# CHEM 154 Study Guide - Final Guide: Valence Electron, Niels Bohr, Ionic Radius

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12 Aug 2016
School
Department
Course
Professor
CHEMISTRY 154
The Periodic Table & Atomic Structure
Atomic Structure:
Particle Charge Mass (u)
= mass of 12C
Proton
Found in the nucleus
Atomic Number
+1 1.0073
Neutron
Found in the nucleus 0 1.0087
Electron
Found outside nucleus/
“Simply Exist”
Determines reactivity
-1 0.0005486
Notation:
E = Element Symbol
A = Mass Number
(# protons\ + # neutrons)
Z = Atomic Number (# protons)
Hydrogen
(H)
1H Z = 1 n = 0
e- = 1
Deuterium
(D)
2H Z = 1 n = 1
e- = 1
Tritium
(T)
3H Z = 1 n = 2
e- = 1
# Protons = # Electrons (for neutral species)
Ex. 2- Mass # = 26
Protons = 16
Electrons = 18
(- charge adds 2 e- relative to neutral atom)
Neutrons = 10
Periodic Table:
Group: Columns in the periodic table
Period: Rows in the periodic table
Bohr Model – Simple Atomic Model
-A model proposed by Niels Bohr
-Electrons orbit around the nucleus in perfectly defined orbits
oIssue: e- should constantly lose energy & not perfectly orbit
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-Energy is quantized (discrete)
-Excellent at predicting energy of one electron species
-Insufficient in dealing with multi-electron species
Equation to determine maximum #e- in a particular orbit:
#e-n = 2n2  where n = 1,2,3…
Shell (n) Sub-shells Maximum #e- in
subshells
1 S 2 2(1)2
2 S,P 8 2(2)2
3 S,P,D 18 2(3)2
4 S,P,D,F 32 2(4)2
Electrons & Energy
-Electrons that are closer to the nucleus have lower energy than
electrons that are further from the nucleus
-Electrons in lower n are more stable than those in higher n
E = - 2.178 x 10-18 J x (Z2/N2)  only applies to 1 electron species
Infinitely separated e-  moves closer =electrostatic  more -
Greater nuclear charge = smaller gap between energy levels
Electrons Excitation – Absorption
-An electron in a low energy orbit Ni can be “excited” to a higher
energy orbit Nf
-Electron absorbs energy = to energy difference between Nf & Ni
Electron Relaxation – Emission
-An electron in high energy orbit Ni can “relax” to a lower energy
orbit Nf
-Electron emits light of energy equal to energy difference between
Nf & Ni
-Colored flame is a result of emission
Energy & Light – “Photon of Light”
-Exposing an atom to electromagnetic radiation (light) excites it
Matter, Light, & Color
-Color can arise through absorbance
White light travels through a sample containing atoms/
molecules that absorb light of only a given wavelength (color)
Light with wavelength corresponding to energy difference
between orbits will be absorbed
Emission
-When atoms in gas phase are excited, they relax & emit light
of wavelength equal to the energy difference between the orbits
Valence Electrons
-The number of electrons in the S & P subshells of the
occupied electronic shell with the largest n for the neutral element
-Valence Shell
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