Electrons in Atoms
By: Ms. Buroker
Okay …
We now know that an element’s identity lies in its number of protons … but there is another particle which is very important as well …
The electron
Electrons control behavior and reactivity in substances.
The Electromagnetic Spectrum
Visible light is a kind of
electromagnetic radiation … a form of energy that exhibits wavelike behavior as it travels through space.
Electromagnetic Radiation
We measure electromagnetic radiation in two ways … 1.) Wavelength2.) Frequency
When multiplied together … these two thing ALWAYS equal the speed of light!!!
C= 3.00 x 108m/s
Wavelength (l)
Wavelength is the distance from one wave crest or wave trough to the next
Electromagnetic Spectrum
Wavelength is measured in meters … but more commonly, nanometers.
Frequency (u)
Frequency is defined as the number of waves that pass a given point in a specific time … usually, a second.
Typically expressed as a Hertz (Hz) … one wave
per second
Remember What We Said …
All Wavelengths and their frequency’s when multiplied are equal to the speed of light; so …
C = lu
Let’s Re-Visit This …
Violet … shortest wavelength and highest frequency
Red … longest wavelength and lowest frequency
The Particle Nature of Light
Matter can loose energy only in small, specific amounts called quanta … so a quantum is the minimum amount of energy that can be gained or lost by an atom.
Energy
Plank’s Constant6.626 X 10-34 J.s
Frequency
Equantum = hn
The Photoelectric Effect
Einstein suggested that electromagnetic radiation can be viewed as a stream of particles called photons.
Ephoton = hn = hc
lPhotoelectric Effect: refers to the phenomenon in which electrons are emitted from the surface a metal when light strikes it.
Photoelectric Effect Continued
1.) Each metal has a threshold frequency.2.) For light with frequency lower than the
threshold, no e- are emitted.3.) For light with frequency greater than threshold,
the number of e- emitted increases with the intensity of the light.
4.) For light with frequency greater than the threshold frequency, the kinetic energy, of the emitted e- increases linearly with the frequency of light.
Let’s Practice!!
Tiny water drops in the air disperse the white light of the sun into a rainbow. What is the energy of a photon from the violet portion of the rainbow if it has a frequency of 7.23 x 1014
Hz?
Ephoton = hnEphoton = (6.626 x 10-34 J.s)(7.23 x 1014/s)Ephoton = 4.79 x 10-19J
Atomic Emission Spectra
The set of frequencies of the electromagnetic waves emitted by atoms of an element.
* They are unique to the element!
Quantum Theory and the Atom
Bohr Model of the Atom* Working with the hydrogen atom, he proposed that the hydrogen atom has only certain allowable energy states.
Lowest allowed energy state = ground state Atoms gain energy = excited state
Bohr Model Continued …Electrons moving around the
nucleus in only certain allowed circular orbits
Bohr Model Continued …Assigned each orbital a quantum
number, n
Smaller the orbital = lower energyHigher orbitals = higher energy
de Broglie Equation
de Broglie predicted that all moving particles have wave characteristics.
l = h mvMass Velocity
Heisenberg Uncertainty Principle
States that it is impossible to know the precisely both the velocity and position of a particle at the same time.
Hydrogen’s Atomic Orbitals
n = Principle Quantum Number
Energy sub levels = s, p, d, f
Shape = s: sphericalp: dumbbell shapedd,f: don’t all have the
same shape
S orbitals
2 e-
p orbitals6 e-
d orbitals10 e-
f orbitals14 e-
In SummaryPrincipleQuantum
Number (n)
Sublevels(types of orbitals)
Present
Number ofOrbitals related
To sublevel
1 s 1
2 sp
13
3 spd
135
4 spdf
1357
Electron Configuration
The arrangement of electrons in an atom.
There are three main rules that govern how
electrons can be arranged in an atom.
The aufbau principle
e- occupy the lowest energy level FIRST!
The Pauli exclusion principle
A maximum of two e- may occupy a single atomic orbital, but only if the electrons have opposite spins!
orbital Total # of e-
s 2
p 6
d 10
f 14
Hund’s rule
Single e- with the same spin must occupy each equal energy orbital before additional e- with opposite spin can occupy the same orbital.
Atomic Orbitals and Quantum Numbers
n= The principle quantum # (1, 2, 3, ….)* relates to the size and
energy of the orbitall = The angular momentum quantum number (0 to n-1)
l = 0 s orbitall = 1 p orbital
l = 2 d orbital l = 3 f orbital
Atomic Orbitals and Quantum Numbers
Ml = magnetic quantum number (l to –l)* related to the orientation of the
orbital in space relative to the other orbitals in the atom.
Ms = Electron spin quantum number
(+1/2 or -1/2)
Valance Electrons
Valence electrons are those electrons that are in the outer most energy level of an atom … it is these electrons that are responsible for the reactivity of an element.
Example:Write the electron configuration for Carbon.1s2 2s2 2p2 … the outer most energy level for Carbon is n=2, right? So to find out how many valance electrons carbon has, you simply count how many electrons are in level 2.
Carbon has 4 valance electrons!
Believe it or not … you can look at the periodic table and determine how many valence electrons an atom has and how that affects the properties of that element.
All elements with the exception of the transition metals are called the representative elements. The representative elements follow the following rules …
The period number tells you the energy level the valence
electrons are in!!
The group number tells you how many valence electrons
there are!!