Investigating Atoms and Atomic Investigating Atoms and Atomic TheoryTheory

EQ’s: EQ’s: How has the model of the atom evolved over How has the model of the atom evolved over

time? time?

What important discoveries did scientists What important discoveries did scientists perform to help construct the modern atomic perform to help construct the modern atomic model accepted today?model accepted today?

AtomosAtomos::

The History of Atomic TheoryThe History of Atomic Theory

Atomic ModelsAtomic Models A model is another word A model is another word

for a theory. It uses for a theory. It uses familiar ideas to explain familiar ideas to explain unfamiliar facts observed unfamiliar facts observed in nature.in nature.

A model can be changed A model can be changed as new information is as new information is collected.collected.

You will be looking at 5 You will be looking at 5 difference atomic models difference atomic models that evolved over time.that evolved over time.

The Greek Model The Greek Model DemocritusDemocritus

This is the Greek This is the Greek philosopher Democritus philosopher Democritus who began the search for who began the search for a description of matter a description of matter more than more than 24002400 years years ago.ago.

He asked: Could matter He asked: Could matter be divided into smaller be divided into smaller and smaller pieces and smaller pieces forever? He just thought forever? He just thought about it. He didn’t do any about it. He didn’t do any experiments.experiments.

400 BC

AtomosAtomos

His theory: Matter could not His theory: Matter could not be divided into smaller and be divided into smaller and smaller pieces forever, smaller pieces forever, eventually the smallest eventually the smallest possible piece would be possible piece would be obtained.obtained.

This piece would be This piece would be indivisible.indivisible.

He named the smallest He named the smallest piece of matter “atomos,” piece of matter “atomos,” meaning “not to be cut.”meaning “not to be cut.”

AtomosAtomos

To Democritus, atoms To Democritus, atoms were were smallsmall, hard , hard particles that were all particles that were all made of the same made of the same material but were material but were differentdifferent shapes and shapes and sizes.sizes.

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

Dalton’s ModelDalton’s ModelIn the early 1800s, the In the early 1800s, the English Chemist John English Chemist John Dalton Dalton and other scientists and other scientists measured the masses of measured the masses of reactants and products in reactants and products in chemical reactions. They chemical reactions. They found that atoms always found that atoms always react in whole number react in whole number ratios. This with other ratios. This with other experiments with gases led experiments with gases led to Dalton’s Model for the to Dalton’s Model for the atom.atom.

Dalton’s Theory:Dalton’s Theory:

1.1. Elements are made of tiny Elements are made of tiny particles called atoms.particles called atoms.

2.2. All atoms of a given element All atoms of a given element are the same mass.are the same mass.

3.3. All atoms of different atoms are All atoms of different atoms are different masses.different masses.

4.4. Atoms combine to form Atoms combine to form compounds that always have compounds that always have the same whole number ratio.the same whole number ratio.

5.5. Atoms are indivisible and Atoms are indivisible and indestructible and cannot be indestructible and cannot be created or destroyed.created or destroyed.

Picture of Dalton’s Atomic Model

..This theory became one of the foundations of modern chemistry.

However, Dalton did not know that there were any subatomic particles and believed that all atoms of an element were exactly alike which was not true.

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

In In 18971897, the , the English scientist English scientist J.J. Thomson J.J. Thomson provided the first provided the first hint that an atom hint that an atom is made of even is made of even smallersmaller particles. particles.

Thomson ModelThomson Model

Thomson studied the Thomson studied the passage passage of an of an electric current electric current through a gas.through a gas.

As the current As the current passed through the passed through the gas, it gave off rays gas, it gave off rays of of negatively negatively charged particles.charged particles. Click here to view

animation

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom, but he could never find them.

He also didn’t know about the nucleus or any arrangement of the electrons.

Thomson ModelThomson Model He proposed a model He proposed a model

of the atom that is of the atom that is sometimes called the sometimes called the ““PlumPlum PuddingPudding” model. ” model.

Atoms were made Atoms were made from a positively from a positively chargedcharged substancesubstance with negatively with negatively charged electrons charged electrons scatteredscattered about, like about, like raisins in rice pudding.raisins in rice pudding.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

In 1908, the English In 1908, the English physicist Ernest physicist Ernest Rutherford was hard Rutherford was hard at work on an at work on an experiment that experiment that seemed to have little seemed to have little to do with unraveling to do with unraveling the mysteries of the the mysteries of the atomic structure.atomic structure.

Rutherford’s experiment Involved firing Rutherford’s experiment Involved firing a stream of tiny a stream of tiny positively chargedpositively charged particles at a thin sheet of particles at a thin sheet of gold foilgold foil (2000 atoms thick)(2000 atoms thick)

Rutherford’s Discovered:Rutherford’s Discovered: MostMost of the positively of the positively

charged “bullets” passed charged “bullets” passed right through the gold right through the gold atoms in the sheet of atoms in the sheet of gold gold foilfoil without changing without changing course at all.course at all.

SomeSome of the positively of the positively charged “bullets,” however, charged “bullets,” however, did bounce away from the did bounce away from the gold sheet as if they had hit gold sheet as if they had hit something something solidsolid. He knew . He knew that positive charges that positive charges repelrepel positive charges.positive charges.

Animated Video Clip of Animated Video Clip of Rutherford’s ExperimentRutherford’s Experiment

Click on this link to see more information

This could only mean that the gold atoms in the This could only mean that the gold atoms in the sheet were mostly sheet were mostly open spaceopen space. Atoms were . Atoms were notnot a pudding filled with a positively charged a pudding filled with a positively charged material.material.

Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged centerdense, positively charged center that that repelledrepelled his positively charged “bullets.”his positively charged “bullets.”

He called the center of the atom the “He called the center of the atom the “nucleusnucleus”” The nucleus is The nucleus is tiny tiny compared to the atom as a compared to the atom as a

whole. whole.

Rutherford’s ModelRutherford’s Model

Rutherford reasoned Rutherford reasoned that all of an atom’s that all of an atom’s positively charged positively charged particles were particles were containedcontained in the in the nucleus. The nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus around the nucleus around the atom’s atom’s edgeedge..

Rutherford’s ModelRutherford’s Model

Rutherford however didn’t Rutherford however didn’t know how the electrons know how the electrons were arranged or why they were arranged or why they didn’t fall into the nucleus didn’t fall into the nucleus since the nucleus was since the nucleus was positive and the electrons positive and the electrons were negative and were negative and opposites attract.opposites attract.

Bohr ModelBohr ModelIn the early 1900s, In the early 1900s,

the Danish the Danish scientist Niels scientist Niels Bohr did Bohr did experiments with experiments with the spectra of the spectra of elements.elements.

Click here to explore information about where spectra come from

Bohr ModelBohr ModelIn 1913, the In 1913, the

Danish scientist Danish scientist he proposed an he proposed an improvement. In improvement. In his model, he his model, he placed each placed each electron in a electron in a specificspecific energy energy level.level.

Click to watch a Click to watch a brief animation on brief animation on where a spectrum where a spectrum comes from.comes from.

Bohr ModelBohr Model According to Bohr’s According to Bohr’s

atomic model, atomic model, electrons move in electrons move in definite definite orbitsorbits around around the nucleus, much the nucleus, much like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or energy energy levelslevels, are , are located at certainlocated at certain distances from the distances from the nucleus.nucleus.

The Bohr Mode looks something like this:

Bohr ModelBohr ModelBohr’s Model does a pretty good Bohr’s Model does a pretty good job of giving a simple view of the job of giving a simple view of the atom. In the next unit we will learn, atom. In the next unit we will learn, however, that the Bohr Model only however, that the Bohr Model only worked for hydrogen and not any worked for hydrogen and not any other atoms and electrons other atoms and electrons sometimes act like waves instead sometimes act like waves instead of particles.of particles.

IndivisibleIndivisible ElectronElectron NucleusNucleus Electrons Electrons in Orbitsin Orbits

GreekGreek XX

DaltonDalton XX

ThomsonThomson XX

RutherfordRutherford XX XX

BohrBohr XX XX XX