Developing a Model of Developing a Model of the Atomthe Atom

The History of Atomic TheoryThe History of Atomic Theory

What should a Model look What should a Model look like?like?

Scientific models Scientific models may not always may not always

look like the actual look like the actual object. A model is object. A model is an attempt to use an attempt to use familiar ideas to familiar ideas to

describe unfamiliar describe unfamiliar things in a visual things in a visual

way.way.This is a painting of a young This is a painting of a young woman by Pablo Picasso. Does woman by Pablo Picasso. Does it actually look like a young it actually look like a young woman?woman?

Can a Model be Changed?Can a Model be Changed?

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

From the early Greek concept to the From the early Greek concept to the modern atomic theory, scientists modern atomic theory, scientists

have built upon and modified have built upon and modified existing models of the atom.existing models of the atom.

Atomic ModelsAtomic Models This model of the This model of the

atom may look atom may look familiar to you. This is familiar to you. This is the Bohr model. In the Bohr model. In this model, the this model, the nucleus is orbited by nucleus is orbited by electrons, which are electrons, which are in different energy in different energy levels. levels.

Name that Scientist!Name that Scientist!

In this lesson, we’ll learn about the scientists whose quests for knowledge about the fundamental nature of the universe helped define our views.

Democritus (460 – 370 B.C.)Democritus (460 – 370 B.C.)

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 atoms were atoms 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 sizes.shapes and sizes.

Atoms were always Atoms were always moving and capable of moving and capable of joining together.joining together.

He also believed that He also believed that there is empty space there is empty space between them.between them.

Aristotle (384 – 322 B.C.)Aristotle (384 – 322 B.C.)The eminent The eminent

philosophers of the philosophers of the time, time, AristotleAristotle and and Plato, had a more Plato, had a more respected, (and respected, (and ultimately ultimately wrongwrong) ) theory.theory.

Aristotle and Plato favored the Aristotle and Plato favored the earth,earth, firefire, , airair and and waterwater approach to the approach to the nature of matter. The nature of matter. The atomosatomos idea idea was buried for approximately 2000 was buried for approximately 2000 years. years.

Dalton (1766 – 1844)Dalton (1766 – 1844)

In the early 1800s, In the early 1800s, the English the English Chemist John Chemist John DaltonDalton performed a performed a number of number of experiments that experiments that eventually led to eventually led to the acceptance of the acceptance of the idea of atoms.the idea of atoms.

Dalton’s TheoryDalton’s Theory1.1. All elements are composed of All elements are composed of

submicroscopic indivisible parts submicroscopic indivisible parts called atoms.called atoms.

2.2. Atoms of the same element have Atoms of the same element have identical properties. identical properties.

3.3. Atoms of different elements have Atoms of different elements have different properties.different properties.

4.4. Atoms of different elements Atoms of different elements combine in constant ratios to form combine in constant ratios to form compounds – LAW OF compounds – LAW OF CONSTANT COMPOSITIONCONSTANT COMPOSITION

5.5. Chemical reactions involve the Chemical reactions involve the rearrangement of atoms. No new rearrangement of atoms. No new atoms are created or destroyed – atoms are created or destroyed – LAW OF CONSERVATION OF LAW OF CONSERVATION OF MASSMASS

J.J. THOMSON (1856 – 1940)J.J. THOMSON (1856 – 1940)

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’s ExperimentThomson’s ExperimentThomson studied Thomson studied

the 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 passed through the gas, it gave off the gas, it gave off rays of rays of negatively negatively charged particles.charged particles.

Thomson ExperimentThomson Experiment

This surprised This surprised Thomson, Thomson, because the because the atoms of the gas atoms of the gas were uncharged. were uncharged. Where had the Where had the negative charges negative charges come from?come from?

Where did they come from?

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.

Thomson’s ModelThomson’s Model He proposed a He proposed a

model of the atom model of the atom that is sometimes that is sometimes called the “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 a pudding.raisins in a pudding.

Ernest Rutherford (1871 – Ernest Rutherford (1871 – 1937)1937)

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 Rutherford’s experiment involved firing a stream of involved firing a stream of tiny 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 Gold Foil Rutherford’s Gold Foil ExperimentExperiment

What he expected to see:What he expected to see:

Why ? -Positive charges were spread out evenly. Alone they were not enough to stop an alpha particle

What he saw:What he saw:

MostMost of the positively charged “bullets” passed right of the positively charged “bullets” passed right through the gold atoms in the sheet of through the gold atoms in the sheet of gold foilgold foil without changing course at all.without changing course at all.

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

How he explained itHow he explained it

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..

Bohr ModelBohr ModelIn 1913, the Danish In 1913, the Danish

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

Bohr ModelBohr ModelAccording to Bohr’s atomic According to Bohr’s atomic

model, electrons move in model, electrons move in definite definite orbitsorbits around the around the nucleus, much like planets nucleus, much like planets circle the suncircle the sun

These orbits, or energy These orbits, or energy levelslevels, , are located at certainare located at certain distances from the nucleus.distances from the nucleus.

Energy levels were like steps Energy levels were like steps on a ladderon a ladder An electron can only be at any given An electron can only be at any given

step at any given timestep at any given time