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ELECTRONS IN ATOMS PART 2 – QUANTUM MECHANICAL MODEL QUANTUM MECHANICAL MODEL • The Bohr Model of the atom has several short comings • Cannot work beyond element 21 very well • Doesn’t describe certain behaviour of atoms or molecules • The model currently used to describe the atom is the Quantum Mechanical Model of the atom • This is the current theoretical framework that is used to describe all of the information we have about atoms and how they function BASIC DEFINITIONS • Quantum (plural ‘quanta’) • A finite amount of energy • i.e. – an energy level in an atom • The amount of energy required to move an electron from its present energy level to the next higher one • Mechanical • Movement of parts in relation to a whole • i.e. – electrons in an atom • Hence the Quantum Mechanical Model deals with the movement and location of electrons in an atom UNCERTAINTY PRINCIPLE • We cannot know where an electron is and where it is going at the same time • The more precisely the position of an electron is known, the less precisely its momentum is known and vice versa • Because of this, we use probability to determine where an electron is most likely to be ELECTRON CLOUDS • Using the electron probabilities, we find areas where electrons are most likely to be • These areas are called electron clouds where the probabilities of finding electrons is very high • The shapes and distance from the nucleus of these electron clouds depends on several factors QUANTUM NUMBERS Principal Quantum Number • • • Energy level Distance from the nucleus Represents the PERIOD on the periodic table Angular Quantum Number • The shape of the orbital; represented the letters s, p, d, and f. Magnetic Quantum Number • Determines the orientation of the orbital in space • Spin Quantum Number • • Which axis it lies on Specifies the value for spin Electrons in the same orbital must spin in opposite directions QUANTUM NUMBERS • Principal Quantum Number • Energy level • Distance away from the nucleus • As # increases, distance from the nucleus also increases • As the number increases, so does the energy of the electrons in those orbitals • Represented by integers 1,2,3,4,5,6,7 that correspond to the seven horizontal rows on the periodic table • Determined by counting as you move down (top to bottom) the periodic table QUANTUM NUMBERS • Angular Quantum Number • Also known as “sub-shells” • Refer to the shape of the orbital • There are four (4) different shapes • S, P, D, F • These correspond to the s, p, d, f blocks on the periodic table QUANTUM NUMBER “S” Sub-shell Spherical shape Only one (1) orbital per energy level The 1 sub shell can hold 2 electrons One with +1/2 spin One with -1/2 spin QUANTUM NUMBERS “P” Sub-shell Dumbbell shape Three (3) orbitals per energy level mL and be from –L to +L Each shell can hold 2 electrons 3 orbitals mean the pshell can hold up to 6 electrons QUANTUM NUMBERS • “D” Sub-shell • Tend to have a clover-leaf shape • Five (5) orbitals per energy level • Each can hold a maximum of two (2) electrons • Can hold a max of 10 electrons QUANTUM NUMBERS • “F” Sub-shell • Shape contains 6 lobes for the most part • Seven (7) orbitals per energy level • Each can hold a maximum of two (2) electrons • Fourteen (14) electrons total at each energy level TO SUMMARIZE Sub-shell Number of sub-shells at a level S Energy level (n) in which it is first found 1 1 Number of electrons in these subshells 2 P 2 3 6 D 3 5 10 F 4 7 14 QUANTUM NUMBERS • Spin Quantum Number • Remember, in each sub-shell there can be two (2) electrons • These electrons must have spins that go in opposite directions • Represented by arrows pointing in opposite directions REPRESENTING ELECTRONS USING THE QUANTUM MECHANICAL MODEL • There are two (2) different types of notation used to represent the quantum mechanical model: • Orbital Notation • Electron Configuration Notation ORBITAL NOTATION • Illustrates the following quantum numbers: principal, second (shape), and spin • Use the template to draw and “fill” the sub-shells with electrons • Order of filling electrons is governed by three (3) rules: • Aufbau Principle • Pauli Exclusion Principle • Hund’s Rule ORBITAL NOTATION Aufbau Principle: Electrons enter sub-shells of lowest energy first 1st energy level fills up before the next Pauli Exclusion Principle: All atomic sub-shells contain a maximum of two (2) electrons. Each MUST have a different spin Hund’s Rule: when electrons occupy sub-shells of equal energy, ONE electron enters EACH sub-shell until all the subshells contain one electron with identical directions Electrons are added to sub-shells so that a maximum number of unpaired electrons result EXAMPLES • Oxygen • Titanium • Strontium ELECTRON CONFIGURATION • Illustrates the following quantum numbers: principal, and angular (shape) • Does not indicate the spin, but it does indicate the number of electrons • Easier to use because there is less to draw ELECTRON CONFIGURATION EXAMPLES: • Nitrogen • Chlorine • Copper • Tin NOBLE GAS NOTATION • An even more simplified and shorthand method for representing electron configuration. • Emphasizes the outermost energy level only • Instead of listing every energy level and amount of electrons individually, it utilizes the nearest noble gas element of the energy level below as a representation of the inner energy levels NOBLE GAS NOTATION • For Example: Sulfur • Electron configuration would be: • 1s22s22p63s23p4 • Its Noble Gas Notation would be: • [Ne] 3s23p4 • …this is because we know that the electron configuration of Ne is: 1s22s22p6, therefore there is no need to write it all out.