Chapter 3 Section 1 NotesAtomic Structure and History of the Atomic Theory

Do theories in science always stay Do theories in science always stay the same?the same? No, theories are changed or replaced as new

discoveries are made. Theories of the atom (Atomic Theory) have

changed many times over the past couple thousand years based on new knowledge that has been gained through experimenting.

What are Atoms?

Atoms are tiny units that determine the properties of matter

Atoms are EVERYWHERE and make up EVERYTHING around you!

Democritus (460-370 BC)

In the 4th Century B.C., the Greek philosopher Democritus suggested that the universe was made of indivisible units that he called atoms.

“Atom” comes from the Greek word atomos, which means “unable to be cut or divided”

Democritus (460-370 BC)

Democritus concluded that there was a limit to how far matter could be divided; eventually, you would end up with a piece of matter that could not be cut.

What did people think of Democritus? People did not believe Democritus because he had no evidence to support his theory.

John Dalton (1766-1844)

In 1808, two thousand years after Democritus, an English school teacher named John Dalton proposed a revised atomic theory.

His theory was developed based on scientific evidence and some parts of his theory still hold true today!

John Dalton (1766-1844)

John Dalton proposed 3 new ideas about the atom:

1. Every element is made of tiny, unique particles called atoms that cannot be subdivided.

2. Atoms of the same element are exactly alike. 3. Atoms of different elements can join to form

molecules.

Evidence for Dalton

Unlike Democritus, Dalton based his theory on experimental evidence.

Today, Dalton’s theory is considered the foundation for the modern atomic theory. Some parts of this theory turned out to be

correct, but his theory could not explain all of the experimental evidence.

J.J. Thomson (1856-1940)

In 1897, J.J. Thomson, a British scientist, performed an experiment that suggested that atoms were not indivisible.

J.J. Thomson (1856-1940)

What did Thomson discover? While investigating with electricity and cathode

ray tubes, Thomson discovered that atoms are made of smaller, negatively charged particles called electrons.

The stream of electrons is attracted to positively

charged plate.

Thomson’s Atomic Theory

In his model, electrons are spread throughout an atom, just like blueberries are spread throughout a muffin.

His model is called the plum-pudding model, which was a dessert popular in his time.

In this model, electrons are

scattered throughout the

atom.

Ernest Rutherford (1871-1937)

Was a student of J.J. Thomson but disagreed with the “Plum Pudding Model”.

Devised an experiment to investigate the structure of positive and negative charges in the atom.

What did Rutherford Propose?

Rutherford proposed that most of the mass of the atom was in the atom’s center.

An interactive model of Rutherford’s Gold-foil Experiment:

http://micro.magnet.fsu.edu/electromag/java/rutherford

Rutherford’s Gold-Foil Experiment

What did most of the alpha particles shot at the gold foil do? Most of the particles traveled straight through the gold

foil. What was the surprising behavior of a few of the

particles? A few of the particles were deflected and some even

bounced back. http://cwx.prenhall.com/petrucci/medialib/media_po

rtfolio/text_images/006_RUTHERFORD.MOV

Rutherford’s Atomic Model

What did Rutherford discover? Rutherford discovered

the nucleus, which is the dense core of an atom.

What is Rutherford’s Atomic Model? Negatively charged

electrons orbit the nucleus like planets orbit the sun.

Niels Bohr (1885-1962)Niels Bohr (1885-1962)

In 1913, the Danish scientist Bohr suggested that electrons in an atom move in set paths around the nucleus like planets orbiting the sun.

In his model, every electron has a certain energy level that is determined by its path around the nucleus.

Modern Atomic Theory (1925)

This is the theory that we currently believe. The modern atomic theory believes that

electrons behave more like waves on a vibrating string.

Do atoms really look like this?

Well…..

It is a much easier way to fit it on paper! If a nucleus were as big as a marble, the

whole atom would be the size of a football stadium! The nucleus is very, very small in comparison to the atom.

So, can atoms be subdivided?So, can atoms be subdivided?

Both Democritus in the 4th century and later Dalton in the 19th century believed that the atom was the smallest particle and could not be subdivided.

We now know that this is NOT TRUE!!!!-We now know that this is NOT TRUE!!!!- the atom can be divided into subatomic particles

What’s in an atom?What’s in an atom?

An atom has 3 subatomic particles in it: Protons: have a

positive charge and are located in the nucleus

Neutrons: have no charge and are located in the nucleus

Electrons: have a negative charge and are located orbiting around outside the nucleus

Parts of an Atom

Particle Location Mass Charge

Proton Nucleus 1.67 x 10-27 kg +1

Neutron Nucleus 1.67 x 10-27 kg 0

Electron Outside the nucleus

9.11 x 10-31 kg -1

Overall Charge of an Atom

The overall charge of an atom is ZERO. Atoms have equal numbers of protons and electrons, and their charges exactly cancel.

Charge of Atom Example

A certain atom has 2 protons, 3 neutrons, and 5 electrons, what is the overall charge of the atom?

Answer: 2 protons= +2 charge3 neutrons= no charge5 electrons= -5 charge

The overall charge of the atom is (+2)+(-5)= -3 charge-3 charge

The nucleus

What two particles are located in the nucleus? Protons and Neutrons

What is the overall charge of the nucleus ONLY? The nucleus has a positive charge because it is

made of protons, which are positive, and neutrons, which have no charge.

Energy Levels

According to Modern Atomic Theory, it is nearly impossible to determine the exact location of an electron.

1st Energy Level: can hold up to 2 e-

2nd Energy Level: can hold up to 8 e-

3rd Energy Level: can hold up to 18 e-

*Note: Each lower level needs to be filled before going to the next energy level.

Orbitals

An orbital is a region in an atom where there is a high probability of finding an electron.

There are 4 orbitals: s orbital p orbital d orbital f orbital

*These orbitals are in order from lowest to highest energy.

Valence Electrons

A Valence Electron is an electron that is located in the outer most energy level.