Chemistry Notes Atomic Theory
Atomic Structure
Nucleus Electron Cloud
Protons (+) about the electrons (-)
same mass 1/1840 mass of a proton Neutrons (0)
Electrons move at extremely high speeds.
Atomic Number: Equal to the number of protons within an atom. Also equal to the number of electrons in the atom.
Atomic Mass: Number of protons + number of neutrons
Isotopes: Individuals of the same element (same number of protons), but they have a different atomic mass (different number of neutrons).
5 p 5 p
5 n Isotopes 6 n
10 amu 11 amu
Wave Nature of Light
Speed (v) of light in a vacuum is 3.00 x 108 m/s and is symbolized with the letter “c.”
Frequency (f) represents how often a pulse of the wave is produced each second. (Hz-hertz, or s-1).
Wavelength (λ) is the distance from crest to crest on a wave. (m or nm).
Wave Equation: v = f λ
Energy (E) E = h f or E = hc/ λ. h = 6.626 x 10-34 Js
Examples:
1. A violet light source has a wavelength of 3.8 x 10-7 m. a. What is the frequency of the light?
b. What is the energy in a violet light photon?
2. Green light has a frequency of 6.12 x 1014 Hz.
a. What is the wavelength of the light in meters and nanometers?
b. How much energy does a green light photon contain?
Photoelectric Effect: The photoelectric effect explains how we can convert light energy into electrical energy.
When a photon hits an electron in an atom, it has the ability to give it enough energy to leave the atom and still have energy to travel. This is governed by the following equation:
Kmax = hf – ф
Kmax is the maximum possible kinetic energy (moving energy) of the electron that was struck by the photon.
Recall: hf is the energy of the photon.
ф is the “work function” for the atom. It is equivalent to the amount of energy required to remove the electron from the atom. This number is related to the ionization energy of the
atom.
If the total energy of the photon (hf) is absorbed by the electron, some of the energy goes to freeing the electron (ф) from the atom and the rest goes to kinetic energy (Kmax).
The maximum voltage (Vmax) that is produced from a photoelectric system is:
Vmax = Kmax ÷ 1.6 x 10-19
Examples:
1. Light having a frequency of 4.5 x 1014 Hz strikes a metal surface having a work function of 3.2 x 10-20 J.
a. Determine the maximum kinetic energy of electrons being freed from the metal.
b. What is the maximum voltage produced by this system?
c. How would changing the metal, but not changing the frequency of light alter this system?
2. Light having a wavelength of 550 nm impinges on a metal surface having a work function of 1.1 x 10-19 J.
a. What is the frequency of the light?
b. What is the maximum voltage of the system?
Particle Nature of Light
As we have noted thus far, light travels as waves. As we understand it now, light travels in small pulses of waves called photons.
deBroglie’s Wavelength
As it so happens, matter has a wave nature to it. As particles move at high speeds, they can often exhibit wave tendencies that can be measured. One of these properties is its wavelength. The equation for this wavelength is as follows:
λ = h
/m v v is the speed of the particle moving.
Example: What is the deBroglie wavelength of an electron as it travels 3.5 x 105 m/s?
Additional Practice:
1. An isotope of aluminum has an atomic mass of 27. How many protons, neutrons, and electrons does this isotope contain?
a. Protons: _____ b. Neutrons: _____ c. Electrons: _____
2. A photon has a wavelength of 15 nm. Does it have more energy or less energy than a photon having a wavelength of 30 nm? Explain your answer.
3. A photon of light has a frequency of 5.0 x 1014 Hz. a. What is the photon’s energy?
b. What is the wavelength of this photon?
Spectral Lines and Emissions:
1. Photons of light can “excite” electrons within an atom.
2. When the electrons are no longer “excited,” they give off energy in the form of photons.
3. Practice: For the electron energy levels of the atom below, determine the
following:
Electron Energy Levels for a hypothetical atom
Hypothetical Energies Ionized State
-0.8 eV n = 6
-1.1 eV n = 5
-1.7 eV n = 4
-3.0 eV n = 3
-6.0 eV n = 2
-15.0 eV n = 1
Ground State
a. How much energy would be needed to raise an electron of this atom from the ground state to n = 4?
b. Photons having a wavelength of 92.0 nm are totally absorbed by electrons of this atom.
i. What are the energies of the photons?
ii. What is the maximum energy level the electrons can rise to? iii. Draw arrows on the diagram above showing all possible energy
transitions.
iv. What wavelengths will be emitted?
Note:
1 eV = 1.6 x 10-19 J
hc = 1240 eV nm
Practice:
1. An electron travels 2.1 x 106 m/s. What is its deBroglie wavelength?
2. For the electron energy levels of the atom below, determine the following:
Electron Energy Levels for a Hypothetical Atom
Hypothetical Energies Ionized State
- 0.5 eV n = 6
-1.0 eV n = 5
-2.0 eV n = 4
-4.0 eV n = 3
-8.0 eV n = 2
-20.0 eV n = 1
Ground State
i. How much energy would be needed to raise an electron of this atom from the ground state to n = 5?
ii. Photons having a wavelength of 75 nm are used to bombard electrons of this atom.
1. What are the energies of these photons?
2. What is the maximum energy level the electrons can rise to?
3. Draw arrows on the diagram above showing all possible energy transitions.
3. Determine the energy of a light wave having a wavelength of 650 nm. Is this light visible light?
4. The maximum voltage produced for a metal plate as a monochromatic light falls on it is 6.3 V. The work function for the metal is 7.68 x 10-19 J. What are the following values for the light striking the metal?
a. Energy
b. Frequency
c. Wavelength
5. The threshold frequency (the lowest frequency at which a photon can barely cause electrons to leave) for a metal plate is 2.1 x 1014 Hz. Light having a frequency of 3.4 x 1014 Hz impinges on the metal plate. Determine the following values:
a. The work function of the metal
b. The maximum kinetic energy of the electrons moved by the light
Orbital Configurations: How electrons are arranged in the electron cloud.
n = 1 n = 2 n = 3 n = 4 n = 5 n = 6 n = 7
s2 s2p6 s2p6d10 s2p6d10f14 s2p6d10f14 s2p6d10f14 s2p6d10f14
Principle Quantum Number (n): Energy level of the electron.
Orbital Shape (s, p, d, f)
Spin:
Example:
Orbital Configuration for Phosphorus:
____ ____ ____ ____ ____ ____ ____ ____ ____ 1s 2s 2p 3s 3p
Orbital Notation for Phosphorus
1s22s2p63s2p3
Noble Gas Notation for Phosphorus (see periodic chart)
[Ne] 3s2p3
Practice:
1. Determine the following for Bromine: a. Noble Gas Notation
b. Full Orbital Notation c. Orbital Configuration d. Diagram
Valence Electrons: For most atoms, this is the combined electrons found in the “s” and
“p” orbitals of an atom.
Question: As a class, determine the number of valence electrons, ionic charge, and electron exchange for the following atoms.
Valence e- Ionic Charge Electron Exchange a. Magnesium _____ _____ _____
b. Nitrogen _____ _____ _____ c. Hydrogen _____ _____ _____
d. Neon _____ _____ _____
e. Potassium _____ _____ _____ f. Aluminum _____ _____ _____ g. Fluorine _____ _____ _____ h. Carbon _____ _____ _____
Ions:
a. Valence electrons found in metals are often given up during chemical reactions, which turns the metal’s atom into a _____________ ion.
b. Nonmetals generally take electrons from metals during a chemical reaction, which turns the nonmetal atom into a _______________ ion.
c. When metallic positive ions and nonmetal negative ions combine, they form ionic bonds.
Molecules:
a. When two nonmetals combine, they form molecules through covalent bonds. Covalent bonds form when two or more atoms _______________ valence electrons.
b. Examples: H20, CO2, NH3, and PCl3.
Lewis Dot Diagrams:
a. Lewis Dot Diagrams are used to show the interactions of valence electrons in a fairly simple manner.
b. As a class, draw Lewis Dot Diagrams for the following atoms. i. Calcium
ii. Lithium iii. Bromine iv. Helium v. Phosphorus vi. Carbon vii. Boron Practice:
a. Draw Lewis Dot Diagrams for the following atoms: Se, Ar, Sr, Cs, As. b. _______ bonds form when a metal gives at least one electron to a nonmetal,
while ________ bonds form molecules as atoms share electrons. c. Periodic Puns: Examples
What do you do when the preacher comes to your house? H Boy, this class sure is B.
Periodic Chart and its Trends
1. Locate the following on the periodic chart: a. Metals
b. Nonmetals c. Metalloids
d. Transition Elements e. Alkali Metals
f. Alkaline Earth Metals g. Halogens
h. Noble Gases
2. Characteristics: a. Metals
b. Nonmetals
c. Metalloids
3. How do the following change over the periodic chart vertically and horizontally? a. Atomic Radius
b. Ionic Radius
c. Ionization Energy
d. Electronegativity
4. The Octet Rule
