Electron Configuration

Introduction

Chemists are particularly interested in the electrons in an atom's electron cloud. This is because the electrons determine the chemical properties of the elements, such as what compounds the element will form and which reactions it will participate in. In this section, we will learn where the electrons are in atoms.

Energy Levels

Niels Bohr and Albert Einstein in 1925. Bohr received the Nobel prize for physics in 1922.

The key idea in Bohr’s model of the atom is that electrons occupy definite orbits that require the electron to have a specific amount of energy. In order for an electron to be in the electron cloud of an atom, it must be in one of the allowable orbits and it must have the precise energy required for that orbit. Orbits closer to the nucleus would require smaller amounts of energy for an electron and orbits farther from the nucleus would require the electrons to have a greater amount of energy. The possible orbits are known as energy levels. One of the weaknesses of Bohr’s model was that he could not offer a reason why only certain energy levels or orbits were allowed.

Bohr hypothesized that the only way electrons could gain or lose energy would be to move from one energy level to another, thus gaining or losing precise amounts of energy. The energy levels are quantized, meaning that only specific amounts are possible. It would be like a ladder that had rungs only at certain heights. The only way you can be on that ladder is to be on one of the rungs and the only way you could move up or down would be to move to one of the other rungs. Suppose we had such a ladder with 10 rungs. Other rules for the ladder are that only one person can be on a rung and in normal state, the ladder occupants must be on the lowest rung available. If the ladder had five people on it, they would be on the lowest five rungs. In this situation, no person could move down because all the lower rungs are full. Bohr worked out rules for the maximum number of electrons that could be in each energy level in his model and required that an atom is in its normal state (ground state) had all electrons in the lowest energy levels available. Under these circumstances, no electron could lose energy because no electron could move down to a lower energy level. In this way, Bohr’s model explained why electrons circling the nucleus did not emit energy and spiral into the nucleus.

Electron Energy Levels

Although Bohr’s model was particularly useful for hydrogen, it did not work well all the other elements. However, other physicists built on his model to create one that worked for all elements. It was found that the energy levels used for hydrogen were further composed of sublevels of different shapes. These sublevels were composed of orbitals in which the electrons were located.

The shape of p-orbitals.

The shape of d-orbitals.

The following table below summarizes the possible energy levels and sublevels, including the number of orbitals that compose each sublevel and the maximum number of electrons the sublevel can hold.

There are several patterns to notice when looking at the table of energy levels. Each energy level has one more sublevel than the level before it. Also, each new sublevel has two more orbitals. Can you predict what the

energy level would look like?

When determining where the electrons in an atom are located, a couple of rules must be followed:

• 1. Each added electron enters the lowest orbital of the lowest energy shell available.

  2. No more than two electrons can be placed in any orbital.

The Electron Configuration

It would be convenient if the sublevels were filled in the order listed in the table, such as , etc. However, this is not the order the electrons fill the sublevels. Remember, the electrons will always go to the lowest energy shell available. When that is taken into account, the actual filling order is:

. The next two would occupy . , with three orbitals, can hold its next 6 electrons. Gallium continues to fill up its orbitals, finally putting 1 electron in . The electron configuration for gallium would be:

Although you can choose to memorize the list and how many electrons fit in each sublevel for the purpose of writing electron configurations, there is a way for us to find this order by simply using our periodic table.

Look at the different sections of the periodic table (Figure below).

You may have noticed that there are several natural sections of the periodic table. The first 2 columns on the left make up the first section; the six columns on the right make up the next section; the middle ten columns make up another section; and finally the bottom fourteen columns compose the last section. Note the significance of these numbers: 2 electrons fit in any s sublevel, 6 electrons fit in any p sublevel, 10 electrons fit in any d sublevel, and 14 electrons fit in any f sublevel. The four sections described previously are known as the

. Moving onto the second row, is filled first followed by . Continuing with the filling order, you generate the list:

Sample Question

Write the electron configurations for

(a) potassium, K

(b) arsenic, As

(c) phosphorus, P

Solution

(a)

(b)

(c)

Abbreviated Electron Configuration

The electron configuration for potassium is . As the electron configurations become longer and longer, it becomes tedious to write them out. A shortcut has been devised so that writing the configurations is less tedious. The electron configuration for potassium is the same as the electron configuration for argon, except that potassium has one more electron than argon. The electron configuration for argon is

, and in order to write the electron configuration for potassium, we need to add . It is acceptable to use [Ar] to represent the electron configuration for argon and [Ar] to represent the code for potassium. Using this shortcut, the abbreviated electron configuration for calcium would be [Ar]

, and the electron configuration for scandium would be [Ar].

Even though the periodic table was organized according to the chemical behavior of the elements, you can now see that the shape and design of the table is a perfect reflection of the electron configuration of the atoms. This is because the chemical behavior of the elements is also caused by the electron configuration of the atoms.

Sample Question

Write the abbreviated electron configurations for

(a) potassium, K

(b) arsenic, As

(c) phosphorus, P

Solution

(a)

(b)

• 1. Which element will have the following electron configuration: ?

Write electron configurations for each of the following neutral atoms:

• 1. Magnesium

  2. Nitrogen

  3. Indium

  4. Yttrium

  5. Tin

  6. Xenon

  7. Cesium

  8. Uranium

Write the abbreviated electron configuration for each of the following neutral atoms:

• 1. Fluorine

  2. Aluminum

  3. Titanium

  4. Arsenic

  5. Rubidium

http://www.ck12.org/book/Introductory-Chemistry/r1/section/2.5/Atoms-and-Electromagnetic-Spectra-%253A%253Aof%253A%253A-The-Structure-of-the-Atom/