CHEM 110 Chapter Notes - Chapter 2: Unified Atomic Mass Unit, Mass Spectrometry, Atomic Mass
10 May 2018
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Chapter 2
Imaging and Moving Individual Atoms
- Scanning Tunnelling Miscrosopy
o Image and move atoms and molecules
o Invited by Gerd Binnig and Heinrich Rohrer
o Works by moving an electrode over a surface and measuring tunnelling current.
o Sensitive to the distance between the top and the surface
▪ If the current starts to drop, the tip is moved down toward the surface to
increase the current
▪ If the current starts increase a bit, the top is moved up, away from the
surface
o By measuring the up-and-down movement, the microscope creates an image,
showing the location of individual atoms on that surface
o Discovering a microscope that could see atoms
o Now the STM can pick up and move individual atoms
- An Atom is the smallest identifiable unit of an element
Modern Atomic Theory and the Laws that Led to It
- Law of Conservation of Mass
o Antoine Lavoisier
▪ Matter is neither created nor Destroyed
▪ When a chemical reaction occurs, the total mass of the substance does not
change
▪ Na + Cl2 NaCl
• The mass of the sodium and chlorine that reacts, equals the mass of
the sodium chloride that forms
- Law of Definite proportions
o Joseph Proust
▪ All samples of a given compound, have the same proportions of their
constituent elements
▪ Example
• 18.0g of water results in 16.0g of oxygen and 2.0g of hydrogen
o Now you calculate the mass ratio of the elements
• 16.0g O/2.0 g H = 8:1 mass ratio of oxygen to hydrogen
▪ Example 2
• 14.0g of nitrogen for every 3.0g of hydrogen
• 14.0g N/3.0 g H = 4.7
- Law of Multiple Proportions
o John Dalton
▪ When two elements form two different compounds, the masses of element
B that combine with 1g of Element A can be expressed as a ratio of small
whole numbers
▪ Example
• Carbon combines with oxygen to form CO & CO2
o CO: Mass of Carbon (12.01) Mass of Oxygen (16.00)
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o CO2 Mass of Carbon (12.01) Mass of Oxygen (32.00)
o Ratio of Oxygen in CO2 to O in CO is 2:1
- Atomic Theory
o Each element is composed of time indestructible particles called atoms
o All atoms of a given element have same mass and physical/chemical proeprties
that distinguish them from atoms of other elements
o Atoms combine in simple who number ratios to form compounds
o Atoms of one element cannot change into atoms of another element
▪ Atoms only change the way they are bound together
Atomic Structure
- The Discovery of the Electron
o JJ Thomson found a beam of particles called cathode rays
o These particles travelled in straight lines
▪ Independent of the composition of the material from where they originated
▪ Carried a negative electrical charge
• Electrical charge is a property that compose atoms and results in
attractive and repulsive forces (Electrostatic forces)
o Electron is the negatively charged, low mass particle
o Proton is the positively charged, high mass particle
o Millikan experiment
▪ Sprayed oil using an atomizer
▪ During the fall, the drop acquired electrons that had been produced by
bombarding the air in the chamber with ionizing radiation
▪ Electrons created a negative charge to the drops and Millikan created an
electric field between the metal plates.
▪ With the lower plate being negatively charged, the free fall of the
negatively charged drop could be slowed
- Discovery of the Nucleus
o Ernest Rutherford
▪ Radioactivity, the emission of small energetic particles from the core of
certain atoms
• Three types of Radioactivity
o Alpha Particles
o Beta particles
o Gamma Rays
• Gold Foil Experiment
o Protons were to act as probes of the gold atoms’ structure
o These probes should pass right through the foil with
minimum deflection
o Matter must not be as uniform as it appears
o He proposed the nuclear theory of the atom with 2 parts
▪ Most of the atom’s mass and charge are contined in
a core called the nucleus
▪ Most of the volume of the atom is empty space
Protons, the Atomic Number and Neutrons
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Document Summary
If the current starts to drop, the tip is moved down toward the surface to increase the current. An atom is the smallest identifiable unit of an element. Modern atomic theory and the laws that led to it. Law of conservation of mass: antoine lavoisier, matter is neither created nor destroyed, when a chemical reaction occurs, the total mass of the substance does not change, na + cl2. Nacl: the mass of the sodium and chlorine that reacts, equals the mass of. Law of multiple proportions: john dalton, when two elements form two different compounds, the masses of element. The discovery of the electron: jj thomson found a beam of particles called cathode rays, these particles travelled in straight lines. Positive charge in a nucleus is protons. Number of protons in a nucleus is atomic number (symbol z) Each element is represented with a unique chemical symbol. Neutrons are neutral particles in the nucleus.
