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ELECTRON CONFIGURATIONS Atomic Models Dalton’s Model Thomson’s Model Rutherford’s Model (Plum-pudding) Rutherford’s atomic model lacked detail on how electrons occupied the space around nucleus Why don’t electrons crash into nucleus? Chemical Behavior of elements • Light • At high temperatures or voltages, elements in the gaseous state emit light of different colors. • When the light is passed through a prism or diffraction grating a line spectrum results. Bohrs Atomic model Bohr model : After Rutherford's discovery, Bohr proposed that electrons travel in definite orbits around the nucleus. (planetary model) •Explained hydrogen’s spectrum but not for other elements. Each element has its own unique set of spectral emission lines that distinguish it from other elements. Line spectrum of hydrogen. Each line corresponds to the wavelength of the energy emitted when the electron of a hydrogen atom, which has absorbed energy falls back to a lower principal energy level. Modern View Movement of electrons is not completely understood The atom is mostly empty space Two regions Nucleus protons and neutrons Electron cloud region where you might find an electron Quantum mechanical model Modern atomic theory described the electronic structure of the atom as the probability of finding electrons within certain regions of space. • Instead of being located in orbits, the electrons are located in orbitals. • An orbital is a region around the nucleus where there is a high probability of finding an electron, can hold a maximum of 2 electrons. Quantum Numbers Four Quantum Numbers: Specify the “address” of each electron in an atom Principal Quantum Number ( n ) Angular Momentum Quantum number ( l ) Magnetic Quantum Number ( ml ) Spin Quantum Number ( ms ) 1. Principal Quantum Number ( n ) Indicates the number of the energy level As n increase, size of electron cloud increases. 1s Energy increases as n increases. (electrons closer to nucleus have less energy) 2s 2n2 = maximum # of electrons possible in the energy level Ex. if n=1 (energy level 1) it can only have 2 electrons n 3s Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem 2. Angular Momentum Quantum Number ( l ) Describes the sublevel within each energy level # of sublevels = value of principal quantum number of that level Ex. n=1, has 1 sublevel n=2, has 2 sublevels lowest sublevel : s second sublevel : p third sublevel : d fourth sublevel : f There is just one s sublevel , thus it has one orbital that can hold only 2 electrons. There are three p sublevels and thus it has s three orbitals; each can hold 2 electrons. There are five d sublevels and thus it has five orbitals; each can hold 2 electrons. There are seven f sublevels and thus it has seven orbitals; each can hold 2 electrons. Too complicated to show with drawings Classwork P 118 # 6 and p122 # 7,8 3. Magnetic Quantum Number ( ml ) Specifies the exact orbital within each sublevel 4. Spin Quantum Number ( ms ) An orbital can hold 2 electrons that spin in opposite directions. Indicated by arrows (in opposite direction): General Rules For Writing Electron Configurations 1. Pauli Exclusion Principle Each orbital can hold TWO electrons with opposite spins. In the following diagrams boxes represent orbitals. • Electrons are indicated by arrows: ↑ or ↓. 2. Aufbau Principle Electrons fill the lowest energy orbitals first. The number represents n, the principal quantum number 3. Hund’s Rule Within a sublevel, place one e- per orbital before pairing them. WRONG RIGHT Notation Orbital Diagram O 8e- 1s 2s 2p Electron Configuration 1s 2 2s2 2p4 C. Johannesson Classwork p 128 #14 Electron Dot Diagrams The electrons in the outer energy level (greatest value of n ) called valence electrons are the most important electrons for chemical reactions. Lewis electron dot diagrams are used to represent these outer electrons around the symbol of an element. Examples Lithium Electron configuration: 1s22s1 Select electrons that are in the outer energy level (the ones with the largest principal quantum number): 1s22s1 Largest principal quantum number is 2 and there is 1 electron in this level Li Valence electron 1. Symbol of element represents nucleus and all electrons except those in outer level 2. Write the electron configuration of element to determine valence electrons. 3. Each side of symbol represents an orbital, draw dots to represent electrons in that orbital. Oxygen: 1s22s2 2p4 Oxygen: 1s2 2s2 2p4 Oxygen: has 6 valence electrons (2 +4) O Krypton: 1s22s2 2p6 3s23p6 4s2 3d10 4p6 Krypton: 1s22s2 2p6 3s23p6 4s2 3d10 4p6 krypton: has 8 valence electrons (2 +6) Kr Classwork p 130 # 15 (Z= atomic number)
"unit 3 atomic structure electron configurations"