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Chemistry Wilbraham Staley Matta Waterman Chapter 5: Electrons in Atoms Copyright © 2005 Pearson Education & Prentice-Hall, Inc. How do we make fireworks? Fireworks! The brilliant colors of fireworks are produced by using compounds containing different elements. In this chapter, you will learn how elements can emit light of different colors. The chemistry of Fireworks http://www.youtube.com/watch?v=OKklDcS3FsA&feature=pla yer_embedded 5.1 Revising the Atomic Model Energy levels in Atoms We know atoms consist of protons and neutron making up the nucleus surrounded by electrons. Limitations of Rutherford’s Atomic Model What were the limitations? Limitations of Rutherford’s Atomic Model Explained a few simple properties of atoms. Could not explain the chemical properties of elements. Could not explain why metals or compounds of metals give off characteristic colors when heated in a flame. Could not explain why an object such as iron heated first glows dull red, then yellow, then white when heated to higher and higher temperatures. The Bohr Model In 1913, Niels Bohr, a Danish physicist and a student of Rutherford, developed a new atomic model. Incorporated newer discoveries about how the energy of an atom changes when the atom absorbs or emits light. He considered the simplest atom, hydrogen, which has one electron. Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. Key points of Bohr’s model Each possible electron orbit has a fixed energy. The fixed energies an electron can have are called energy levels. Think of these like the rungs of a ladder. Rungs of ladders The lowest rung of the ladder corresponds to the lower energy. A person can climb up or down the ladder by stepping from rung to rung. Similarly, an electron can move from one energy level to another. To move up A person on the ladder cannot stand between rungs. Similarly, the electrons in an atom cannot exist between energy levels. To move from one rung to another, a person climbing the ladder must move just the right distance. To move from one energy to another, an electron must gain or lost just the right amount of energy. A quantum of energy is the amount of energy required to move an electron from one energy level to another energy level. The word quantity has the same root as quantum and means “a definite amount or number”. The energy of an electron is therefore said to be quantized. The amount of energy an electron gains or losses in an atom is not always the same. Energy levels The rungs of a ladder are somewhat like the energy levels of Bohr’s model of the atom. In an ordinary ladder, the rungs are equally spaced. The energy levels in atoms are unequally spaced, like the rungs in this unusual ladders. The higher energy levels are closer together. The energy levels in an atom are not equally spaced. The higher energy levels are closer together. It takes less energy to climb from one rung to another near the top of the ladder. Bohr fail to explain the energies absorbed and emitted by atoms with more than one electron. The Quantum Mechanical model The modern description of the electrons in atoms, the quantum mechanical model, came from the mathematical solutions to the Schrodinger equation. The quantum mechanical model does not specify an exact path the electron takes around the nucleus. The quantum mechanical model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus of an atom. Probability describes how likely it is to find an electron in a particular location around the nucleus of an atom. Probability demonstration Describe how electrons move around the nucleus is similar to a description of how the blades of a windmill rotate. The blurry region they produce in the picture, but you cannot predict their exact locations at any instant. The probability of finding an electron within a certain volume of space surrounding the nucleus can be represented as a fuzzy cloud-like region. The cloud is more dense where the probability of finding the electron is high less dense where the probability of finding the electron is low. No boundary to the cloud because there is a slight change of finding the electron at a considerable distance from the nucleus. Atomic Orbitals How do sublevels of principal energy levels differ? Atomic Orbitals Schrodinger equation also leads to a mathematical expression, called an atomic orbital. The atomic orbital, describes the probability of finding an electron at various locations around the nucleus. pictorial representation shows a region of space in which there is a high probability of finding an electron. The energy levels of electrons in the quantum mechanical model are labeled by principal quantum numbers (n). These numbers are assigned the values n= 1, 2, 3, 4, and so forth. Each energy sublevel corresponds to one or more orbitals of different shapes. The orbitals describe where an electron is likely to be found. Denoted by letters. http://www.slideshare.net/wsautter/elect ron-configuration-of-atoms S orbitals are spherical P orbitals are dumbbell-shaped Three kinds of p orbitals have different orientations in space. Four or five kinds of d orbitals are in your book on page 131. Shapes of s and p orbitals For a given principal energy level greater than 1, there is one s orbital and three p orbitals. Shapes of d orbitals Four of the five d orbitals have the same shape but different orientations. How are the orientations of dxy and dx2-y2 orbitals similar? How are they different? d orbitals Electron Orbitals Periodic Table S Orbitals Each orbital holds 2 electrons. Left side of the periodic table are the ‘s orbitals’. P Orbitals There are 3 sub orbitals Each contains 2 electrons totals electrons = 6 groups 13-18 on the periodic table are p orbitals D Orbitals There are 5 sub orbitals. Each contains 2 electrons. Total electrons = 10 Groups 3-12 on periodic table are D orbitals. F Orbital There are 5 sub orbitals. Each orbital contains 2 electron. Total electrons = 10. Groups at the very bottom of the periodic table are F orbitals. Sorting Periodic Table into Orbital Types What type of electrons are in elements Electron Configuration Worksheet for HW An angular node is a flat plane such as the ones shown in the diagram above. The ℓ quantum number determines the number of angular nodes an orbital will have. A radial node is a circular ring that occurs as the principle quantum number increases. Thus, n tells us how many radial nodes an orbital will have and is calculable with the equation: Total # of nodes = n-1. Problems 1. Which orbital would the electrons fill first? The 2s or 2p orbital? 2. How many d orbitals are there in the d subshell? 3. How many electrons can the p orbital hold? 4. Determine the number of angular and radial nodes of a 4f orbital. 5. What is the shape of an orbital with 4 radial nodes and 1 angular node in the xy plane? Solutions 1. 2. 3. 4. 5. The 2s orbital would be filled before the 2p orbital because orbitals that are lower in energy are filled first. The 2s orbital is lower in energy than the 2p orbital. There are 5 d orbitals in the d subshell. A p orbital can hold 6 electrons. Based off of the given information, n=4 and ℓ=3. Thus, there are 3 angular nodes present. The total number of nodes in this orbital is: 4-1=3, which means there are no radial nodes present. 1 angular node means ℓ=1 which tells us that we have a p subshell, specifically the pz orbital because the angular node is on the xy plane. The total number of nodes in this orbital is: 4 radial nodes +1 angular node=5 nodes. To find n, solve the equation: nodes=n-1; in this case, 5=n-1, so n=6. This gives us a: 6pz orbital Electron Configuration http://www.germane-software.com/~dcaley/atom/Atom.html