2.1 Atoms and Their Structure

ObjectiveRelate historic experiments to the development of the modern model of the atom.Illustrate the modern model of the atom.Interpret the information available in an element block of the periodic table.

Early Ideas About Matter Our current model of the composition of

matter is based on hundreds of years of work.

About 2500 years ago, the Greek philosophers thought about the nature of matter and its composition.

They proposed that matter was composed of four fundamental elements…

And these were… Air Water Fire Earth

Greek Philosophers They also debated whether matter

could be divided into smaller and smaller pieces endlessly or whether there was an ultimate smallest particle that could no longer be divided.

They were excellent observers, but they did not test their theories with experiments.

Democritus, 460-370 B.C. Proposed that the world is made up a

empty space and tiny particles called atoms.

He thought that atoms were the smallest particles of matter and that different types of atoms exist for every type of matter.

Atomic Theory- matter is made up of fundamental particles called atoms.

Antoine Lavoisier (1743-1794) French chemist In 1782 he made

measurements of a chemical change in a sealed container.

Lavoisier’s Conclusion He observed that the mass of the reactants

before the chemical reaction were equal to the mass of the products after the reaction. In a sealed container 2.0 g of hydrogen gas reacted

with 16.0 g of oxygen gas to produce 18.0 g of water

Matter was neither created nor destroyed during a chemical reaction, but changed.

This became known as the law of conservation of matter

Law of Conservation of Mass

Joseph Proust Joseph Proust, French chemist September 26, 1754 – July 5, 1826

Proust’s Contribution Observed that the composition of water is

always 11% hydrogen and 89% oxygen by mass. Regardless of its source

He observed many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This is referred to as law of definite proportions.

John Dalton

Dalton’s Atomic Theory John Dalton (1766-1844), an

English schoolteacher and chemist Studied the experiments of

Lavoisier, Proust, and many others. Based on this he developed his

atomic theory.

Dalton’s Atomic Theory

1. All matter is made up of atoms.2. Atoms are indestructible and

cannot be divided into smaller particles(Atoms are indivisible)

3. All atoms of one element are exactly alike, but they are different from atoms of other elements.

Dalton’s Atomic Theory This gave chemist a model of the

particle nature of matter, but it also raised a lot of questions.

If all elements are made up of atoms then why are there so many different types of elements?

What makes one atom different from another?

Dalton’s Atomic Theory Experiments performed late in the

19th century began to explain the properties and behavior of substances

This was done by the discovery of three smaller particles. Protons, electrons and neutrons

Atomic Theory, Conservation of Matter and Recycling What happens to the stuff you

throw away? What happens to the atoms?

As you have already learned matter is neither created nor destroyed, so what happens.

What happens? When waste is incinerated or

buried the atoms may combine with oxygen and other substances to form new compounds.

The atoms are “recycled”

Recycling Recycling has become a part of life

now. Much of what you buy is either

recycled or can be recycled. We have found the advantage of

recycling materials and therefore atoms. We mimic what nature does and conserve our natural resources.

Hypotheses Hypothesis – testable prediction to

explain observations. Hypotheses are based on

observations. They can be proven correct or

incorrect by the experiments that are designed to test them.

Hypothesis

Theories Theory- an explanation based on

many observations and supported by the results of many experiments.

As scientists gather new information, a theory may be revised or replaced.

Theory

Laws Scientific Law – a fact of nature that is

observed so often that it is accepted as the truth.

Examples: Sun rises in the east

A law can generally be used to make predictions but does not explain why something happens.

Theories explain laws. One part of Dalton’s atomic theory explains why

the law of conservation of matter is true.

Review What is the difference between a

theory and a hypothesis? What is the Law of Conservation of

Mass?

The Discovery of Atomic Structure Dalton’s atomic theory was almost true. He assumed that atoms are the ultimate

particles of matter and can’t be broken up into smaller particles and that all atoms of the same element are identical.

His theory needed to be modified with the discovery of electrons, protons and neutrons.

The Electrons In 1897, JJ Thomson a British

physicist discovered that the solid ball of model was not accurate.

His experiments involved the use of a cathode ray.

Cathode Ray Composed of a vacuum tube At the end of the tube is a piece of metal

called on electrode which is connected to a metal terminal on the outside of the tube.

The electrode becomes electrically charged when connected to a high voltage.

When the electrode is charged rays travel in the tube from the negative electrode(the cathode) to the positive electrode (the anode)

Cathode Ray

Cathode Ray

Thomson’s Discovery What he found that the ray would bend

towards a positive charged plate and away from a negatively charged plate.

He concluded that the rays are composed of invisible negatively charged particles he called electrons

Electron – negatively-charged particle

Early Atomic Model The early atomic model was

referred to as the “plum pudding model”; you could more closely relate it to a chocolate chip cookie

Scientists believed that atoms were balls of positive charge with the negatively charged particles embedded in them.

Changes in the Atomic Model

Solid BallModel

Plum PuddingModel

Ernest Rutherford In 1909 he carried

out the first of the experiments that would reveal an arrangement far different from the “plum pudding” model

Rutherford’s Experiment Gold Foil Experiment He set up a lead-shielded box containing

polonium, which emitted a positively charged beam of alpha particles.

When the beam struck a sheet of gold foil, most of the particles passed straight through the foil

However, some of the particles from the beam were deflected. Some were only slightly deflected and some bounced straight back.

Gold Foil Experiment

Gold Foil Experiment

The Nuclear Model of the Atom Based on Rutherford’s work the team

devised a new model for the atom. Because some of the particles

bounced straight back they concluded that atoms must have a dense central core called the nucleus

Nucleus – a small, dense, positively charged central core of an atom

Nuclear Model

Changes in the Atomic Model

Solid BallModel

Plum PuddingModel

Nuclear Model

Atomic Model It was hard for people to grasp that atoms

contained a lot of empty space. When they looked at a rock, it was very

difficult to see how most of this object could be empty space.

If you were to enlarge an atom of hydrogen so the nucleus was the size of a golf ball, the electron would be a mile a way.

In one drop of water there is 6,500,000,000,000,000,000,000 (6.5 sextillion) atoms

Atomic Numbers and Masses The nucleus of an atom is composed

of protons and neutrons. Electrons move in the space around the nucleus.

Atomic Number – the number of protons in the nucleus of an atom. Every element has a unique atomic

number. Therefore, it is the number of protons that determines the identity of an element.

Atoms Protons - positively charged subatomic

particles. Neutrons – subatomic particle that does

not have a charge, it is neutral Atoms have no overall charge. Therefore the number of protons and

electrons must be equal. If you elements atomic number is 2

2 protons 2 electrons

Mass Number Mass number - the number of protons

and neutrons in the nucleus of an atom.

Isotope – atom of an element with a different number of neutrons and therefore a different mass number Ne-20 = neon 20(10 protons, 10 neutrons) Ne-21 = neon 21(10 protons, 11 neutrons)

Neon

Composition of Atoms You can

determine the composition of any element if you know the Atomic number Mass number

Atomic Mass Since elements have different isotopes

the atomic mass is the weighted average.

Located under the symbol on the periodic table.

You can use the atomic mass to determine the mass number of a element You take the atomic mass rounded up

Determining Atomic Mass

62.930 X 0.6917 = 43.52964.928 X 0.3083 = 20.017

43.529 + 20.017 = 63.546

Review What information does the atomic

number give us? How do we determine the number

of neutrons of any element? Who discovered the electron? What discovery did the Gold Foil

Experiment led to?