A review of the Evolution of the Model of the Atom Chapter 13-1 Models of the Atom A review of the Evolution of the Model of the Atom Chapter 13-1

The Different Models of the Atom 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. Thomson Model

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.

Electron Configuration In the atom, electrons and the nucleus interact to make the most stable arrangement possible. The way in which electrons arranged around the nuclei are called electron configurations.

Principal Quantum numbers (n) Also known as Principal energy level. Each Principal quantum number refers to a energy level in an atom. Look at the periodic table. Along the left side 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 …

2s (1orbital) 2p (3 orbitals) Principal energy Level Number of sublevels Type of sublevel 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

Electrons enter orbitals of lowest energy first. 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. And so on

Notation for Quantum Numbers Principle Quantum Number (n) 1s2 Number of Electrons Angular Momentum

1s2 In other words This would the second electron The first energy level 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

Only after all the orbitals are filled will electrons pair-up. An electron enters each orbital one at a time until all the orbitals contain one electron. Only after all the orbitals are filled will electrons pair-up. For example: Carbon has 6 electron so it’s configuration is: 1s2 2s2 p2 n=1 and it fills in this order n=2

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Sample Test Question Write the complete electron configuration for each atom. Argon Carbon

Answers 1S2 2S2 2P6 3S2 3P6 1S2 2S2 2P2 Argon Carbon