The History of Atomic The History of Atomic TheoryTheory

The atomThe atom

We know the atom is made up of 3 particles... We know the atom is made up of 3 particles...

What are they?What are they?

3 Subatomic Particles3 Subatomic Particles

ProProtons = tons = PosPositively chargeditively charged

NeutNeutrons = rons = NeutNeutrally charged (no charge)rally charged (no charge)

Electrons = Negatively chargedElectrons = Negatively charged

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

The atomThe atomElectrons orbit Electrons orbit around the nucleus in around the nucleus in different ENERGY different ENERGY LEVELSLEVELS

Atomic ModelsAtomic 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 AtomModels of the AtomOver the last 2400 years, the model of the Over the last 2400 years, the model of the atom has gone through 7 major changesatom has gone through 7 major changes

You need to know:You need to know:Who came up with each modelWho came up with each modelWhat it looks likeWhat it looks likeHow they came up with itHow they came up with it

DemocritusDemocritusIn 400 B.C. Greek Philosopher Democritus came up with a theory:Matter could not be divided into smaller and smaller pieces foreverEventually the smallest possible piece would be obtainedThis piece would be indivisible

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

Why?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 ModelDalton’s Model

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

Dalton’s TheoryDalton’s TheoryHe 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 ModelThomson’s Plum Pudding ModelIn 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 He proposed a model of the atom that is called the “Plum Pudding” that is called the “Plum Pudding” modelmodel. .

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

Thomson ModelThomson ModelThomson 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 ModelThomson Model

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.

Where did they come from?

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil ExperimentIn 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

RutherfordRutherford

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 ModelBohr 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 ModelBohr 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 ModelThe Wave Model

The current atomic model is known as the The current atomic model is known as the “wave model”“wave model”

While many things added up to form the model, While many things added up to form the model, Schrodinger was the biggest contributor was the biggest contributor

The Wave ModelThe Wave ModelThe 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 It is impossible to determine the EXACT location of an the EXACT location of an electron. electron.

The Wave ModelThe 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 CloudElectron CloudThe 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 What does the periodic table tell us about the structure of the atom?about the structure of the atom?

Atomic Number = the number of protonsAtomic Number = the number of protons

Atomic Number also = the number of electronsAtomic Number also = the number of electrons

Atomic Mass = Number of Protons + Number of Atomic Mass = Number of Protons + Number of

NeutronsNeutrons

Neutrons = Atomic Mass – Atomic NumberNeutrons = Atomic Mass – Atomic Number