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Section 4–3: Modern Atomic Theory Coach Kelsoe Physical Science Pages 113–118 Objectives • Describe Bohr’s model of the atom and the evidence for energy levels. • Explain how the electron cloud model represents the behavior and locations of electrons in atoms. • Distinguish the ground state from excited states of an atom based on electron configurations. Bohr’s Model of the Atom • Niels Bohr, a Danish physicist, created a new and updated model of the atom. • His new model resembled the solar system, with electrons orbiting the nucleus like planets orbit the sun. Bohr’s Model of the Atom • Bohr, who worked with Ernest Rutherford, agreed with Rutherford’s model of a nucleus surrounded by a large volume of space. • Bohr’s model added to Rutherford’s by focusing on electrons. Energy Levels • In Bohr’s model, electrons move with constant speed in fixed orbits around the nucleus. • Each electron in an atom has a specific amount of energy. • If an atom gains or loses energy, the energy of an electron can change. • The possible energies that electrons in an atom can have are called energy levels. Energy Levels • Energy levels are a lot like a staircase. You can measure your position by the distance. • No two elements will have the same set of energy levels. Energy Levels • An electron in an atom can move from one energy level to another when the atom gains or loses energy. • Electrons can move up multiple energy levels at a time if it gains the right amount of energy. The size of the jump between energy levels determines the amount of energy gained or lost. Evidence for Energy Levels • Scientists can measure the energy gained when electrons absorb energy and move to a higher energy level and they can measure the energy released when the electron returns to a lower energy level. Fireworks • The movement of electrons between energy levels explains the light you see when fireworks explode. • When the explosion happens, electrons absorb the energy and the electrons jump to higher energy levels. When they fall to a lower level, they emit different colors of light. Electron Cloud Model • Bohr’s model was improved over time – he was right about energy levels, but was wrong in assuming that electrons moved like planets in the solar system. Electrons are much less predictable than this. Electron Cloud Model • An electron cloud is a visual model of the most likely locations for electrons in an atom. The cloud is denser at those locations where the probability of finding an electron is high. Electron Cloud Model • Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus. • This cloud is much like the blur you see around a spinning propeller. Atomic Orbitals • The electron cloud represents all the orbitals in an atom. • An orbital is a region of space around the nucleus where an electron is likely to be found. • Your textbook compares an orbital to the locations you visit most at school. Atomic Orbitals • An electron cloud is a good approximation of how electrons behave in their orbitals. • Each energy level has different numbers of orbitals, and each increases as the distance from the nucleus increases. Energy Levels, Orbitals, and Electrons Energy Level (n) Number of Orbitals (n2) Maximum Number of Electrons (2n2) 1 1 2 2 4 8 3 9 18 4 16 32 Electron Configurations • An electron configuration is the arrangement of electrons in the orbitals of an atom. • The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies. Electron Configurations • When all the electrons in an atom have the lowest possible energies, the atom is said to be in its ground state. • For example, a lithium atom with 3 electrons is in ground state when two of its electrons are in the orbital in the first energy level and the other is in the first orbital in the second energy level. Vocabulary • • • • • Energy levels Electron cloud Orbital Electron configuration Ground state