# Electrons in Atoms

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```					                         Chapter 5: Electrons in Atoms
5.1 Light and Quantized Energy
 Electromagnetic radiation is a form of energy that exhibits wavelike behavior

o Wavelength

o Frequency

o Energy

   Waves didn’t explain all of light’s behaviors
   The quantum concept
o Matter can gain or lose energy only small, specific amounts called
QUANTA
   Wave-particle duality
o PHOTONS are tiny bundles of energy

o Radiation is now defined as a stream of photons that move in a wavelike
pattern

5.2 Quantum Theory and the Atom
 All of an atom’s properties revolves around its electrons

   GROUND STATE is the lowest allowable energy state for an electron
o If an electron gains energy, it will jump up to an EXCITED STATE
 It has to gain a specific quantum of energy

   It will be only be excited for a moment
   It will fall back to the ground state, releasing the same amount of
energy it initially gained

o Many photons have the same wavelength as visible light
 When an electron “falls” the energy released (in the form of a
photon) often can be seen as visible light

   An ATOMIC ORBITAL is a 3-D region around the nucleus that describes the
probable location of an electron
o PRINCIPLE ENERGY LEVEL is how close an electron is located to the
nucleus
 n=1 (close), n=4 (far away)
 Also defines how much energy the electron has
 n=1 (little energy), n=4 (lots of energy)
   Each level has SUBLEVELS; Each sublevel has ORBITALS
o 4 different sublevels (s, p, d, f), each with a different
number of orbitals.
o s = 1 orbital
o p = 3 orbitals
o d = 5 orbitals
o f = 7 orbitals
   Each ORBITAL can have TWO ELECTRONS at most

   n=1

   n=2

   n=3

   n=4

5.3 Electron Configurations

   Electron configuration is the arrangement of the electrons in the atom
o Usually in the ground state, which is the most stable

   Aufbau Principle: Each electron occupies the LOWEST ENERGY ORBITAL
available
o Aufbau diagram
   All orbitals related to an energy sublevel are of equal energy
o i.e., all three 2p orbitals are of equal energy

   The energy sublevels within a principle energy level have different energies
o i.e., the 2s orbital has less energy than the three 2p orbitals

   The sequence of energy sublevels within a principle energy level (in order of
increasing energy) is s … p … d … f

   Orbitals in one energy level can overlap the orbitals in another energy level
o i.e., 4s orbital has less energy than the five 3d orbitals

   Pauli exclusion principle: A maximum of two electrons may occupy a single
atomic orbital
o Only if the electrons have opposite spins

   Hund’s rule: Single electrons with the same spin must occupy each equal-energy
orbital BEFORE additional electrons with opposite spins can occupy the same
orbital
   Practice Problems p. 139 18 (a-c with boxes; e-f without boxes)

   Helpful Hint for the Representative Groups 1-8
o Groups 1 and 2 have their last electrons in the s orbital. The principle
energy level is related to the period number of the element.
 Na = 3s1           K = 4 s1
2
 Be = 2s            Sr = 5s2
o Groups 3-8 have their last electrons in the p orbital. Principle energy level
is also related to the period number of the element.
 B = 2p1            In = 5p1
2
 Si = 3p            Pb = 6p2
3
 N = 2p
 S = 3p4
 I = 5p5
 Ar = 3p6
   Noble Gas Configuration
o This is a shorthand way to show electron configurations
o Find the first NOBLE GAS that has an atomic number lower than your
given element
o Put that symbol in brackets: [He]
o Complete the rest of the configuration for all of the electrons after the
noble gas: Magnesium – [Ne] 3s2
o Phosphorus

o Practice Problem p. 139 18 (in noble gas configuration), 21, 22

   VALENCE ELECTRONS
o Remember the electrons determine the chemical properties of an element
o Valence electrons are the OUTERMOST electrons (highest principle
energy level)

o These are the electrons that participate in BONDING
o OCTET RULE

o ELECTRON DOT (LEWIS DOT) STRUCTURES

o Practice Problems p.141 23

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