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Atomic Structure
Atomic Theory:
Substances and elements are made up of small units called atoms. Atoms are the “building blocks” of matter.
Democritus. Ancient Greek philosopher who advocated the atomic theory of matter.
John Dalton’s atomic theory:
1. All matter is made up of very tiny particles called atoms
2. Atoms of the same element are alike…same size, mass, and other properties.
3. Atoms are not subdivided, created, or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.
The Modern Atomic Theory reflects more of what we know today such as….
 individual atoms of an element may not all have the same mass…different # of neutrons. However, the
atoms of an element have a definite average mass that is characteristic of the element
 atoms can be broken down into subatomic particles…protons, neutrons and electrons
Some basic laws we use:
Law of Conservation of Mass: Mass is neither created nor destroyed during ordinary chemical reactions or physical
changes.
Example: If the mass of the reactants is 10 g in a reaction, then the mass of the products will be 10 g.
If you start with 2 atoms of Mg as part of your reactants, you will have 2 atoms of Mg in your product.
Law of Definite Proportions: A chemical compound contains the same elements in exactly the same proportions by
mass regardless of the amount of the compound.
Example: The formula for water is H2O. It does not matter if you have a bucket or a glass, the formula is still H 2O.
Law of Multiple Proportions:
Whenever two elements form more than one compound, the different masses of one element that combine with the same
mass of the other element are in the ratio of small whole numbers.
Example 1: Carbon reacts with oxygen to form two compounds. Compound A contains 2.41 g of carbon for each
3.22 g of oxygen. Compound B contains 6.71 g of carbon for each 17.9 g of oxygen. What is the lowest wholenumber mass ratio of carbon that combines with a given mass of oxygen?
Example 2: In the compound nitrous oxide, also known as laughing gas, and used as an anesthetic in dentistry,
the mass ratio of nitrogen to oxygen is 7:4. A 68-g sample of a compound composed of nitrogen and oxygen
contains 42 g of nitrogen. Is this sample nitrous oxide?
Structure of the Atom
The atom consists of a tiny, massive central nucleus surrounded by electrons.
There are three sub-atomic particles inside the atom:
Particle
Relative Charge
Relative Mass
Proton
+1
1
Neutron
0
1
Electron
–1
0 (1/1840)
Location
nucleus
nucleus
Electron cloud
Atoms of different elements contain different numbers of protons.
Atoms have no overall charge. Therefore the number of protons is the same as the number of electrons.
I. The Electron
A. Discovery:
J. J. Thomson. Studied the newly invented cathode ray tube (a TV screen is a cathode ray tube). He
showed that atoms could be further subdivided into negative (which he named electrons) and positive
components. He postulated a "Plum Pudding" model for atoms. He calculated the charge to mass ratio
e/m for the electron by careful observations of the curvature of an electron beam in cathode ray tubes in a
magnetic field.
B. Inferences from the properties of electrons
1. Atoms are neutral, so there must be positive charges to balance the negatives
2. Electrons have little mass, so atoms must contain other particles that account for most of the mass
C. Millikan. He calculated the charge on the electron with his famous oil drop experiment. He measured the
static electrical charge on microscopic oil droplets by balancing droplets between charged plates.
D. Properties:
 negative charge, mass of 9.11 x10-31kg…it is the smallest of the three subatomic particles
 The electrons are fast moving.
 Electrons are arranged in energy levels around the nucleus.
 Each energy level can only contain a set number of electrons. Once an energy level is full any extra
electrons go into the next level.
II. The Nucleus
A. Discovery: Ernest Rutherford: The Gold Foil Experiment (1911)
1. Alpha particles (helium nuclei) fired at a thin sheet of gold
Assumed that the positively charged particles were bounced back if they approached a
positively charged atomic nucleus head-on (Like charges repel one another)
2. Very few particles were greatly deflected back from the gold sheet
a. nucleus is very small, dense and positively charged
b. most of the atom is empty space
B. Properties:
 Contains the protons and neutrons.
 It is positively charged (protons are positively charged and neutrons have no charge).
 It is the heavy part of the atom (protons and neutrons have much greater mass than electrons).
 It is small in comparison to the rest of the atom.
III. Structure of the Nucleus
1. Protons
a. Positive charge, mass of 1.673 x10-27kg
b. The number of protons in the nucleus determines the atom's identity & is called the atomic number
2. Neutrons
a. No charge, mass of 1.675x10-27kg
3. Nuclear Forces
a. Short range attractive forces:
neutron-to-neutron, proton-to-proton, proton-to-neutron
Questions:
1. What part of the atom has a positive charge? ___________ Why? ________________
2. What is the atomic number of helium? ___ fluorine? _____ manganese? ____ silver?____
3. How many protons does helium have? ____ fluorine? _____ manganese? ____ silver?____
4. How many electrons does helium have? ____ fluorine? _____manganese? ____ silver?____
5. Who discovered the nucleus? _________________________
6. Who discovered the electron?_________________________
7. Who discovered the charge on an electron? _______________________
8. Who conducted a gold foil experiment?_________________________
9 Who had the “plum pudding” idea for the atom?___________________
10. Who conducted an “oil drop” experiment?______________________
11. Where are neutron located?__________________
12. Nitrogen and oxygen can combine to form nitrogen monoxide as well as nitrogen dioxide. What concept
explains why the same mass of nitrogen can combine with different masses of oxygen to form different
compounds? ___________________________________