Chapter 16: The Atom

The Atomic Model

Model of the Atom 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. Conceptual Mathematical

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

History of the Atomic Model Leucippus

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

History of the Atomic Model Leucippus Democritus

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.

What did Democritus do? He discovered the atom. He theorized that atoms exist. He discovered the electron. He discovered the proton.

Democritus was right when he said matter is composed of atoms. atoms are indestructible. atoms are large. matter is composed of protons and electrons.

The Atom Rediscovered 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.

History of the Atomic Model Leucippus Democritus Dalton

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.

heat envelope core

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

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

History of the Atomic Model Leucippus Democritus Dalton Thomson

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

Thomson’s Model 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.

atom positive “pudding” negative electrons “plums”

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

Thomson’s model looked like peanut brittle. roast beef. plum pudding. a hot dog.

Thomson’s cathode rays were protons. neutrons. quarks. electrons.

History of the Atomic Model Leucippus Democritus Dalton Thomson Rutherford

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+).

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

Rutherford’s Model 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.

electrons occupy space around nucleus tiny dense positive nucleus

Rutherford’s experiment involved a cathode ray. alpha particles. neutron emission. atomic absorption diffraction.

Rutherford discovered protons. neutrons. quarks. electrons.

History of the Atomic Model Leucippus Democritus Dalton Thomson Rutherford Chadwick

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.

History of the Atomic Model Leucippus Democritus Dalton Thomson Rutherford Chadwick Bohr

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

Flame Tests

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’s Model 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.

Kr

History of the Atomic Model Leucippus Democritus Dalton Thomson Rutherford Chadwick Bohr Quantum model

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.