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Bohr Atom and Electron Configuration Study Guide

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Updated on Sep 24, 2011

Introduction

An electron's position in an atom or ion can be described by determining its electron configuration and orbital diagram. These representations of an atom or ion can explain physical and chemical properties of the substance, including magnetic attraction.

Bohr Atom

Niels Bohr's "planetary" model of the hydrogen atom—in which a nucleus is surrounded by orbits of electrons—resembles the solar system. Electrons could be excited by quanta of energy and move to an outer orbit (excited level). They could also emit radiation when falling to their original orbit (ground state). Basic components of the Bohr model include the following:

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Energy levels: Energy levels are the volume of space where certain electrons of specific energy are restricted to move around the nucleus. Energy levels consist of one or more orbitals. Energy levels are categorized by the letter n using whole numbers (n = 1, 2, 3, 4 . . . ).
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Orbitals: An orbital is the space where one or two paired electrons can be located or the probability of an electron's location. These are mathematical functions (i.e., figures) with specific shapes (s orbitals: spherical; p orbitals: dumbbell, etc.; see Figure 10.1) and restricted zones (called nodes; see Figure 10.2). The nodes represent areas where the probability of an electron is zero.
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Outer or valence shell: The valance shell is the last energy level containing loosely held electrons. These are the electrons that engage into bonding and are therefore characteristic of the element's chemical properties.

Figure 10.1

Figure 10.2

Electron Configuration

Electron configurations describe the exact arrangement of electrons (given as a superscript) in successive energy levels or shells (1, 2, 3, etc.) and orbitals (s, p, d, f) of an atom, starting with the innermost electrons. For example, a lithium atom's configuration is 1s22s1. The superscripts mean two electrons are in the 1s orbital and one electron is in the 2s orbital. Several "rules" are applied to the filling of electrons:

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Pauli exclusion principle: The Pauli exclusion principle states that an orbital can hold a maximum of two electrons if they are of opposite spins. In other words, every electron has a unique set of quantum numbers.
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Hund's rule: Hund's rule states that the most stable arrangement of electrons in the same energy level in which electrons have parallel spins (same orientation).
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Aufbau principle: The Aufbau principle is based on the Pauli exclusion principle and states that electrons are placed in the most stable orbital. Aufbau means "building up" in German (e.g., 1s2 2s2 2p6).

So how is an electron configuration written? First, the number of total electrons must be determined. This is the equal to the mass number for neutral atoms. For ions, the total electron is corrected for the charge (add electrons for anions; subtract electrons for cations). The electrons are "added" according to Hund's rule and the Aufbau principle. Figure 10.3 describes the order in which the electrons are added. Keep in mind the maximum number of electrons in each type of orbital: s orbitals hold two electrons, p orbitals hold six electrons, d orbitals hold ten electrons, and f orbitals hold 14 electrons.

Figure 10.3 Electron-filling sequence

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p →6 s → 4f → 5d → 6p → 7s → 5d → 6p ...

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