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Models of the Atom A review of the Evolution of the Model of the Atom Chapter 13-1 The Different Models of the Atom Thomson Model In the XIX, Thomson described the atom as a ball of positive charge with a number of electrons. Said the atom was like plum pudding. Rutherford Model In the Early XX, Rutherford showed that the most of the atom’s mass is concentrated in a small, positively charged nucleus. Atom is mostly empty space. Bohr Model After Rutherford, Bohr proposed that electrons travel in definite orbits around the nucleus. Quantum Mechanical Model Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions in space. Energy Levels/Shells Energy Level Quantum The energy level of an electron is the region around the nucleus where the electron is likely to be moving. It is the amount of energy required to move an electron from its present energy level to the next higher one. Electron Configuration What is it? The configuration is described by 4 quantum numbers. Just like a point can be described by the (x,y) coordinates An electron can be described by 4 quantum numbers. the atom, electrons Configuration and the nucleus interact to make the most stable arrangement possible. The way in which electrons arranged around the nuclei are called electron configurations. Electron In Principal Each Principal quantum number refers to a Quantum numbers (n) energy level in an atom. Look at the periodic Also known table. as Principal Along the left side energy level. you will see numbers from 1 to 7. These represent the energy levels of electrons for that row of atoms. n=1, n=2 , n=3 … Principal energy Level Number Type of sublevel of sublevels n=1 1 1s (1 orbital) n=2 2 2s (1orbital) 2p (3 orbitals) n=3 3 3s (1 orbital) 3p (3orbitals) 3d (5 orbitals) n=4 4 4s (1 orbital) 4p (3 orbitals) 4d (5 orbitals) 4f (7 orbitals) Electron Arrangement in Atoms The Three Rules that tell you how to find the electron configuration of atoms Aufbau Principle Pauli exclusion Principle Hund’s rule Aufbau Principle Pauli Principle Electrons enter orbitals of lowest energy first. An atomic orbital may describe at most two electrons To occupy the same orbital, the two electrons must have opposite spins Hund’s Rule When electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron. Only after all the orbitals are filled will electrons pair-up. • This applies to the p, d and f sublevels. Electrons enter orbitals of lowest energy first. n=1 n=2 n=3 n=4 And so on Notation for Quantum Numbers Principle Quantum Number (n) 1s 2 Number of Electrons Angular Momentum In other words This would the second electron 1s The first energy level 2 the s sublevel of the 1st energy level An atomic orbital may at most describe two electrons. See pg 365 figs 13-4 and 13-5 • s orbital can hold 2 electrons – it has 1 orbital • p orbitals can hold 6 electrons – each of the 3 orbitals can hold 2 electrons • d orbitals can hold 10 electrons – each of the 5 orbitals can hold 2 electrons • f orbitals can hold 14 electrons – each of the 7 orbitals can hold 2 electrons electron Only after all the orbitals are filled enters each will electrons pairorbital one up. at a time For example: until all the Carbon has 6 electron orbitals so it’s configuration is: 1s2 2s2 p2 contain one electron. n=1 An and it fills in this order n=2 THE ULTIMATE TOOL Sample Test Question Write the complete electron configuration for each atom. a. b. Argon Carbon Answers 1S2 2S2 2P6 a. Argon a. 2 Carbon 1S 2S2 3S2 3P6 2P2