Electrons in Atoms Bohr Orbits vs. Electron-cloud Orbitals Electron Configurations and Orbital Diagrams 1 s 2 2 p 3 Periodic Patterns
Bohr Model of the Atom • Electrons are NOT randomly arranged • Energy of electrons is quantized Vocabulary: quantized
The Bohr Diagram The simplicity of the Bohr Model of the atom makes it useful for representing atoms of different elements. Bohr Diagrams place electrons on concentric rings surrounding the nucleus.
Bohr Diagrams Show the important features of an atom • number of protons • number of electrons • number of neutrons • arrangement of electrons
The Bohr Diagram The following table shows how many electrons can occupy a given energy level in a Bohr Diagram. Shell Number Energy Level Number of Electrons Allowed 1 2 3 4 2 8 18 32 Vocabulary: Shell number
The Bohr Diagram for the first 5 elements:
Learning Check Draw the Bohr Diagram for Phosphorous (P) remaining e-
Valence Electrons • Valence electrons: the electrons in the outermost occupied shell (orbital) • Valence electrons are in the highest energy shell (orbital) • Valence electrons are the electrons that will participate in a chemical reaction • The number of valence electrons largely influences the properties of an element Vocabulary: valence electron
Valence Electrons The valence electrons in our Bohr diagrams are the electrons on the outermost ring of the diagram, in this example the 4 electrons on the third ring. 14 P 14 N
Learning Check How many valence electrons does Cl have?
Learning Check Cl has 7 valence electrons. (Count the e- on the outermost ring)
Ground State Lowest energy state of an electron Normal resting state most stable The ground state is what you are familiar with: Vocabulary: ground state, excited state
Excited States Electrons have multiple possible excited states. Each one can produce a different line in the emission spectrum.
Excited States Excited states are NOT stable. Excited states are higher energy.
Ground State vs. Excited State An electron in the excited state will ALWAYS return to the ground state. Examples: firecrackers and glowing burner
Emission Spectra While investigating atoms and their properties, scientists discovered that atoms of different elements emit light in distinct patterns. These patterns are called Line Emission Spectra, and are unique for each element, based on its electronic structure.
Each element has a "fingerprint" in terms of its line emission spectrum, as illustrated by the examples below. Line spectrum for hydrogen. Line spectrum for helium. Line spectrum for neon.
Bohr Model vs. Electron Cloud • Both models agree that the arrangement of the electrons matters • Both models have quantized energy levels for the electrons • The Bohr model is simpler to understand Why do we need the Electron cloud model?
Particle Behavior (Expectation before the Bohr model) Electrons are subatomic particles of atoms. Electrons are matter. Matter is governed by certain laws of physics. You can predict the trajectory of a particle. It does not bend around corners. Matter has phases (solid, liquid, gas) with different amounts of kinetic energy.
Emission of Light (Caused change in thinking) • Electrons emit light when returning from an excited state to the ground state. • Emission of light indicates that electrons have wavelike properties. • Different colors of light correspond to different wavelengths and energies. • Line spectra exist for the different elements
Waves Electromagnetic radiation propagates through space as a wave moving at the speed of light. c = speed of light, a constant (3. 00 x 108 m/s) = frequency, in units of hertz (hz, sec-1) = wavelength, in meters
Long Wavelength = Low Frequency = Low ENERGY Short Wavelength = High Frequency = High ENERGY E = h
The Electromagnetic Spectrum
Wave Behavior Light is a wave. Waves are NOT matter. Light CAN bend around corners. Light has different properties from matter.
Electron transitions from excited state to ground state involve jumps of quantized amounts of energy. This produces bands of light with definite wavelengths. What happens if the atom gets less than the needed amount of energy to reach the n = 2
Proof of the Wave Nature of Electrons: Emission of Light ? Photons of light are emitted as an electron drops back to its ground state after being excited.
Flame Tests Atoms become excited when they are heated in a flame. Their electrons move from their ground state to higher energy levels. As they return to their ground state they emit photons of very specific energy. The energy of the emitted photons corresponds to particular wavelengths of light, producing different colors of light.
This picture illustrates the distinctive colors produced by burning particular elements.
The Dilemma of the Atom The Bohr Model predicted the observed line spectrum for H. However, the Bohr Model does NOT correctly predict the line spectrum of any atom besides H. Why did the Bohr Model fail for atoms with more than one electron?
Wave-Particle Duality JJ Thomson won the Nobel prize (1906) for describing the electron as a particle. His son, George Thomson won the Nobel prize (1937) for describing the wave-like nature of the electron. The electron is a particle! The electron is an energy wave!
Wave-Particle Duality Electrons have BOTH particle and wave behavior. This is known as the wave-particle duality. All matter has both wave and particle properties, but we can usually only observe one or the other. Only the Electron Cloud model of the atom uses the dual nature of the electron.
• Quantum Mechanics Energy Levels Quantum mechanics uses a principal quantum number, “n”. It represents the energy level just like in the Bohr diagram. • n = 1 describes the first energy level • n = 2 describes the second energy level • Each energy level is represented on the periodic table, and approximately correlates to a row. Red n = 1 Orange n = 2 Yellow n = 3 Green n = 4 Blue n = 5
Sublevels = Specific Atomic Energy levels have sublevels which describe Orbitals the specific types of atomic orbitals for that level. • n = 1 has 1 subshell • n = 2 has 2 subshells orbitals) • n = 3 has 3 subshells • n = 4 has 4 subshells “f”) (the “s” orbital) (“s” and “p” (“s”, “p” and “d”) (“s”, “p”, “d” and Vocabulary: subshell or
Energy Level and Orbital Size Representation of how the energy level, n (shell), affects the size of an orbital.
Shells, Subshells, and Orbitals Shell = energy level n = 1, n = 2, etc. Subshell = type of orbital np) Orbital = exact orbital s, p, d, f (ns, 2 px
Shells, Subshells, and Orbitals Shells define the energy level Subshells define the sublevels within a shell Orbitals exist for each sublevel Shell s Orbital s Subshell Orbital s
Subshells of the energy levels n = 1 1 subshell (s) red elements n = 2 2 subshells (s, p) orange elements 1 n = 3 3 subshells (s, p, d) yellow 2 Red n = 1 elements 3 Orange n = 2 Yellow n = 3 Green n = 4 Blue n = 5 Indigo n = 6 4 3 4
QM Atomic Orbitals s 1 spherical orbital per energy level p 3 dumbbell shaped orbitals per energy level d 5 multi-lobed orbitals per energy level f 7 multi-lobed orbitals per energy level Each type of orbital corresponds to a block on the periodic table. Each atomic orbital can hold 2 e- (or fewer).
QM Atomic Orbitals
QM Atomic Orbitals The quantum mechanical orbitals are identifiable by their shape. You should recognize each of the following orbitals.
Orbitals s p d • In the s block of the periodic table, electrons fill s orbitals. • In the p block, the s orbitals are full. New electrons begin filling the p orbitals. • In the d block, the s and p orbitals are full. New electrons are added to the d orbitals. • What about the f block?
Periodic Table & Electron Configuration
Orbital Diagrams and Electron Configurations Orbital diagrams are a simple way of representing the complete electron arrangement in an atom. It contains more information than an electron configuration. An electron configuration is a simple way to give the basic electron arrangement in an atom, and emphasizes the valence 2 2 1 s 2 s electrons. 2 p 3
Capacity of the Levels Energy Sub. Level levels Total Orbitals n = 1 s 1 (1 s orbital) 2 2 n = 2 s p 1 (2 s orbital) 3 (2 p orbitals) 2 6 8 s p d 1 (3 s orbital) 3 (3 p orbitals) 5 (3 d orbitals) 1 (4 s orbital) 3 (4 p orbitals) 5 (4 d orbitals) 2 6 10 n = 3 n = 4 Total Electrons s per Energy Level 18 32
Rules for Electron The Aufbau Principle states that each Configurations electron occupies the lowest energy orbital available. Note the axis label: E
Orbital Filling Sequence Note the axis label: E Note that 4 d orbital is higher E than 5 s orbital Shells (energy levels) ALWAYS the lowest energy orbital
Aufbau Principle Electrons fill orbitals starting at the lowest available energy level before filling higher levels 1 s is ALWAYS
Aufbau Diagram Start with 1 s and follow the arrows.
Aufbau Diagram Energy Levels: 1, 2, 3, 4… Sublevels: s, p, d, f Orbitals: 1 s, 2 p, 3 s, 3 p, 4 s, 3 d, 4 p…
Learning Check Use the Aufbau Principle to order the orbitals occupied by electrons for Co.
Learning Check • Co has 27 electrons • Every orbital can hold 2 e- 27/2 = 13. 5 orbitals • Orbitals are filled beginning with the lowest energy (1 s) • Using the Aufbau Diagram: 1 s, 2 p, 3 s, 3 p, 4 s, 3 d 1 + 3+ 1 + 3 + 1 + 5 = 15 orbitals Co will end in the 3 d orbitals
Learning Check Co:
Fill Lower Energy Orbitals http: //www. meta. Each line represents FIRST an orbital. 1 (s), 3 (p), 5 (d), 7 (f) High Energy Low Energy • The Aufbau Principle states that electrons enter the lowest energy orbitals first. synthesis. com/webbook/34_qn/qn 3. jpg • The lower the principal quantum number (n) the lower the energy. • Within an energy level, s orbitals are the lowest energy, followed by p, d and then f. F orbitals are the highest energy for that level.
Pauli Exclusion Principle The Pauli Exclusion Principle states that a maximum of two electrons can occupy a single orbital, but only if the paired electrons have opposite spins. What orbitals are shown? How many electrons are in each orbital? What is the valence energy level for Ne?
Pauli Exclusion Principle An orbital can hold 0, 1, or 2 electrons Full orbital contains 2 electrons Half full orbital contains 1 electron If there are two electrons paired in the orbital, they must have opposite spins.
Pauli Exclusion Principle When we draw electrons, we use up and down arrows. If electrons are paired in a box, one arrow is up and the second must be down. Incorrect; the paired electrons must have opposite spin, and unpaired electrons should have the same spin Correct; the paired electrons have opposite spin, and the unpaired electrons have the same spin
Learning Check 1. Write the correct orbital diagram for Si. 2. What is wrong with the following orbital diagram? Correct the orbital diagram and identify the element. 3 p 3 s 2 p 2 s 1 s
Learning Check 1. Write the correct orbital diagram for Si. 1 s 2 s 2 p 3 s 3 p 2. What is wrong with the following orbital diagram? Correct the orbital diagram and identify the element. 3 p Na 3 s 2 p 2 s 1 s
No more than 2 Electrons in Any Orbital…ever! • The Pauli Exclusion Principle states that an atomic orbital may have up to 2 electrons and then it is full. • The spins have to be paired. • We usually represent this with an up arrow and a down arrow.
Hund’s Rule states that single electrons with the same spin must occupy each degenerate orbital before electrons with opposite spins pair to occupy the same energy level orbitals. Each 3 p orbital has the same energy. They are degenerate. Vocabulary: degenerate
Hund’s Rule When filling sublevels, electrons are placed individually in degenerate orbitals before they are paired up. Think of the charge of an electron. Like charges repel. Thus, when a degenerate orbital is available, the electrons prefer to be unpaired. degenerate orbitals
Hund’s Rule So when working with the p sublevel, electrons fill like this. . up, up. . . down, down atom orbital box diagram B C N O F 1 s 2 s 2 p 2 p 2 p 1 s 2 s 2 p
Learning Check Which of the following options is correct? Apply Hund’s Rule (and what else? )
Hund’s Rule http: //intro. chem. okstate. edu/AP/2004 Norman/Chapter 7/Lec 111000. html Don’t pair up the 2 p electrons until all 3 orbitals are half full. • Hund’s Rule requires that you half fill degenerate orbitals before pairing. Begin pairing the electrons only when all orbitals are halfway full. Degenerate orbitals = orbitals that have the same energy • The 3 p orbitals on each level are degenerate they all have the same energy. • Similarly, the d and f orbitals are degenerate, too.
Summary: Rules for Filling Orbitals 1. Lower-energy orbitals fill first (Aufbau Principle). 2. An orbital can hold only 2 electrons with opposite spins (Pauli Exclusion Principle). 3. If degenerate orbitals are available, electrons remain unpaired with like spin in each orbital until all are half-full (Hund's Rule).
Chapter Vocabulary Aufbau Principle Pauli Exclusion Principle Week 3 (new) Hund’s Rule Degenerate Shell Subshell (sublevel) Week 2 Orbital Ground state, Excited state Quantized Valence electron Week 1 Shell number
Electron Configurations and Orbital Diagrams Orbital diagrams and electron configurations are closely related. The orbital diagram for N is The electron configuration for N is 1 s 22 p 3 Note that the number of electrons in the degenerative boxes adds up to the number in the orbital subshell of the electron configuration.
4 d 5 s 4 p 4 s s 3 p 3 s 2 p 2 s 1 2 3 4 5 6 7 3 d 1 2 p 1 2 3 4 5 6 d 1 2 3 4 5 6 7 8 9 10 f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 s + 1 s 1 That’s right: it goes in the 1 s sublevel. And its e- config is 1 s 1. Notice in the table above where H is – in the area designated as 1 s. So where does the next electron go?
4 d 5 s 4 p 4 s s 3 p 3 s 2 p 2 s 1 2 3 4 5 6 7 3 d 1 2 p 1 2 3 4 5 6 d 1 2 3 4 5 6 7 8 9 10 f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 s + 1 s 2 If you were thinking it went in the 2 s, then you forgot that each orbital can hold up to two electrons. Note how He is right here in the area designated as 1 s 2 and so its e- config is 1 s 2.
4 d 5 s 4 p 4 s s 3 p 3 s 2 p 2 s 1 2 3 4 5 6 7 3 d 1 2 p 1 2 3 4 5 6 d 1 2 3 4 5 6 7 8 9 10 f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 s + 1 s 2 2 s 1 Now that the 1 s is filled, the next electron goes in the next sublevel – the 2 s. Again note how Li is in 2 s 1. Its full e- config is 1 s 2 2 s 1. What is the e- config of Be?
4 d 5 s 4 p 4 s s 3 p 3 s 2 p 2 s 1 2 3 4 5 6 7 3 d 1 2 p 1 2 3 4 5 6 d 1 2 3 4 5 6 7 8 9 10 f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 s + 1 s 2 2 p 1 Is this what you were thinking? Notice how B is in the 2 p 1 spot. So its full e- config is 1 s 2 2 p 1. What’s next?
4 d 5 s 4 p 4 s s 3 p 3 s 2 p 2 s 1 2 3 4 5 6 7 3 d 1 2 p 1 2 3 4 5 6 d 1 2 3 4 5 6 7 8 9 10 f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 s + 1 s 2 2 p 2 Is this what you were thinking? Notice how C is in the 2 p 2 spot. So its e- config is 1 s 2 2 p 2 Notice also how we fill a sublevel…Hund’s Rule
Electron Configurations Element Configuration H Z=1 1 s 1 He Z=2 1 s 2 Li Z=3 1 s 22 s 1 Be Z=4 1 s 22 s 2 B Z=5 1 s 22 p 1 C Z=6 1 s 22 p 2 N Z=7 1 s 22 p 3 O Z=8 1 s 22 p 4 F Z=9 1 s 22 p 5 Ne Z=10 1 s 22 p 6 (2 p is now full) Cl Z=17 1 s 22 p 63 s 23 p 5 Na Z=11 1 s 22 p 63 s 1 K Z=19 1 s 22 p 63 s 23 p 64 s 1 Sc Z=21 1 s 22 p 63 s 23 p 64 s 23 d 1 Fe Z=26 1 s 22 p 63 s 23 p 64 s 23 d 6 Br Z=35 1 s 22 p 63 s 23 p 64 s 23 d 104 p 5 Note that the numbers in the electron configuration add up to the atomic number for that element. Ex: for Ne (Z=10),
Periodic Pattern • Look again at the electron configurations of hydrogen, lithium, sodium and potassium. • Do you see any similarities? • Since H and Li and Na and K are all in Group 1 A, they all have the same ending. (s 1) Element Configuration H Z=1 1 s 1 Li Z=3 1 s 22 s 1 Na Z=11 1 s 22 p 63 s 1 K Z=19 1 s 22 p 63 s 23 p 64 s 1
Periodic Pattern This similar electron configuration for members of a group (a column) causes them to behave the same chemically. It’s for that reason the periodic table is arranged as it is. Each group will have the same ending configuration, in this case the electron configuration always ends in s 1.
Periodic Table & Electron Configuration
Noble Gas Electron Configurations Simplified method of writing an electron configuration abbreviated form 1. Use noble gas symbol from last full shell 2. Place noble gas symbol in square brackets Represents the core (inner) electrons 3. Write the remaining valence electrons after the bracketed noble gas symbol
Noble Gas Electron Configurations Ca electron configuration: 1 s 22 p 63 s 23 p 64 s 2 Ca abbreviated electron configuration (Noble gas electron configuration): [Ar]4 s 2 [Ar] = the 18 core electrons (1 s 22 p 63 s 23 p 6) 4 s 2 represents the 2 valence electrons (in n = 4 shell)
Learning Check Write the abbreviated electron configuration of Al.
Learning Check Abbreviated electron configuration of Al: Al is in the n = 3 shell and has 13 electrons Last full shell is n = 2 shell Noble gas element for n = 2 shell is Ne Ne has 10 electrons; we need to add 3 electrons [Ne]3 s 23 p 1
Learning Check Write the abbreviated electron configuration of Sn. Sn is atomic number 50
Learning Check Abbreviated electron configuration of Sn: Sn has 50 electrons, and is in the n = 5 shell Use the noble gas symbol from n = 4 Kr [Kr]5 s 24 d 105 p 2
Lewis Dot Diagrams • Use only the valence electrons • Gives fundamental information about chemical behavior of the element • Simple representation that shows periodic trends
Lewis Dot Diagrams Steps: 1. Write the chemical symbol for element 2. Count the valence electrons 3. Use one dot for each valence electron 4. Dots should be placed following the basic rules governing orbital diagrams: do not pair if not needed and place as far away as possible (electrons repel each other)
Lewis Dot Diagrams Examples: Element Column IA Column IIA Li Be B 1 2 3 Column VA Column VIIA Column VIIIA C N O F Ne 4 5 6 7 8 Column IIIA Column IVA Lewis Structure number of valence electrons
Learning Check Write the Lewis dot diagrams for Si, S, and Ar. Si = ? Valence electrons S = ? Valence electrons Ar = ? Valence electrons
Learning Check Write the Lewis dot diagrams for Si, S, and Ar. 4 valence 6 valence 8 valence electrons