CHAPTER 1ATOMS, ELEMENTS, COMPOUNDS AND MIXTURES…..

SEC 1MODEL OF AN ATOM

THE STUDENTS WILL:• Explain how scientists discovered

subatomic particles.• Explain how today’s model of the

atom developed.• Describe the structure of the nuclear

atom.

Terms to Know:

• Atoms• Element• Electrons• Protons• Neutron• Electron cloud

What is an Element?

• A substance that cannot be broken down into simpler substances is called an element.

• Ex. Gold (Periodic Table of Element)

WHO ARE THESE MEN?

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

What is an Atom?- “ATOMOS” Greek name- means uncuttable or

something that cannot be divided further-A basic unit of matter- All matter is made up of atoms.

History of Atom….

1. Democritus developed the idea of atoms.

He pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called ATOMA

460 BC

2. JOHN DALTON

1808

-suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them ATOMS.

DALTON’S CONCEPT OF ATOMS (Atomic Theory)• Matter is made up of Atoms.• Atoms cannot be divided into smaller

pieces.• All the atoms of an element are exactly

alike.• Different elements are made up of different

kinds of atom

William Crooke• 1870 Experimented with a Glass

Tube using Electrodes• Cathode Ray Tube (CRT)

- Crooke hypothesized that:the green glow inside the tube

was caused by rays or streams of particles….called cathode rays.

3. JOSEPH JOHN THOMPSON-English Physicist - Repeated Crooke’s Experiment- Cathode Rays are negatively charged

1898

THOMSON’S MODEL

Thomson studied the passage of 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 atoms of the gas were uncharged. Where had the negative charges 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!

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• 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.

4. Ernest Rutherford

• Atoms are mostly empty space.• Negative electrons orbit a

positive nucleus.

1910

Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil….-

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

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

• Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus.

• He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction

Parts of an ATOM:PROTONS- positively charged particles inside

the nucleus of an atom.ELECTRONS- negatively charged particles

moving around the nucleus.NEUTRONS- neutral charge (same mass as

the protons)inside the nucleus.

ELECTRON CLOUD

• Area around the nucleus of an atom where electrons travel.

Atomic Model