Post on 02-Jan-2016
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Chapter 2 Matter is Made Chapter 2 Matter is Made of Atomsof Atoms2.2 Electrons in Atoms
ObjectiveObjective
Objectives: Relate the electron to modern atomic theory, Compare electron energy levels in an atom, Illustrate valence electrons by Lewis electron dot structures
Electrons in motionElectrons in motionElectrons are in motion in the
outer part of an atomWhen two atoms come near,
these electrons interactElectrons and their arrangement
in the atom greatly affect an element’s properties and behavior
Electrons in motionElectrons in motionNiels Bohr (early 20th century)-
Proposed e-‘s must have enough energy to keep them in constant motion around the nucleus (enables e-‘s to overcome the attraction of the positive nucleus)
Bohr’s model (1913)- Planetary model The well-defined orbits are not
correct. Electron motion within an atom is irregular and an electron may be close to the nucleus at times.
Electromagnetic spectrumElectromagnetic spectrum
Electromagnetic radiation (radiant energy) travels in the form of waves- have both electrical and magnetic properties
Electromagnetic waves can travel through empty space
Ex: radiant energy from the sun travels to Earth
Electromagnetic spectrumElectromagnetic spectrum
Electromagnetic waves travel through space at the speed of light, approx. 300 million meters/sec
Electromagnetic spectrumElectromagnetic spectrumProperties of waves: Frequency and
wavelength
Frequency-number of vibrations/ second (Hertz-Hz)
Wavelength- distance between two corresponding points on two consecutive
waves
Wavelength is determined by frequencyLow frequency = long wavelength (low energy)High frequency = short wavelength (high
energy)
Electromagnetic spectrumElectromagnetic spectrum
Electromagnetic spectrumElectromagnetic spectrumAll the forms of radiant energy
are parts of a whole range of electromagnetic radiation
The electromagnetic spectrum includes gamma rays, X rays, ultraviolet (UV), Visible, Infrared (IR), microwaves, radio waves
Electromagnetic spectrumElectromagnetic spectrum
Electromagnetic spectrumElectromagnetic spectrumGamma Rays- highest frequency,
shortest wavelength, most energetic waves, can pass through most substances
X rays- lower frequency than gamma, but still high energy waves, can pass through soft body tissue, but stopped by bone
Ultraviolet-slightly more energetic than visible, can cause sunburns, mostly absorbed by ozone
Electromagnetic spectrumElectromagnetic spectrumVisible- part of the spectrum to
which our eyes are sensitive, Our eyes and brains interpret different frequencies as different colors
ROY G BIV - NewtonRed- low energy
Violet- high energy
Electromagnetic spectrumElectromagnetic spectrumInfrared- less energy than visible,
given off by most “warm objects”
Microwaves/ Radio waves- lowest frequency
Electrons and LightElectrons and Light EMISSION SPECTRUM: Spectrum of light
released from excited atoms of an element
Electrons can have only certain amounts of energy
Electrons can move around the nucleus only at distances that correspond to those amounts of energy
When e- absorb energy, they move to a higher energy state
When they fall back to a lower energy state- they emit energy (Light)
Electrons and LightElectrons and LightENERGY LEVEL: Region of space in which e-‘s can move about the nucleus of an atom
ELECTRON CLOUD: Space around the nucleus of an atom where the atom’s e-‘s are found Each energy level can hold a limited number of electronshttp://www.youtube.com/watch?v=QI50GBUJ48s&safety_mode=true&persist_safety_mode=1&safe=active
# of electrons in each # of electrons in each levellevelLowest energy level is the smallest
and closest to the nucleus, Holds two electrons max
Second energy level is larger because it is farther away, Holds eight electrons max
Max # of e-‘s: 2, 8, 8,18, 32, 50, 78, 91
# of electrons in each # of electrons in each levellevelBohr Model Examples:
H-1 (1 p, 1 e, 0 n)
Li-7 (3 p, 3 e, 4 n)
0-16 (8 p, 8 n, 8 e)
VALENCE ELECTRONSVALENCE ELECTRONSThe electrons in the outermost energy
level Valence electrons are important
because they interact when atoms come near each other
Chemical and physical properties of an element are directly related to the number and arrangement of valence electrons (v.e.)
VALENCE ELECTRONSVALENCE ELECTRONSAtoms in group 1 have one valence electronAtoms in group 2 have two valence electronsAtoms in group 13 have three valence
electronsAtoms in group 14 have four valence electronsAtoms in group 15 have five valence electronsAtoms in group 16 have six valence electronsAtoms in group 17 have seven valence
electronsAtoms in group 18 have eight valence
electrons
LEWIS DOT DIAGRAMLEWIS DOT DIAGRAM Illustrates valence electrons as dots
around the chemical symbol of an element.
Each dot represents one valence
electron (max # of v.e. is 8) The symbol represents the core of the
atom. Ex: . .
Li Be B
LEWIS DOT DIAGRAMLEWIS DOT DIAGRAMLi has 2 e- in the first energy level
and 1 in the second, Be has 2 e- in the first energy
level and 2 in the second, B has 2 e- in the first energy level
and 3 in the second More examples: C, N, O, F, Ne