The History of Atomic Theory

The History of Atomic Theory

The atom We know the atom is made up of 3 particles... What are they?

3 Subatomic Particles Protons = Positively charged Neutrons = Neutrally charged (no charge) Electrons = Negatively charged

The atom Protons and Neutrons are in the nucleus… so what charge does the nucleus have?

The atom Electrons orbit around the nucleus in different ENERGY LEVELS

Atomic Models This is the Bohr model of the atom, this is actually NOT how the atom looks but it’s pretty close, so we can use it for now…

Models of the Atom Over the last 2400 years, the model of the atom has gone through 7 major changes You need to know: • Who came up with each model • What it looks like • How they came up with it

Democritus In 400 B.C. Greek Philosopher Democritus came up with a theory: • Matter could not be divided into smaller and smaller pieces forever • Eventually the smallest possible piece would be obtained • This piece would be indivisible

Atomos He named the smallest piece of matter “atomos,” meaning “not to be cut.” We now know these as atoms. To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together.

This theory was ignored and forgotten for more than 2000 years!

Why? Aristotle and Plato believed all matter was made up of earth,fire, air and water. They were more well known so their ideas were believed to be correct for 2000 years.

Dalton’s Model In the early 1800s, John Dalton performed experiments that eventually led to the acceptance of the idea of atoms.

Dalton’s Theory He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. Atoms of the same element are exactly alike while atoms of different elements are different. This theory became one of the foundations of modern chemistry

Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles. He proposed a model of the atom that is called the “Plum Pudding” model.

Thomson Model Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

Thomson Model Thomson was doing an experiment- running an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. This surprised Thomson, because the gas had no charge Where did the negative charge come from?

Thomson Model Where did they come from? Thomson concluded that the negative charges came from within the atom. Thomson found the electron! Since the gas had no charge, he thought that there must also be positively charged particles in the atom, but he could never find them.

Rutherford’s Gold Foil Experiment In 1908, Ernest Rutherford was doing an experiment involving firing a stream of positively charged particles at a thin sheet of gold foil Most of the particles passed through the foil but some of the particles bounced off the gold foil. He knew that positive charges repel positive charges so there must be a positive charge in the atom

Rutherford He concluded that MOST of the atom is empty space. Rutherford concluded that an atom had a small, positively charged center, he called it the nucleus. The nucleus is tiny compared to the atom as a whole.

Bohr Model In 1913, Danish scientist Niels Bohr proposed an improvement to the model. In the Bohr model, each electron is placed in a specific energy level.

Bohr Model In the Bohr model, electrons move in definite orbits around the nucleus, like the planets circling the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

The Wave Model The current atomic model is known as the “wave model” While many things added up to form the model, Schrodinger was the biggest contributor

The Wave Model The most current atomic model is based on the principles of wave mechanics. According to the theory of wave mechanics, electrons do not orbit the atom like planets around the sun. It is impossible to determine the EXACT location of an electron.

The Wave Model Instead we can know the general location of an electron is based on how much energy the electron has. According to the wave model, the atom has a small positively charged nucleus surrounded by a large electron cloud

Electron Cloud The electron cloud is a space in which electrons are likely to be found. Depending on how much energy the electron has, the electron is locked into a certain area in the cloud. Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

What does the periodic table tell us about the structure of the atom? Atomic Number = the number of protons Atomic Number also = the number of electrons Atomic Mass = Number of Protons + Number of Neutrons Neutrons = Atomic Mass – Atomic Number

The History of Atomic Theory