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Atomic Theory and the Periodic Table

Chapter 3

Tiffany Johnson

on 21 September 2012

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Transcript of Atomic Theory and the Periodic Table

Chapter 3 Atomic Theory History Democritus: 460 – 370 B.C.
Proposed the world is empty space and atoms.
Gave the name atom meaning indivisible
Explained why changes in matter were possible: the atoms were being rearranged. John Dalton’s “Atomic Theory of Matter” (1803)
All matter is composed of atoms
Atoms of an element are similar in size & mass, but different elements are have a different mass & size
Atoms cannot be subdivided, created, or destroyed
Different elements combine to make compounds
Atoms can combine, separate or rearrange in reactions Ancient Greeks believed all matter was made up of four basic elements: fire, earth, water, and air. JJ Thomson (1897)
Experiment: Cathode Ray Tube
Thomson released atomic particles from one side of the tube and they were attracted to the positive plate. Cathode Ray led to the hypothesis:
Atoms are composed of electrically charged particles (subatomic particles).
The particles that were attracted to the positive plate were _________ (positive or negative?). Thomson's Theory
Atoms are composed of charged particles (subatomic particles).
The particles that were attracted to the positive plate were negative.
These were called “electrons”
Protons were discovered the same way. Discovery of Atomic Structure Rutherford (1909)
Experiment: Gold Foil Experiment
Most particles pass through, but some are Model: Rutherford explained that atoms must be mostly empty space with a small, concentrated center of positive charge, called the nucleus. Chadwick (1932)
Discovered the neutron
Neutron is a subatomic particle roughly the size of a proton (large compared to electrons). Discovery of Atomic Structure Bohr (1913)
Model: proposed the “electron cloud” in which electrons orbit at a given distance from the nucleus.
Small orbits = low energy.
Big orbits = high energy. Quantum mechanical Model

Mathematical model that predicts the location of electrons in the electron cloud. The Atom Major Parts of the Atom Nucleus: dense, central part of the atom
Protons and neutrons are found in the nucleus. Electron cloud: large area outside of the nucleus
Electrons occupy the electron cloud. Parts of the Atom Subatomic
Particles Electrons:
located in e-cloud, negatively charged, tiny in size. Neutron:
In the nucleus, no charge and large size Proton:
In the nucleus, positive charge, large in size Atoms and the Periodic Table What do all the numbers mean????? What do all the numbers mean? Atoms and the Periodic Table Atomic Number – the whole number in an element’s box on the periodic table.
Atomic # = # protons = # electrons

The atomic # for Hydrogen is:

The atomic number determines an element’s identity! The atomic # for Chlorine (Cl) is ____.

Chlorine has ____ protons.

Chlorine has ____ electrons. An atom has 24 protons. What element is it? Mass Number
the sum of the protons and neutrons;
This number isn’t on the periodic table, because the number of neutrons can vary (these are called isotopes) Atomic Mass
The decimal number on the periodic table. The average mass of all isotopes of that element.

The atomic mass for sulfur is _____. Isotopes Bohr Model Atoms of the same element that have different
Typical Hydrogen atom has one proton
Naturally occurring Hydrogen atoms have different numbers of neutrons The Isotope of an element has the same number of protons and electrons but different numbers of electrons
In all three forms of hydrogen the positive charge of the proton is balanced by the negative charge of the electron Mass Number The total number of protons and neutrons in the nucleus of an isotope How to write isotopes
2 methods Method 1:
Subscript/Superscript method
The atomic # is your subscript (below) and the mass # is the superscript (above), both on the left side of the symbol. Example: 235 U 235 – mass # and 92 – atomic #
92 Method 2:
Hyphen-notation method

The symbol is written, then hyphen, then mass #. Example U-235.
Mass # = 235.
Atomic # = ?? More Practice What is the atomic number of zinc? ___ How many electrons does silver have? ____ How many protons does iodine have? ____ How many neutrons does Fe-53 have? ____ What is the mass number of Chlorine-37? ____ How many electrons does Chlorine-37 have? _____ What is the mass number of tritium? ____ How many neutrons does neon-20 have? _____ How many neutrons does deuterium have? ____ Bohr Diagrams 1. Find your element on the periodic table. 2. Determine the number of electrons – it is the same as the atomic number. 3. This is how many electrons you will draw. 1. Find out which period (row) your element is in.
2. Elements in the 1st period have one energy level.
3. Elements in the 2nd period have two energy levels, and so on. 1. Draw a nucleus with the element symbol inside.
2. Carbon is in the 2nd period, so it has two energy levels, or shells.
3. Draw the shells around the nucleus. 1. Add the electrons.
2. Carbon has 6 electrons.
3. The first shell can only hold 2 electrons. 1. Since you have 2 electrons already drawn, you need to add 4 more.
2. These go in the 2nd shell.
3. Add one at a time -starting on the right side and going counter clock-wise. 1. Check your work.
2. You should have 6 total electrons for Carbon.
3. Only two electrons can fit in the 1st shell.
4. The 2nd shell can hold up to 8 electrons.
5. The 3rd shell can hold 18, but the elements in the first few periods only use 8 electrons. Practice Try the following elements on your own:
K You should know how to draw a Bohr Diagram for the first 20 elements. The Mole
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