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THE TRUE MYSTERY OF THE WORLD IS THE VISIBLE, NOT THE INVISIBLE. - Oscar Wilde -

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ATOMIC STRUCTURE "I think I can safely say that nobody understands quantum mechanics." - Richard Feynman- - Richard Feynman-

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QUANTUM OF ENERGY – THE ENERGY REQUIRED TO MOVE AN ELECTRON FROM ONE ORBIT OR ENERGY LEVEL TO ANOTHER. PHOTONS OF LIGHT REPRESENT DISCRETE PACKETS OF ENERGY WITH THE WAVELENGTH OF LIGHT DETERMINING THE AMOUNT OF ENERGY E = hc/ where h = constant, c = speed of light = wavelength of light = wavelength of light ERWIN SCHRODINGER USED THESE RELATIONSHIPS TO DESCRIBE THE BEHAVIOR OF ELECTRONS IN ATOMS. ATOMIC ORBITAL – A REGION IN SPACE WITH A HIGH PROBABILITY OF FINDING AN ELECTRON.

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WHERE DO OUR CURRENT IDEAS OF ATOMIC STRUCTURE COME FROM? de Broglie put forward that all objects in motion have wave nature. The smaller an object is, the greater the wave nature. This means that the electron would behave as much like a wave as a particle. Schrodinger came up with a differential wave equation that would describe the motion of an electron about a nucleus in three dimensions. Chemists call these wave functions “orbitals”.

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The math involved is very complex. However, the results do an excellent job of describing why atoms of different elements behave as they do, and they are very useful in describing how atoms interact to form molecules. They even do a good job helping to describe the shapes of molecules. This is very important in biochemistry and medicine.

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From the quantum theory, there are four quantum numbers that describe electron orbitals. n = principal quantum number - what shell the electron goes in (n = 1, 2, 3...) l = angular momentum quantum number - what subshell the electron goes in (0 < l < n-1) m l = magnetic quantum number – how many orbitals the subshell is broken into (-l < m l < l) m s = spin quantum number (- 1/2 or + 1/2)

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Principal Quantum Number (shell) Number of SubshellsType of Subshell n = 111s (1 orbital) n = 222s (1 orbital) 2p (3 orbitals) n = 333s (1 orbital) 3p (3 orbitals) 3d (5 orbitals) n = 444s (1 orbital) 4p (3 orbitals) 4d(5 orbitals) 4f (7 orbitals)

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The quantum numbers, in a sense, tell us how many electrons can go where. There are some rules that tell us how electrons fill shells.

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ELECTRON FILLING RULES Aufbau (build-up) Principal - Aufbau (build-up) Principal - Electrons enter and fill lower energy orbitals before higher energy orbitals. Pauli Exclusion Principal - orbitals can contain a maximum of two electrons which must be of opposite spin. Hund’s Rule - when there are orbitals of equal energy, electrons will enter the orbitals one- at-a time until all of the orbitals are half filled and only then will pairing occur.

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So, let’s take a look at how electrons go into orbitals as we move through the periodic table. Hydrogen has only one electron, so it would go in the first shell. The first shell has only one subshell, the 1s. So, we’d write that as: Hydrogen, H 1s 1

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Helium has an atomic number of 2, so it has two electrons. This second electron would also go in the 1s subshell, but it would have opposite spin. Hydrogen, H 1s 1 Helium, He 1s 2 This fills up this shell and subshell.

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Notice H and He are in the first row or first period. Electrons are going into the first shell.

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Lithium has an atomic number of 3, so this means that it has 3 electrons. The first shell is filled, so the third electron has to go in the second shell. It will go in the 2s subshell, as this has the lowest energy. Hydrogen, H 1s 1 Helium, He 1s 2 Lithium, Li 1s 2 2s 1

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Beryllium has an atomic number of 4, so 4 electrons. Two in the first shell, and two in the 2s subshell. Hydrogen, H 1s 1 Helium, He 1s 2 Lithium, Li 1s 2 2s 1 Beryllium, Be 1s 2 2s 2 This fills the 2s subshell. The next electrons will have to go in the 2p subshell.

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Boron has an atomic number of 5, so it has 5 electrons. B 1s 2 2s 2 2p 1 There are actually 3 2p subshells, all of equal energy, p x, p y, p z. One electron will go into each of these. After each has an electron, the the additional three electrons will pair up. Right now, don’t be too concerned. Just remember that 6 electrons can go into the p subshells.

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So, for the rest of the second row (period), we have: Lithium, Li 1s 2 2s 1 Beryllium, Be 1s 2 2s 2 Boron, B 1s 2 2s 2 2p 1 Carbon, C 1s 2 2s 2 2p 2 Nitrogen, N 1s 2 2s 2 2p 3 Oxygen, O 1s 2 2s 2 2p 4 Fluorine, F 1s 2 2s 2 2p 5 Neon, Ne 1s 2 2s 2 2p 6 This gives us a filled first shell and a filled second shell.

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The order in which the third shell would fill would be: Sodium, Na 1s 2 2s 2 2p 6 3s 1 Magnesium, Mg 1s 2 2s 2 2p 6 3s 2 Aluminum, Al 1s 2 2s 2 2p 6 3s 2 3p 1 Silicon, Si 1s 2 2s 2 2p 6 3s 2 3p 2 Phosphorus, P 1s 2 2s 2 2p 6 3s 2 3p 3 Sulfur, S 1s 2 2s 2 2p 6 3s 2 3p 4 Chlorine, Cl 1s 2 2s 2 2p 6 3s 2 3p 5 Argon, Ar 1s 2 2s 2 2p 6 3s 2 3p 6

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There is one very important reason we are going through this. It is the number of electrons in the outer shell (valence electrons) that determine the properties of the element. If you look at the periodic table, you will see that all of the elements in a given column have the same number of valence electrons. So, they will have similar properties. A column is called a family.

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A row is called a period. The period number tells what shell is being filled. The octet rule says that the maximum number of electrons that can occur in any outer shell is 8, except for the first shell, which is 2. This represents a stable configuration, and elements will react by losing, gaining or sharing electrons to obtain this configuration.

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