4.1 studying atoms section 1: the development of atomic theory preview key ideas bellringer the...

4.1 Studying Atoms

Section 1: The Development of Atomic Theory

Preview• Key Ideas • Bellringer• The Beginnings of Atomic Theory• Dalton’s Atomic Theory• Thomson’s Model of the Atom• Rutherford’s Model of the Atom

4.1 Studying Atoms

Key Ideas〉Who came up with the first theory of atoms?

〉What did Dalton add to the atomic theory?

〉How did Thomson discover the electron?

〉What is Rutherford’s atomic model?

4.1 Studying Atoms

Bellringer

When scientists wanted to find out what an atom was, they were not able to look directly at what the atom was made of. They had to make inferences from the results of many different experiments. It was like trying to describe a picture, such as the one on the next slide, with only small portions visible.

4.1 Studying Atoms

Bellringer, continued

1. Write four sentences describing what you can see of the above picture.

2. What information or parts of the picture would make your descriptions more accurate without revealing the entire picture?

4.1 Studying Atoms

Studying the structure of atoms is a little like studying wind. Because you cannot see air, you must use indirect evidence to tell the direction of the wind. Atoms pose a similar problem because they are extremely small. Even with a microscope, scientists cannot see the structure of an atom.

4.1 Studying Atoms

If you cut a piece of aluminum foil in half, you have two smaller pieces of the same shiny, flexible substance. You could cut the pieces again and again. Can you keep dividing the aluminum into smaller pieces? Greek philosophers debated a similar question about 2500 years ago.

Ancient Greek Models of Atoms

4.1 Studying Atoms

The philosopher Democritus believed that all matter consisted of extremely small particles that could not be divided. He called these particles atoms from the Greek word atomos, which means “uncut” or “indivisible.”


4.1 Studying Atoms

Aristotle thought that all substances were made of only four elements—earth, air, fire, and water. He did not think there was a limit to the division of matter. For many centuries, most people accepted Aristotle’s views on the structure of matter. By the 1800s, scientists had enough experimental data to support an atomic model.


4.1 Studying Atoms

The Beginnings of Atomic Theory

〉Who came up with the first theory of atoms?

〉In the fourth century BCE, the Greek philosopher Democritus suggested that the universe was made of indivisible units called atoms.

• Democritus did not have evidence for his atomic theory.

4.1 Studying Atoms

What was Dalton’s theory of the structure of matter?

Dalton’s Atomic Theory

Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

4.1 Studying Atoms

Evidence for Atoms

John Dalton studied the behavior of gases in air. Based on the way gases exert pressure, Dalton correctly concluded that a gas consists of individual particles.

Dalton measured masses of elements that combine when compounds form. The ratio of the masses of the elements in each compound was always the same. In other words, compounds have a fixed composition.


4.1 Studying Atoms

Dalton’s Atomic Theory, continued

• Dalton used experimental evidence.– Law of definite proportions: A chemical compound always

contains the same elements in exactly the same proportions by weight or mass.

• Dalton’s theory did not fit all observations

4.1 Studying Atoms

When magnesium burns, it combines with oxygen. In magnesium oxide, the ratio of the mass of magnesium to the mass of oxygen is always about 3 : 2. Magnesium dioxide has a fixed composition.


4.1 Studying Atoms

Dalton’s Theory• All elements are composed of atoms.• All atoms of the same element have the

same mass, and atoms of different elements have different masses.

• Compounds contain atoms of more than one element.

• In a particular compound, atoms of different elements always combine in the same way.


4.1 Studying Atoms

Dalton made these wooden spheres as a model to represent the atoms of different elements. A tiny, solid sphere with a different mass represents each type of atom.


4.1 Studying Atoms

A theory must explain the data from many experiments. Because Dalton’s atomic theory met that goal, the theory became widely accepted.

Over time, scientists found that not all of Dalton’s ideas about atoms were completely correct. They revised the theory to take into account new discoveries.


4.1 Studying Atoms

What contributions did Thomson make to the development of atomic theory?

Thomson’s Model of the Atom

Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.

4.1 Studying Atoms


〉How did Thomson discover the electron?

〉Thomson’s cathode-ray tube experiment suggested that cathode rays were made of negatively charged particles that came from inside atoms.

4.1 Studying Atoms

When some materials are rubbed, they gain the ability to attract or repel other materials. Such materials are said to have either a positive or a negative electric charge.

• Objects with like charges repel, or push apart. • Objects with opposite charges attract, or pull

together.


4.1 Studying Atoms

Amber is the hardened form of a sticky, viscous liquid that protects trees from insects and disease. If amber is rubbed with wool, it becomes charged and can attract a feather.


4.1 Studying Atoms

Thomson’s Experiments

In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of gas.


Sealed tube filled with gas at low pressure

Glowing beam

Metal disk Source of electric current

Metal disk


4.1 Studying Atoms

Thomson’s Experiments

In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of gas.


Sealed tube filled with gas at low pressure

Glowing beam


Metal disk


Positive plate

Negative plate

4.1 Studying Atoms

When the current was turned on, the disks became charged, and a glowing beam appeared in the tube.

• Thomson hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and caused the air to glow.

• Thomson observed that the beam was repelled by the negatively charged plate and attracted by the positively charged plate.


4.1 Studying Atoms

Evidence for Subatomic Particles

Thomson concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. He hypothesized that the particles came from inside atoms because

• no matter what metal Thomson used for the disk, the particles produced were identical.

• the particles had about 1/2000 the mass of a hydrogen atom, the lightest atom.


4.1 Studying Atoms

Thomson’s Model

Thomson revised Dalton’s model to account for these subatomic particles.

• The atom has neither a positive nor a negative charge, but there must always be some positive charge in the atom.

• The atom is filled with a positively charged mass of matter that has negative charges evenly scattered throughout it.


4.1 Studying Atoms

Thomson’s model is called the “plum pudding” model. Today, it might be called the “chocolate chip ice cream” model.

The chips represent negatively charged particles, which are spread evenly through a mass of positively charged matter—the vanilla ice cream.


4.1 Studying Atoms

Thomson’s Model of the Atom, continued

• Thomson developed the plum-pudding model.

– In his cathode-ray tube experiment, Thomson had discovered electrons.

• electron: a subatomic particle that has a negative charge

– Thomson’s plum-pudding model: electrons are spread throughout the atom, like blueberries in a muffin

4.1 Studying Atoms

What contributions did Rutherford make to the development of atomic theory?

Rutherford’s Atomic Theory

According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus.

4.1 Studying Atoms

Rutherford’s Model of the Atom, continued

• Rutherford conducted the gold-foil experiment.

• Rutherford discovered the nucleus.

– nucleus: an atom’s central region, which is made up of protons and neutrons

4.1 Studying Atoms

Rutherford’s Hypothesis

Ernest Rutherford designed an experiment to find out what happens to alpha particles when they pass through a thin sheet of gold. Alpha particles are fast-moving, positively charged particles.

• Based on Thomson’s model, Rutherford hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle.

• He predicted that most particles would travel in a straight path from their source to a screen that lit up when struck.


4.1 Studying Atoms

The Gold Foil Experiment


Beam of alpha particles

Source of alpha particles

Slit

Deflected particle

Undeflected particle

Screen

Gold atoms

Alpha particles

Alpha particles

Nucleus

4.1 Studying Atoms

Discovery of the Nucleus

The alpha particles whose paths were deflected must have come close to another charged object. The closer they came, the greater the deflection.

However, many alpha particles passed through the gold without being deflected. These particles did not pass close to a charged object.


4.1 Studying Atoms

Thomson’s model did not explain all of the evidence from Rutherford's experiment. Rutherford proposed a new model.

• The positive charge of an atom is not evenly spread throughout the atom.

• Positive charge is concentrated in a very small, central area.

• The nucleus of the atom is a dense, positively charged mass located in the center of the atom.


4.1 Studying Atoms

The Houston Astrodome occupies more than nine acres and seats 60,000 people. If the stadium were a model for an atom, a marble could represent its nucleus. The total volume of an atom is about a trillion (1012) times the volume of its nucleus.


4.1 Studying Atoms

Assessment Questions

1. Dalton’s theory did not include which of the following points? a. All elements are composed of atoms.

b. Most of an atom’s mass is in its nucleus.

c. Compounds contain atoms of more than one element.

d. In a specific compound, atoms of different elements always combine in the same way.

4.1 Studying Atoms


1. Dalton’s theory did not include which of the following points? a. All elements are composed of atoms.

b. Most of an atom’s mass is in its nucleus.

c. Compounds contain atoms of more than one element.

d. In a specific compound, atoms of different elements always combine in the same way.

ANS: B

4.1 Studying Atoms


2. J. J. Thomson’s experiments provided the first evidence of a. atoms.b. a nucleus.c. subatomic particles.d. elements.

4.1 Studying Atoms


2. J. J. Thomson’s experiments provided the first evidence of a. atoms.b. a nucleus.c. subatomic particles.d. elements.

ANS: C

4.1 Studying Atoms


1. The concept of an atom as a small particle of matter that cannot be divided was proposed by the ancient Greek philosopher, Democritus.

TrueFalse

4.1 Studying Atoms


1. The concept of an atom as a small particle of matter that cannot be divided was proposed by the ancient Greek philosopher, Democritus.

TrueFalse

ANS: T

4.1 studying atoms section 1: the development of atomic theory preview key ideas bellringer the...

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