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Daily science- pg. 30 Create a table like the following on page 30 in your
notebook. Label this page History of the Atom. Leave a few lines for each scientist.
Scientist/Philosopher
Theory Year Model/Experiment
Democritus
Aristotle
John Dalton
J. J. Thompson
Ernest Rutherford
Niels Bohr
James Chadwick
New Atomic Theory
THE STRUCTURE OF ATOMS
Pg. 29
What makes up an atom?
Atom- smallest unit of an element that still retains that element’s properties
Atoms are made of subatomic particles. The three subatomic particles are the
proton, electron, and neutron.
Structure of an atom
The nucleus is located at the center of the atom.
It contains positively charged particles, called protons, and neutral particles, called neutrons.
Most of the mass is located here.
Structure of an atom
Electrons, or negatively charged particles, exist outside of the nucleus in the electron cloud.
Subatomic Particles chart
Particle Charge Mass Location
Proton + 1 1.67 x 10 -27 In the nucleus
Neutron 0 1.67 x 10 -27 In the nucleus
Electron -1 9.11 X 10 -31 Outside the nucleus
Atomic Number
The periodic table can be used to find the number of protons, electrons, and neutrons.
The atomic number of an element (labeled Z) tells you how many protons there are.
On the periodic table, the elements are listed as neutral elements so the number of electrons=the number of protons.
Atomic number
Z
Mass Number
The mass number (labeled A) is the number of neutrons + protons.
Mass number (if # neutrons is unknown)=atomic mass rounded to the nearest whole number
The atomic mass is located under the element symbol.
You can find the number of neutrons by subtracting the Atomic number (Z) from the Mass number (A)
A-Z = # of neutrons
Mass Number
Writing Atomic number and mass number
There is a way to represent atomic mass and number
Steps: 1-start with the element symbol 2- place the mass number on the top left
side of the symbol 3-place the atomic number on the bottom
left side of the symbol
Symbol
Al27
13
Isotopes
An isotope is an element that has the same number of protons but a different number of neutrons.
Ex. Carbon has 2 common isotopes: Carbon-12 Carbon-14
The numbers represent the mass number
Determining the number of neutrons Ex. Carbon 12 and Carbon 14
COUNTING ATOMS
Pg. 31
Atomic Mass
Atomic mass units (amu) are used to express an atoms mass
Atomic mass units are based off the element Carbon
Found under the element symbol
Average atomic mass
Average atomic mass is a weighted average of the isotopes of a given element.
More common isotopes have a greater effect on the average atomic mass than do less common isotopes.
Ex. Chlorine- 37 and Chlorine- 35 (average atomic mass is 35.45)
Percent Abundance
The atomic mass can be calculated by summing the products of each isotope’s percent abundance and that isotope’s atomic mass.
Mass contribution tells how much of each isotope contributes to the total atomic mass.
Mass contribution = (% abundance) x (atomic mass)
Percent Abundance example Chlorine 35 has an atomic mass of
34.969 amu and its percent abundance 75.770%. What is its mass contribution?
Chlorine 37 has an atomic mass of 36.966 amu and its percent abundance is 24.230%. What is its mass contribution.
Using the two previous answers, what is the average atomic mass of Chlorine?