The Atomic Model

The atomic model is an idea of what an atom looks like. It is not a model like a model airplane. Scientific models are simply working representations. Model of the Atom

There are two different types of models. 1. Conceptual 2. Mathematical

Scientific models of any kind are just tools to help us visualize real-life phenomena. Model of the Atom

History of the Atomic Model 1. Leucippus

Leucippus was a Greek who believed that there is a smallest particle. This belief implies that the atom is indivisible. Leucippus

History of the Atomic Model 1. Leucippus 2. Democritus

Democritus was a student of Leucippus. He called the smallest particle the atom. (Atomos means “indivisible.”) He believed that atoms make up all matter. Democritus

Democritus was right! There is a basic unit of matter called the atom. However, the atom is not indivisible, and it has not always existed. Democritus

Democritus made no observations, so he cannot be called a scientist. He reasoned, not experimented, to create his model. Democritus

Question What did Democritus do? 1.He discovered the atom. 2.He theorized that atoms exist. 3.He discovered the electron. 4.He discovered the proton.

Question Democritus was right when he said 1.matter is composed of atoms. 2.atoms are indestructible. 3.atoms are large. 4.matter is composed of protons and electrons.

As more information was discovered about the atom, scientists made changes to the atomic model. Model change represents a revolution in the way scientists are thinking. The Atom Rediscovered

History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton

Dalton created the first experimental model by weighing substances and applying the Law of Definite Proportions. Dalton

The Law of Definite Proportions, discovered by Joseph Proust, states that every compound has a definite composition by mass. For example, water’s composition by mass is a 1:8 ratio. Dalton

Dalton used Proust’s Law and further experiments to come up with his model called the core-envelope model. Dalton

core heat envelope

Dalton’s Model Atoms are neither created nor destroyed. Atoms cannot be subdivided.

Elements are made only of atoms. All atoms of an element are the same. Atoms of elements differ in their masses. Dalton’s Model

History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson

Thomson found that the atom is not solid—it has negative particles in it. He discovered the electron (e – ). Thomson

The electrons are in a positive (+) gel-like substance. The electrons and the positive substance charges cancel out. Electrons do not vary in mass or charge. Electrons can be removed from the atom. Thomson’s Model

Thomson’s model is known as the plum-pudding model. Thomson’s Model

positive “pudding” negative electrons “plums” atom

Thomson’s model can also be called the chocolate chip cookie model. Thomson’s Model

Question Thomson’s model looked like 1.peanut brittle. 2.roast beef. 3.plum pudding. 4.a hot dog.

Question Thomson’s cathode rays were 1.protons. 2.neutrons. 3.quarks. 4.electrons.

History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford

Rutherford shot alpha particles at thin gold foil.  Alpha particles are relatively heavy and positively charged. Some of the alpha particles bounced straight back, revealing that atoms have a nucleus made of positive protons (p + ). Rutherford

glass plate vacuum chamber microscope zinc sulfide screen alpha particle beam scattered alpha particles gold foil rotating housing

Rutherford’s model is called the nuclear model. Rutherford’s Model

Protons form a small, central nucleus only 1/100,000 the size of the atom. Protons are many times larger than electrons. Electrons are whirling somewhere around the nucleus. Most of the atom is empty space. Rutherford’s Model

tiny dense positive nucleus electrons occupy space around nucleus

Question Rutherford’s experiment involved 1.a cathode ray. 2.alpha particles. 3.neutron emission. 4.atomic absorption diffraction.

Question Rutherford discovered 1.protons. 2.neutrons. 3.quarks. 4.electrons.

History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford 6. Chadwick

Chadwick discovered the neutron (n°). The neutron is as big as the proton but it has no charge. The neutron is also in the nucleus. Chadwick

History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford 6. Chadwick 7. Bohr

Bohr knew that atoms give off colors when heated (like fireworks). Looking through a prism, he could see different colors being emitted. Bohr

Bohr thought that the electrons jumped further from the nucleus when heated and then fell back down, giving off energy as they fell. The electrons occupy specific levels at their normal energies. Bohr

Each atom has a different arrangement of electrons, so it gives off different colors of light. Electrons can jump from level to level. Bohr’s Model

Bohr’s model looked like the solar system, so it was called the planetary model. Bohr’s Model

Kr

History of the Atomic Model 1. Leucippus 2. Democritus 3. Dalton 4. Thomson 5. Rutherford 6. Chadwick 7. Bohr 8. Quantum model

The quantum model replaces Bohr’s exact orbits with regions (like whirling fan blades) where electrons are most likely to be found. These regions are called orbitals. Quantum Model