Chapter 11

Introduction to Atoms

Section 1: Objectives

Describe some of the experiments that led to the current atomic theory.

Compare the different models of the atom.

Explain how the atomic theory has changed as scientists have discovered new information about the atom.

The Atomic Theory

Around 440 BCE, a Greek philosopher (Democritus) started forming this theory.

He thought that if you sliced an atom enough times, then eventually you would end up with something that could not be cut.

The Atomic Theory

Atom= Greek for “atomos” which means not able to be divided.

Democritus thought 2 things about atoms: All are small, hard particles Made from a single material formed

into different shapes and sizes.

The Atomic Theory

Aristotle disagreed with Democritus.

Aristotle believed that you would never have with a particle that could never be cut.

He had such a strong social influence on people that everyone believed he was right.

The Atomic Theory

Democritus was right though. Matter is made up of particles called

atoms. Atom: smallest particle into which

an element can be divided and remain the same substance

Long before actually being able to see atoms, scientists had ideas about them.

The Atomic Theory

By the late 1700s, scientists knew elements combine to form compounds.

John Dalton, a British chemist, experimented with different substances to figure out why they combine and create certain substances.

The Atomic Theory

Dalton’s Theory stated that: All substances are made of atoms Atoms cannot be created nor destroyed Atoms of the same element are alike

and atoms of different elements are not Atoms join with other atoms to make

new substances

The Atomic Theory

Toward the end of the 1800s, scientists agreed with most of Dalton’s Theory.

New information was found that did not fit his ideas.

The Atomic Theory was then changed.

The Atomic Theory

Thomson showed that there was a mistake in Dalton’s theory.

Thomson discovered that there are smaller particles inside atoms.

This means that atoms can be divided into smaller parts.

The Atomic Theory

Thomson experimented and found that negatively charged particles are present inside atoms.

Electrons: subatomic particles with a negative charge

Thomson experimented with a cathode ray tube.

The Atomic Theory

Cathode = energy source.

2 plates placed in the center.

Invisible beam produced.

Straight beam if no charge.

If charged, beam pulled towards positive plate.

The Atomic Theory

After learning that atoms contained electrons, Thomson produced a new model.

Thomson’s new model was called the “Plum-Pudding Model.”

This was a popular desert during this time period.

Thomson thought electrons were mixed throughout the atom, like plums are mixed throughout the pudding.

The Atomic Theory

In 1909, a former student of Thomson, (Rutherford) wanted to test Thomson’s theory.

Rutherford concentrated a beam of small, positively charged particles at a thin sheet of gold foil.

Rutherford put a special coating behind the gold which glowed when hit by positively charged particles.

The Atomic Theory

This allowed Rutherford to see where the particles traveled after contact.

Some of the particles passed straight through, were deflected, and bounced straight back.

The Atomic Theory

In 1913, Niel Bohr studied the way atoms react with light.

He proposed that electrons are located on different “levels” and orbit the nucleus.

Nucleus = protons + neutrons

The Atomic Theory

Electrons do not travel in definite paths.

The exact path of an electron cannot be predicted.

There are regions inside the atom where electrons are likely to be found. (“electron clouds”)

Calculating Charges: Lithium

Calculating Charges: Nitrogen

Calculating Charges: Oxygen

Section 2: Objectives

Describe the size of an atom. Name the parts of an atom. Describe the relationship between

numbers of protons and neutrons and atomic number.

State how isotopes differ. Calculate atomic masses. Describe the forces within an atom.

The Atom

Now that we know what an atom is, let’s discuss what it is made up of.

As tiny as an atom is, it is made up of smaller parts: Protons Neutrons Electrons

The Atom

Protons: subatomic particles that have a positive charge

Neutron: subatomic particle that has no charge

Both protons and neutrons located in the nucleus.

If the nucleus is only made up of protons and neutrons, what does this mean about the charge?

The Atom

Electrons are found around the nucleus within electron clouds.

Electrons are very small in mass. Example: It takes more than 1,800 electrons to

equal the mass of 1 proton.

What happens if the number of protons and electrons are not equal?

The Atom

There are more than 110 elements. Why aren’t all atoms the same?

Because of the numbers of protons, neutrons, and electrons.

Start with the basics: To build the simplest element (hydrogen),

place one proton in the center of an atom. It’s so simple it has no neutron. Then add one electron in the electron cloud. You have just created a hydrogen atom.

The Atom

The Atom

If you were to build an atom that had more than one proton, you must have neutrons.

Neutrons keep the protons from repelling. (like charges)

How many electrons would the atom have to contain to have a neutral charge? You just created helium.

The Atom

How can you tell which elements these atoms represent?

The key is the number of protons.

The atomic number is the number of protons in the nucleus.

The Atom

Atoms of the same element have the same atomic number.

Isotopes: atoms that have the same number of protons but different numbers of neutrons Atoms that are isotopes of each other

have the same number of protons. They have different numbers of

neutrons which give them different masses.

The Atom

In order to sketch an atom: Determine mass number and atomic

number. Subtract atomic number from mass

number. Determine number of electrons. Sketch your atom.