The electron configuration of an atom is a form of notation which shows how the electrons are distributed among the various atomic orbital and energy levels. The format consists of a series of numbers, letters and superscripts.

1s 2

This electron configuration provides us with the following information:  The large number "1" refers to the principle quantum number "n" which stands for the energy level. It tells us that the electrons of helium occupy the first energy level of the atom.  The letter "s" stands for the angular momentum quantum number "l". It tells us that the two electrons of the helium electron occupy an "s" or spherical orbital.  The exponent "2" refers to the total number of electrons in that orbital or sub-shell. In this case, we know that there are two electrons in the spherical orbital at the first energy level.

 The number of sublevels that an energy level can contain is equal to the principle quantum number of that level. So, for example, the second energy level would have two sublevels, and the third energy level would have three sublevels. 

is called an s sublevel

is called a p sublevel.

is called a d sublevel

is called an f sublevel.

Although energy levels that are higher than 4 would contain additional sublevels, these sublevels have not been named because no known atom in its ground state would have electrons that occupy them.

Energy LevelNumber of sublevels Names of sublevels 11s 22s, p 33s, p, d 44s, p, d, f 55s, p, d, f, g

Each type of sublevel holds a different number of orbitals, and therefore, a different number of electrons.

Space occupied by a pair of electrons

 s sublevels have one orbital, which can hold up to two electrons.  p sublevels have three orbitals, each of which can hold 2 electrons, for a total of 6.  d sublevels have 5 orbitals, for a possible total of 10 electrons.  f sublevels, with 7 orbitals, can hold up to 14 electrons.

Sublevel# of orbitals Maximum number of electrons s1 2 p3 6 d5 10 f7 14

 An easy way to calculate the number of orbitals found in an energy level is to use the formula n 2.  For example, the third energy level (n=3) has a total of 3 2, or nine orbitals.  This makes sense because we know that the third energy level would have 3 sublevels;  an s sublevel with one orbital, a p sublevel with 3 orbitals and a d sublevel with 5 orbitals = 9, so the formula n 2 works!

 An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2n 2.  For example, the fourth energy level (n=4) can hold 2(4) 2 = 32 electrons.  This makes sense because the fourth energy level would have four sublevels, one of each of the named types.  The s sublevel hold 2 electrons, the p sublevel holds 6 electrons, the d sublevel holds 10 electrons and the f sublevel holds 14 electrons = 32, so the formula 2n 2 works!

ENERGY LEVELMAX # OF ELECTRONS Max # = …n…

2n 2 Where n is any energy level

A detailed way of showing the order in which electrons fill in around the nucleus

K K: 2s 2 1s 2 4s 1 3s 2 2p 6 3p 6 Bohr Models vs. e - Configs

5f 35f 3 Energy Level Sub-Energy Level # of e - in sub-energy level

Main Energy Level Numbers 1, 2, 2 Sublevels Number of electrons in the sub level 2,2,5 1s 2 2s 2 2p 5

Blocks in the Periodic Table

 Use the last noble gas that is located in the periodic table right before the element.  Write the symbol of the noble gas in brackets.  Write the remaining configuration after the brackets.  Ex: Fluorine: [He] 2s 2 2p 5

 In 1925, Wolfgang Pauli stated: No two electrons in an atom can have the same set of four quantum numbers. This means no atomic orbital can contain more than TWO electrons and the electrons must be of opposite spin if they are to form a pair within an orbital.

The most stable arrangement of electrons is one with the maximum number of unpaired electrons. It minimizes electron-electron repulsions and stabilizes the atom. In a set of orbitals, the electrons will fill the orbitals in a way that would give the maximum number of parallel spins (maximum number of unpaired electrons).

In large families with several children, it is a luxury for each child to have their own room. There is less fussing and fighting if siblings are not forced to share living quarters. The entire household experiences a lower, less frazzled energy state. Electrons find each other very repulsive, so they too, are in a lower energy state if each “gets their own room” or in this case orbital.

Electrons that are single occupants of orbitals have parallel (same direction) spins and are assigned an up arrow. Analogy: Students could fill each seat of a school bus, one person at a time, before doubling up. The second electron to enter the orbital, thus forming an electron pair, is assigned a down arrow to represent opposite spin.

Orbital notation is basically just another way of expressing the electron configuration of an atom.

An orbital can hold only two electrons and they must have opposite spin.