of protons, but DIFFERENT number of neutrons! Remember ISOTOPES are an atom with the same number of protons, but DIFFERENT number of neutrons!

Dalton’s atomic model: a tiny sphere that is indivisible

J.J. Thomson 1 st to suggest that there were particles SMALLER than the atom!!1 st to suggest that there were particles SMALLER than the atom!! Discovered the 1 st subatomic particle…….. the ELECTRON!Discovered the 1 st subatomic particle…….. the ELECTRON! Developed the “plum pudding model.”Developed the “plum pudding model.”

Thomson’s “plum-pudding” model: the atom is composed of electrons surrounded by a cloud of positive charge to balance the electron's negative charge

Thompson vs. Dalton Thompsons model proved Daltons Atomic Theory was wrong….Thompsons model proved Daltons Atomic Theory was wrong…. Atoms are divisible into smaller subatomic particles Atoms are divisible into smaller subatomic particles Remember!

Ernest Rutherford gold foil experimentgold foil experiment discovered the NUCLEUS, and realized that it was very dense and contained positively charged PROTONSdiscovered the NUCLEUS, and realized that it was very dense and contained positively charged PROTONS suggested that most of the atom is empty spacesuggested that most of the atom is empty space

Gold Foil Experiment: Rutherford shot tiny positively-charged alpha particles through a thin sheet of gold foil What Rutherford expected: What really happened:

Gold Foil Experiment: Most of the particle passed straight though, BUT some of the alpha particles bounced back. Some of the alpha particles had run into concentrations of + charge and, since like charges repel, they had been hurled straight back by them.

Rutherford’s atomic model: All of the positive charge is crammed inside a tiny, massive nucleus about ten thousand times smaller than the atom as a whole. The atom is mostly empty space. nucleus

Scientists were looking for missing mass. The protons and electrons in the atom didn’t weigh enough to account for the mass of the atom

James Chadwick Rutherford’s co-workerRutherford’s co-worker proved the existence ofproved the existence of NEUTRONS, neutral particle in the atom’s nucleus.

Chadwick’s model of the atom Protons and neutrons are in the nucleus, which only takes up one millionth of a billionth of the volume of the atom. The electrons are in the electron cloud, which constitutes almost all of the volume of the atom.

Neils Bohr Proposed that electrons are made up of different shells

Subatomic Particles to Know Subatomic Particle LocationMassCharge ProtonNucleus1 amu+1 NeutronNucleus1 amu0 Electron Cloud Practically zero

Foldable Outside 1. 1.Democritus 2. 2.Dalton 3. 3.Thompson 4. 4.Rutherford 5. 5.Chadwick 6. 6.Bohr Inside Include a description of each scientists contribution to the atom. Draw a picture of each model. Example on the board