When electrons fill the energy levels, it fills principal energy levels, sublevels, atomic orbitals from lowest energy first. to view the order in which the sublevels are ordered according to energy. Look carefully and you will see:

some 4 sublevel is lower in energy than a 3 sublevel (i.e. 4s is lower in energy than 3d;) some 5 or 6 sublevel is lower in energy than a 4 sublevel (i.e. 5p and 6s are lower in energy than 4f; )

At first glance it appears that the sequence for electrons to fill the atomic orbitals are of random order. Read on to find an easier way to remember the order of atomic orbitals according to energy.

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3F - Filling Order of the Sublevels

How do we go about remembering the sequence in which electrons fill the sublevels?

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Follow the arrows. The sublevels are magically arranged in the correct sequence from lowest energy. compare the order of filling sublevel sequence with the energy diagram of the sublevels.
The order in which electrons fill the sublevels is easy to remember if you follow these steps:
Write the principal energy levels and their sublevels on separate lines (as shown on the diagram). Draw arrows over the sublevels (see the red diagonal lines on the diagram by placing your mouse over the diagram).Join the diagonal lines from end to end (click on the diagram to see how I have joined the red diagonal lines).
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3G - Electron Configuration Notations

There is a way to represent precisely the electron arrangement in atoms. Let"s take a look at the simplest atom, hydrogen.

A hydrogen atom has 1 electron. That electron will occupy the lowest principal energy level, n = 1, and the only sublevel, s. We denote the electron configuration of hydrogen as

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Similarly,

Helium has 2 electrons; the 2 electrons both occupy the s sublevel in principal energy level 1. Helium"s electron configuration is 1s2 Lithium has 3 electrons; 2 of the 3 electrons occupy the s sublevel in principal energy level 1. The 3rd electron must go in the next available sublevel, 2s. Lithium"s electron configuration is 1s2 2s1 Beryllium has 4 electrons; 2 of the 3 electrons occupy the s sublevel in principal energy level 1. The 3rd and 4th electrons must go in the next available sublevel, 2s. Beryllium"s electron configuration is 1s2 2s2

The table below shows the electron configuration for the first 20 elements on the periodic table.NB: the superscripts add up to the atomic number of the atom.

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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

3H - Electron Configuration and the Periodic Table

There is a pattern between the electron configuration for the elements and their positions on the periodic table. You should take a look at and look closely at the first 20 elements. Compare the electron configuration of an element and its position on the periodic table.

Elements belonging in Group IA (eg - H, Li, Na, K) all have electron configuration ending in ns1 (the superscript of "1" indicates there is 1 valance electron for elements belonging to Group IA). Elements belonging in Group IIA (eg - Be, Mg, Ca) all have electron configuration ending in ns2 (the superscript of "2" indicates there are 2 valence electrons for elements belonging to Group IIA). Elements belonging in Group IIIA (eg - B, Al) all have electron configuration ending in ns2np1 (the superscripts total to "3" indicates there are 3 valence electrons for elements belonging to Group IIIA). Elements belonging in Group IVA (eg - C, Si) all have electron configuration ending in ns2np2 (the superscripts total to "4" indicates there are 4 valence electrons for elements belonging to Group IVA). Elements belonging in Group VA (eg - N, P) all have electron configuration ending in ns2np3 (the superscripts total to "5" indicates there are 5 valence electrons for elements belonging to Group VA). Elements belonging in Group VIA (eg - O, S) all have electron configuration ending in ns2np4 (the superscripts total to "6" indicates there are 6 valence electrons for elements belonging to Group VIA). Elements belonging in Group VIIA (eg - F, Cl) all have electron configuration ending in ns2np5 (the superscripts total to "7" indicates there are 7 valence electrons for elements belonging to Group VIIA). Elements belonging in Group VIIIA (eg - He, Ne, Ar) all have electron configuration ending in ns2np6 (the superscripts total to "8" indicates there are 8 valence electrons for elements belonging to Group VIIIA).BACK TO MAIN PAGAE