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Arrangement of Electrons in Atoms • Principles of electromagnetic radiation led to Bohr’s model of the atom. • Electron location is described using identification numbers called quantum numbers. • Rules for expressing electron location results in a unique electron configuration for each element. Mullis Chemistry Holt Ch.4 1 Building electron configurations for the ground state of an atom Aufbau: Lowest energy level 1st Pauli Exclusion: Only 2 e- per orbital, opposite spin Hund: One electron per orbital until that level is full (same spin) Theater analogy: 1. Enter at front. Climb stairs and sit in first empty section you reach. 2. Each bench seats 2 people. One male and one female per bench. 3. Within a level, ladies first. One lady per bench until each bench is occupied, then one man per bench to fill that level. Mullis Chemistry Holt Ch.4 2 Example: Building electron configurations for the ground state of an atom Phosphorous: 15 electrons: Start with 1s. Each s can hold 2 electrons. Each p can hold 6 electrons. Place one electron per orbital, then pair until that level is full. Only then go to the next level. Mullis Chemistry Holt Ch.4 3 Wave Description of Light • Electromagnetic radiation is a form of energy that exhibits wavelike behavior as it travels through space • Wavelength (λ) – Distance between corresponding points on adjacent waves. – Unit: nm,cm,m • Frequency (ν) – Number of waves that pass a specific point in a given time – Unit: Hz or waves/sec Recall that Speed = Distance/time (m/sec) • Speed of light (c) C=λν Mullis Chemistry Holt Ch.4 4 Behavior of Light • Photoelectric effect – The emission of electrons when light shines on the metal – Scientists found that below a certain frequency, no electrons were emitted. – Light also behaves as a particle: Since hot objects do not emit em energy continuously, they must emit energy in small chunks called quanta. • Quantum – Minimum quantity of energy that can be gained or lost by an atom Mullis Chemistry Holt Ch.4 5 Light as a particle and a wave Planck and Einstein • Max Planck: Relationship between quantum of energy and wave frequency • Planck’s constant h = 6.626 x 10-34 J-s E = hν E is energy, ν is frequency • Albert Einstein: Established dual wave-particle nature of light 1st – Einstein explained PE effect by proposing that EM radiation is absorbed by matter only in whole numbers of photons. – Electron is knocked off metal surface only if struck by one photon with certain minimum energy. Mullis Chemistry Holt Ch.4 6 Quantum Theory • Ground state: An atom’s lowest energy state • Excited state: Higher potential energy than ground state. • Photon: A particle of electromagnetic radiation having zero mass and carrying a quantum of energy (i.e., packet of light) • Only certain wavelengths of light are emitted by hydrogen atoms when electric current is passed through—Why? Mullis Chemistry Holt Ch.4 7 Niels Bohr links hydrogen’s electron with photon emission • Bohr proposed that an electron circles the nucleus in allowed orbits at specific energy levels. – Lowest energy is close to nucleus • Bohr’s theory explained the spectral lines seen in hydrogen’s line emission spectrum, but it did not hold true for other elements. Mullis Chemistry Holt Ch.4 8 Quantum Numbers • • • • Principal quantum number Angular momentum quantum number Magnetic quantum number Spin quantum number Mullis Chemistry Holt Ch.4 9 Quantum numbers 1s ____ 2s ____ 3s _____ Principal quantum number 2p ____ ____ ____ Magnetic quantum number Angular momentum quantum number Mullis Chemistry Holt Ch.4 10 Magnetic quantum number • Magnetic quantum number is the orientation of an orbital around the nucleus. • It is the number of orbitals in a sublevel. Orbitals per sublevel The s sublevel has 1 orbital. The p sublevel has 3 orbitals. The d sublevel has 5 orbitals. The f sublevel has 7 orbitals. Mullis Chemistry Holt Ch.4 s p d f 1 3 5 7 11 Atomic Structure Summary of Contributions • Max Planck – A hot object emits energy in small, specific amounts called quanta. • Albert Einstein – E = mc2 – Light behaves as both a wave and a particle. – Each particle of light carries a photon ( a quantum of energy). • Niels Bohr – Created a model of the atom that showed a single electron of hydrogen orbits the nucleus only in Mullis Chemistry Holt Ch.4 allowed orbits with a fixed energy. 12 Atomic Structure Summary of Contributions, continued • Werner Heisenberg – Uncertainty principle: It is impossible to determine simultaneously both the position and velocity of an electron. • Erwin Schrödinger – Helped lay the foundation for quantum theory with an equation that treats electrons like waves. Mullis Chemistry Holt Ch.4 13 Electron Configuration: The Rules • Aufbau principle – An electron occupies the lowest energy orbital that will receive it. • Pauli exclusion principle – No two electrons in the same atom can have the same set of 4 quantum numbers. – Therefore, electrons can pair in an orbital as long as their spins are opposite. • Hund’s rule – Each of the orbitals at a particular level have one electron before any of them can have two electrons. – All single electrons in the orbitals at a particular level 14 have the same spin.Mullis Chemistry Holt Ch.4