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Chemistry Unit 2: Atomic Theory

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Michelle Durham

on 6 February 2015

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Transcript of Chemistry Unit 2: Atomic Theory

Gram/ mole conversions
contribute to the mass and charge of an atom (determine identify of the atom)
contribute to the mass only
contribute to the charge only (determine bonding and reactivity)
J.J.Thomson also revelealed the mass to charge ratio of an electron

determined the mass of a electron to be 1/2000 that of the smallest known atom (hydrogen)
Atoms: The Building Blocks of Matter
CLE: 3221.1.1~ Compare and Contrast the historical models of the atom
CLE 3221.3.4~ Explain the law of conservation of mass
CLE 3221.1.3~ Describe the atom in terms of its composition and electron characteristics
CLE 3221.Math2~use appropriate math equations and processes to solve chemistry problems
I Can....
Explain the law of definite proportions, and the law of multiple proportions
Summarize the 5 principle of Dalton's atomic theory
Discuss the contributions of Democritus, Thomson, Rutherford, Bohr and Chadwick to the foundations of the atomic theory
Compare and Contrast the proton, neutron, and electron
Differentiate between atomic mass and atomic number
Determine the number of particles in a mole of a substance and how much it weighs
Calculate the number of protons, neutrons and electrons in a nuclide
Given the number o fmoles in a substance calculate the number of atoms
Aristotle v Democritus
all matter is continuous- no such thing as ATOMS
particle theory of matter- matter is divisible down to its smallest particle the "atom"
coined phrase atomus meaning indivisible
400 BC
neither support with experimentation
~by the 1700's
we know that:
elements combine to form compounds with different properties than the elements that made them
an element is defined as a substance that cant be broken down any further
What we DON'T know:
if elements form compound in fixed proportions (ratios)?
Law of Conservation of Mass: mass is neither created not destroyed in a chemical reaction!

experimented with HgO Hg + O
Lavoisier and Proust~
Law of Definite Proportions: compounds contain the same elements in EXACTLY the same proportions by MASS regardless of sample size.

NaCl is ALWAYS 60.66% Na and 39.34 % Cl
Law of Multiple Proportions:

If 2 or more DIFFERENT compounds are composed of the same elements then the ratios of the masses of the second element combined with a certain mass of the first element is always in a whole number ratio
EX:Take an example of two mineral samples iron pyrite (FeS2) and iron troilite (FeS).

Both contain iron and sulfur atoms.

However, for a given fixed amount of iron it requires exactly twice the mass of sulfur needed to make pyrite than that of troilite with the same amount of iron.
CO and CO
O:C ratio O:C ratio
16g: 12g 32g: 12g
1:1 2:1
H O and H O
H:O ratio H:O ratio
1g:16g 2g:32g
2:1 2:2
Dalton's Atomic Theory
creates a cumulative explanation that combines the law of conservation of mass, law of definite proportions and law of multiple proportions
1. all matter is composed of atoms
2. atoms of same element are the identical (size, mass, properties)
3. atoms cannot be divided, created or destroyed
4. atoms of different elements combine in whole mumber ratios to create compounds
5. in chemical reactions, atoms are combined, separated and rearranged
English School Teacher
~ law of conservation of mass
Today- Modern Atomic Theory

some changes have been made to Dalton's original theory.
atoms are divisible (though the law of conservation of mass is always maintained)
atoms of the same element can have different masses (isotpoes)

the important concepts still reamain unchanged.
all matter is composed of atoms
atoms of one element differ in properties from atoms of other elements
Particles smaller than an atom?
cathode-ray tube~ glass tubes containing gas as very low pressure

when electricity was passed through the tube the opposite end glowed, hypothesized that this was caused by a stream of particles they called a cathode ray

these rays were tested and the following discovered:
rays were deflected (moved away from) by a negative charged magnetic field
ratio of the charge of the particles: mass of particles was determined to always be the same regardless of the metal or gas used in the cathode-ray tube
Called these particles that made up the cathode ray~ ELECTRONS
cathode ray experiments provide evidence that:
atoms are divisible!
further inferences regarding the atom:
because atoms are electrically neutral they must also contain some sort of positive charge to balance the negative electron
because electrons have such a small mass there must be other particles/substance present to account for the mass of the atom
Thomson's proposed model of the atom
plum pudding model
electrons spread evenly throughout a positive charged mass
electrons contribute to charge but very little to mass
The Nucleus
Earnest Rutherford ~ 1911
Gold Foil Experiment
bombarded a thin sheet of gold foil with alpha particles
alpha particles: + charged particles 4X mass of H
EXPECTED particles to pass through the foil (due to equal distribution of charge and mass of gold atoms in the foil)
DISCOVERED that 1/8000 particles was deflected back
Rutherford's explanation
deflected particles must have encountered some strong force within the gold atoms
this force must occupy a small space since so few particles were deflected
this force must be dense and + charged to deflect the + alpha particles
Called this + charged dense object the nucleus
Volume of nucleus is small compared to the total volume of the atom.

Mass of nucleus accounts for almost all of the mass of the atom.

Where are the electrons?
**student of Thomson
Neils Bohr (student of Rutherford) proposed a model of the atom that depicted a central, dense + charged nucleus surrounded by - electrons in fixed orbits
The Nucleus
All atoms, except the simplest H atom contain both protons and neutrons in their nucleus
since like charges repel, a nucleus with more than one proton would need some help maintaining stability - the role of the neutron
electrons: exist in "clouds": Wave mechanical Model or Quantum Mecahnical Model
the radius of an atom is measured from nucleus center to the outer most portion of the e- cloud (picometers)
Atomic Number
If atoms are all composed of the same basic particle, why aren't all atoms the same???
Answer: different number of protons
atoms of the same element have the same number of protons (atomic number)
Elements are arranged on the periodic table by increasing atomic number
atoms of the same element with different numbers of neutrons
Does not change the chemical properties of element
only effects the mass
hydrogen exists naturally as 3 isotopes

Protium ~ 1 p+ 0 n 1 e-
Deutrium ~ 1 p+ 1 n 1 e-
Tritium ~ 1 p+ 2 n 1 e-

Mass number
Total number of protons and neutrons
always a whole number!
2 methods:
1. name - mass number
2. nuclear symbol with subscript (atomic number) and superscript (mass number) U
How many protons, neutrons and electrons in the isoptope Chlorine - 37
Relative Atomic Mass
scientists use a standard measure that is recognized universally.
The standard is based on Carbon - 12
atomic mass unit or amu is the measurement standard for atoms
Carbon - 12 is 12 amu's

so.... Hydrogen in 1/12 the mass of a carbon so it measures 1 amu.
from this.... protons and neutrons are measured as 1 amu each and an electron approx 1/2000 amu's
actual mass of carbon - 12 is 12.01amu
actual mass of H is 1.008 amu
actual mass proton 1.007 amu
neutron 1.008 amu
electron .00055 amu
Average atomic mass
most naturally occuring elements exist as mixtures of isotopes
Average Atomic Mass: the weighted average of all known isotopes of an element
multiply the mass of each isotope by the decimal fraction representing it's percentage in the mixture
Copper exists naturally as :
69.15% Copper - 63 atomic mass 62.929 amu
30.85% Copper - 65 atomic mass 64.927 amu

(.6915 X 62.929 amu) + (.3085 X 64.927 amu) = 63.55 amu
~The Mole~
SI unit for the amount of a substance
abbreviated mol
the amount of substance that contains as many particles as there are atoms in exactly 12 g of Carbon - 12
Avagadro's Number
6.022 X 10
**Exactly 12 g of Carbon - 12 contains 6.022 X 10 atoms
one mol of anything is 6.022 X 10 particles
Molar Mass
The mass of one mol of a substance

Written in g/mol

numerically equal to the atomic mass of the element in amu's (rounded 2 decimal places)

contains 1 mol of atoms
How many grams of helium in two moles of He?
multiply by conversion factor to conver between moles and grams
2 mol He X 4.00 g He
1 mol He
molar mass He = 4.00 g/mol
= 8.00 g He
Practice Problems 1 - 4 on page 85!!
James Chadwick: 1932 Discovers the Neutron
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