L07 Chem 151 Lecture Notes - Lecture 2: Electron Configuration, Valence Electron, Atomic Number

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31 August 2015
Lecture 2
I. Structure of an Atom
A. Basics
1. The nucleus contains neutrons and protons (AE and ZE)
a. The mass number depends on the number of protons + neutrons (A)
b. The chemical properties of an element depend on the number of protons (aka the
atomic number) (Z)
2. Electrons exist in orbitals outside the nucleus (probability)
a. Orbitals describe the energy of an electron and probability of finding an electron
in a given location
b. The electron configuration tells us the orbital of each electron
B. The Periodic Table
1. In the early days, elements are arranged on Periodic table according to their chemical
properties
2. Mendeleev is credited with the modern looking periodic table, but left blanks for
elements to be discovered
3. Periods are the rows (there are 7), groups are the columns
4. Periodic Table Blocks:
C. Ground State Electron (GSE) Configurations
1. Elements in the periodic table are arranged according to GSE configurations
2. The GSE is the electrons configuration in which all electrons are in the lowest
possible energy levels
3. Valence electrons are the electrons in the outermost shell of an atom
a. Atoms are most stable when the configuration of the outermost shell is ns2np6
This is the configuration of noble gases
b. Atoms will form chemical bonds to achieve more stable electron configurations
4. The principal quantum number (n) represents the energy shell (1, 2, 3…)
a. As the number increases, energy increases
5. There are s, p, d, and f subshells
a. In the same shell (n) these subshells increase in energy
b. There are degenerate orbitals associated with each subshell (s-1, p-3, d-5, f-7)
6. The filling order can be weird: ie- 4s is lower in energy than 3d, BUT 4s is still at a
greater distance from the nucleus
a. This is important since the outermost electrons are lost first
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