1. Preview Section 1 Development of the Atomic Theory Section 2 The Atom
Introduction to Atoms
Preview
Section 1 Development of the Atomic Theory
Section 2 The Atom
Concept Mapping

2. Section 1 Development of the Atomic Theory
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.

3. The Beginning of the Atomic Theory
Section 1 Development of the Atomic Theory
The Beginning of the Atomic Theory
What Is an Element? Around 440 BC, a Greek philosopher named Democritus thought that you would eventually end up with a particle that could not be cut. He called this particle an atom.
Democritus was a philosopher, not a scientist.
Aristotle, another Greek philosopher, disagreed with Democritus’s ideas. He believed that you would never end up with a particle that could not be cut.

4. The Beginning of the Atomic Theory, continued
Section 1 Development of the Atomic Theory
The Beginning of the Atomic Theory, continued
Democritus was right, though:
Matter is made of particles, which we call atoms.
An atom is the smallest unit of an element that maintains the chemical properties of that element.

5. John Dalton was a scientist. Democritus was not.
Section 1 Development of the Atomic Theory
Dalton’s Theory
John Dalton was a scientist. Democritus was not.
John Dalton published his atomic theory in His theory stated that all substances are made of atoms.
Atoms are small particles that cannot be created, divided, or destroyed (divided into smaller pieces).
He got that last bit wrong. Atoms CAN be divided:
Into protons, neutrons, electrons

6. Dalton’s Theory , continued
Section 1 Development of the Atomic Theory
Dalton’s Theory , continued
Atoms of the same element are exactly alike.
Atoms of different elements are different.
Different numbers of protons, neutrons, and electrons
Atoms join with other atoms to make new substances.

7. Dalton’s Atomic Theory Based on Experiments
Section 1 Development of the Atomic Theory
Dalton’s Atomic Theory Based on Experiments
Dalton Not Quite Correct
So…the atomic theory has changed, as more has been discovered, to describe the atom more correctly.

8. Thomson’s Discovery of Electrons
Section 1 Development of the Atomic Theory
Thomson’s Discovery of Electrons
Negatively Charged Particles JJ Thomson experimented with a cathode-ray tube like the one shown on the next slide. He discovered negatively charged particles that are now known as electrons.
Plum Pudding Model After learning that atoms contain electrons, Thomson proposed a new model of the atom. Thomson thought that electrons were mixed throughout an atom, like plums in a pudding.

9. Section 1 Development of the Atomic Theory

10. Rutherford’s Atomic “Shooting Gallery”
Section 1 Development of the Atomic Theory
Rutherford’s Atomic “Shooting Gallery”
Negatively Charged Particles In 1909, Ernest Rutherford aimed a beam of small, positively charged particles at a thin sheet of gold foil. The next slide shows his experiment.
Surprising Results Rutherford expected the particles to pass right through the gold in a straight line. To Rutherford’s great surprise, some of the particles were deflected.
The particles had hit something, unknown and big!

11. Section 1 Development of the Atomic Theory

12. Where Are the Electrons?
Section 1 Development of the Atomic Theory
Where Are the Electrons?
Electrons are Far from the Nucleus!
Rutherford proposed that in the center of the atom is a tiny, positively charged part called the nucleus.
Bohr’s Electron Levels In 1913, Niels Bohr proposed that electrons orbit the nucleus in certain paths, or energy levels.

13. Where Are the Electrons?
Section 1 Development of the Atomic Theory
Where Are the Electrons?
Bohr’s Electron Levels In 1913, Niels Bohr proposed that electrons orbit the nucleus in certain paths, or energy levels.
We now know that electrons are found in clouds, known as orbitals. They don’t follow specific orbits like planets around the sun.
Werner Heisenberg. Mid-1900s. Orbitals.

14. Where Are the Electrons?, continued
Section 1 Development of the Atomic Theory
Where Are the Electrons?, continued
The Modern Atomic Theory According to the current theory, there are regions inside the atom where electrons are likely to found. These regions are called electron clouds, or orbitals.

15. Comparing Models of the Atom
Section 1 Development of the Atomic Theory
Comparing Models of the Atom
Click below to watch the Visual Concept.
Visual Concept

16. Section 2 The Atom
Bellringer
Answer the following question: An atom is the smallest particle into which an element can be divided and still be that element. Now that scientists have learned that an atom is made up of even smaller particles, is this definition still accurate?
Explain your answer in your science journal.

17. Objectives Describe the size of an atom. Name the parts of an atom.
Section 2 The Atom
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.

18. Section 2 The Atom
How Small Is an Atom?
Three One-Hundred-Millionths of a Centimeter! Scientists know that aluminum is made of average-sized atoms. An aluminum atom has a diameter of about cm.

19. What Is an Atom Made Of? Inside The Nucleus: Protons and Neutrons
Section 2 The Atom
What Is an Atom Made Of?
Inside The Nucleus: Protons and Neutrons
Protons are positively charged particles
Neutrons have no electrical charge.
Outside the Nucleus: Electrons
Electrons are the negatively charged particles Electrons are found around/outside the nucleus within orbitals.

20. Section 2 The Atom

21. How Do Atoms of Different Elements Differ?
Section 2 The Atom
How Do Atoms of Different Elements Differ?
Starting Simply
A hydrogen atom (atomic number = 1) has one proton and one electron.
The helium atom (atomic number = 2) has two protons, two neutrons, and two electrons.

22. How Do Atoms of Different Elements Differ?
Section 2 The Atom
How Do Atoms of Different Elements Differ?
Building Bigger Atoms For bigger atoms, simply add protons, neutrons, and electrons.
Protons and Atomic Number
The number of protons tells you what the element is.
The number of protons in the nucleus = the atomic number of that atom.
All atoms of an element have the same atomic number.

23. Section 2 The Atom
Isotopes
Isotopes are atoms of an element that have the same number of protons but different numbers of neutrons.

24. Section 2 The Atom
Isotopes, continued
Properties of Isotopes An unstable atom is an atom with a nucleus that will change over time. This type of isotope is radioactive.
Telling Isotopes Apart You can identify each isotope of an element by its mass number. The mass number is the sum of the protons and neutrons in an atom.

25. Section 2 The Atom
Isotopes, continued

26. Section 2 The Atom
Isotopes, continued
Naming Isotopes To identify a specific isotope of an element, write the name of the element followed by a hyphen and the mass number of the isotope.
Calculating the Mass of an Element The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element.

27. Section 2 The Atom

28. Section 2 The Atom
Forces in Atoms
Four Basic Forces Four basic forces are at work everywhere, even within the atom. These forces are gravitational force, electromagnetic force, strong force, and weak force.
These forces work together to give an atom its structure and properties.

29. Section 2 The Atom

30. Introduction to Atoms
Concept Mapping
Use the terms below to complete the concept map on the next slide.
a nucleus atoms
mass number electrons
isotopes atomic number
protons

31. Introduction to Atoms

32. Introduction to Atoms