4.1 Studying Atoms Atomic Theory

The Atom If we live in a lego world, then every lego piece is an atom There are all types of atoms of different elements … there are oxygen atoms, hydrogen atoms, etc. What do you know about atoms? How can we study something that we can’t see?

Albert Einstein:

Just for Fun, historical context: Origin of the idea of atoms Aristotle believed that everything in the world was made of the elements Air, Water, Earth and Fire and the idea was commonly accepted at the time

Just for Fun, historical context: Origin of the idea of atoms This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”

The concept of the atom Just how small is an atom? Ted ed Vid. https://www.ted.com/talks/just_how_small_is_an_atom History of atomic theory. Vid. https://www.youtube.com/watch?v=-4Us5PTb4J8 The 2,400-year search for the atom - Theresa Doud https://ed.ted.com/lessons/the-2-400-year-search-for-the-atom-theresa-doud

Models of the atom

Dalton’s Atomic Theory In 1808, an Englishman named John Dalton proposed a theory: 1. every element is made of tiny unique particles called atoms that cannot be subdivided 2. atoms of the same element have the same mass & atoms of different elements have different masses 3. compounds contain atoms of more than one element 4. in any particular compound, atoms of different elements always combine in the same way

Thomson & Rutherford Q: What did Dalton notice that all compounds have in common? A: Compounds contain atoms of more than one element JJ Thomson (1856-1940) used a sealed tube of gas to show that particles are charged

JJ Thomson (1856-1940) used a sealed tube of gas to show that particles are charged

In Thomson’s experiment, the particles repelled the negative plate and were attracted to the positive plate Q: What was the charge on the particles in the beam of the gas? Thomson’s experiments provided the first evidence that atoms are made of even smaller particles Plum-pudding model

Ernest Rutherford (1871-1937) discovered fast moving particles in uranium that he called alpha particles Uranium is a radioactive substance He used a zinc sulfide screen, gold foil and alpha particles to produce a very famous experiment in which he showed that the positive charges are not evenly dispersed throughout the atom

Rutherford concluded that atoms must have a concentrated central core Definition: a nucleus is a dense, positively charged mass located in the center of the atom According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus Today, much of an atom consists of empty space outside the positively charged nucleus

The atom Atoms consist of 3 parts: The nucleus contains 1. Protons: positive (+) charge, atomic weight = 1 2. Neutrons: No charge, atomic weight = 1 amu The outer shells contain 3. Electrons: negative charge (-), atomic weight ~ 0 (negligible)

4.1 Short Response What theory did Dalton propose about the structure of matter? What evidence did J.J. Thompson provide about the structure of an atom? What did Rutherford discover about the structure of an atom? What evidence did Thomson have that his glowing beam contained negative particles? Why was Dalton’s model of the atom changed after Thompson’s experiment? If you observed a beam of particles being bent towards a negatively charged plate, what might you conclude? In the Rutherford experiment, why weren’t all the alpha particles deflected?

4.2 The structure of an atom

The atom Atoms consist of 3 parts: The nucleus contains 1. Protons: positive (+) charge, atomic weight = 1 2. Neutrons: No charge, atomic weight = 1 amu The outer shells contain 3. Electrons: negative charge (-), atomic weight ~ 0 (negligible)

The atom The number of protons is what gives an atom its identity— it's called the “atomic number” In a neutral atom, protons = electrons Neutrons = mass – atomic number

The atom Atomic mass = number of protons + number of neutrons # of Protons = # of Electrons (unless there is a marked charge)

Abbreviations for subatomic particles p+ protons no Neutrons e- electrons

What is an isotope? Isotope (n): Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element. Example: Carbon-14, used in radioactive dating of historical artifacts

How do isotopes of the same element differ from one another? http://education- portal.com/academy/lesson/isotopes-and- calculation-of-a-weighted-average.html

How to read an element

Isotope exercise

Atoms with missing/extra electrons: Ions Cations: atoms with missing electrons. Positively charged due to surplus of protons compared with electrons Anions: atoms with extra electrons. Negatively charged due to deficit of protons compared with electrons.

Ions Exercise

Summary Page: How to read #s associated with an element +1 ←Charge

4.2 The structure of an atom Name 3 subatomic particles Name 3 properties you could use to distinguish a proton from an electron Which characteristic of an atom always varies among atoms of different elements? How are the isotopes of an element different from one another? What do neutrons and protons have in common? How are they different? How can atoms be neutral if they contain charged particles? What is the difference between atoms of oxygen-16 and oxygen- 17? What property do protons and electrons have that neutrons do not? Explain why it isn’t possible for an atom to have a mass number of 10 and an atomic number of 12.