Atomic Theory

Bellringer Suppose someone said they had a present for you. When they gave it to you it was in a bag that you couldn’t see through. What would you do to figure out what was inside the bag without opening it?

What is an atom? An atom is the smallest particle into which something can be divided. Building blocks of matter Over time, the theory of what an atom looks like has kept changing as scientists learn more.

Dalton Theory – late 1700’s Dalton proposed the theory that: All substances are made of atoms which cannot be created, destroyed or divided Atoms of the same element are exactly alike Atoms of different elements are different Atoms join with other atoms to make new substances

Thomson Theory – late 1800’s Electrons Discovered!! Based on his observations of electron beam behavior, Thomson proposed that the particles making up an electron beam have a negative charge. His model of the atom was that is was a solid structure of positive charge with enough electrons mixed in to make it neutral (no charge).

Thomson’s Model A solid mass with the electrons distributed throughout it. Often called the “plum pudding” model. A more modern comparison would be the “chocolate chip ice cream” model.

Rutherford Model – Early 1900’s Nucleus Discovered! Based on the way that positive particles were deflected in his gold foil experiments, Rutherford concluded that there was something extremely dense and positively charged in the center of a particle. His model of the atom was that of a positively charged nucleus at the middle surrounded by tiny electrons that constantly move.

Rutherford Model Dense nucleus in center surrounded by electrons that move constantly. Equal number of protons and electrons to keep it neutral.

Bohr’s Model Bohr proposed that while electrons are traveling around the nucleus, they do so on defined paths called levels. Each level can only hold a certain number of electrons Electrons can jump from one level to the next, but cannot be in between

Bohr’s Model Neutrons and protons located in the nucleus. Electrons were in specific orbits around the nucleus…..not moving randomly.

Electron Cloud Model Current theory is that the electrons are located in an “electron cloud” surrounding the nucleus. While they still exist in levels, it is thought that the path they travel is not quite as defined as Bohr suggested.

Electron Cloud Model (current theory for what an atom looks like)

Evolution of Atomic Theory

Summary Questions In what part of an atom is most of the mass located? What are 2 differences between the atomic theories as described by Thomson and Rutherford? What discovery demonstrated that atoms are not the smallest particles?

Bellringer Rewrite the following numbers in standard form: 3 x 10-8 2 x 1022

How Small is an Atom An atom is so small that their diameter is three hundred-millionths of a centimeter! 0.00000003 cm This means that a single penny contains twenty thousand billion billion atoms 20,000,000,000,000,000,000,000

So What is Inside an Atom? Nucleus made of protons and neutrons Electrons Lots of empty space

Protons Charge: Positive (+) Mass: 1.7 x 10-24 grams 0.0000000000000000000000017 1 atomic mass unit (amu) Location: Nucleus

Neutron Charge: None Mass: 1.7 x 10-24 grams 0.0000000000000000000000017 1 atomic mass unit (amu) Location: Nucleus

Electron Charge: Negative (-) Mass: almost zero Location: Electron clouds around nucleus

How Do Atoms of Different Elements Differ Each element has atoms that have a unique combination of protons, neutrons and electrons.

Law of Electric Charge Opposite charges are attracted to each other while like (same) charges repel each other (push each other away)

Rules for Building an Atom 1. The atomic number of an atom is equal to the number of protons in the atom. 2. All stable atoms have the same number of protons and electrons…so atomic number ALSO equals the number of electrons. 3. The atomic mass of an atom is equal to the number of protons PLUS number of neutrons in the nucleus of an atom.

How to Read Element Squares

Bellringer What is the difference between atomic number and atomic mass?

(Atomic mass = protons + neutrons) Answer Atomic number is the number of protons in the nucleus of an atom. (Atomic # = protons) Atomic mass is the sum of the number of protons AND neutrons in the nucleus. (Atomic mass = protons + neutrons)

Atomic mass is always the largest number