
UNIT # 3
Introduction
Objectives
Reading
3.1 Structure of the Atom 3.2 Isotopes 3.3 Arrangement of Electrons 3.4 Electron Configura tions 3.5 Atomic Mass
Problems 1 \| 2 \| 3

ATOMIC STRUCTURE

3.4 - Electron Configurations of Atoms

3.4.3 - Electron Configuration for Atoms of the First 20 Elements

When the electrons are arranged in their lowest energy state, the atom is in the ground state. The following table summarizes the ground state electron configuration of the first 20 elements on the periodic table.

NB: the superscripts add up to the atomic number of the atom.

Name	Atomic Number	Electron Configuration
Period 1
Hydrogen	1	1s¹
Helium	2	1s²
Period 2
Lithium	3	1s² 2s¹
Beryllium	4	1s² 2s²
Boron	5	1s² 2s²2p¹
Carbon	6	1s² 2s²2p²
Nitrogen	7	1s² 2s²2p³
Oxygen	8	1s² 2s²2p⁴
Fluorine	9	1s² 2s²2p⁵
Neon	10	1s² 2s²2p⁶
Period 3
Sodium	11	1s² 2s²2p⁶3s¹
Magnesium	12	1s² 2s²2p⁶3s²
Aluminum	13	1s² 2s²2p⁶3s²3p¹
Silicon	14	1s² 2s²2p⁶3s²3p²
Phosphorus	15	1s² 2s²2p⁶3s²3p³
Sulfur	16	1s² 2s²2p⁶3s²3p⁴
Chlorine	17	1s² 2s²2p⁶3s²3p⁵
Argon	18	1s² 2s²2p⁶3s²3p⁶
Period 4
Potassium	19	1s² 2s²2p⁶3s²3p⁶4s¹
Calcium	20	1s² 2s²2p⁶3s²3p⁶4s²

Section 4.8
Arrangement of Electrons in Sublevels ..p95

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ATOMIC STRUCTURE 3.4 - Electron Configurations of Atoms

ATOMIC STRUCTURE

3.4 - Electron Configurations of Atoms