Electron Configuration

Electron configuration is a method of describing the orbitals in which electrons occupy, it also states the number of electrons in each orbital. (maximum is 2)

Just to show you what it looks like, here is the electron configuration for silicon:

1s² 2s² 2p⁶ 3s² 3p² 

You see, electrons only exist in certain energy states. When the electrons absorb or emit a specific amount of energy, it will instantly jump to another orbital. The energy level is basically that amount that an electron of an atom can have. This is called "n". 

The energy difference between 2 energy levels is called the quantum of energy.

Atoms can exist in 2 states:

Ground State: all electrons of the atom are at their lowest possible energy level

Excited State: one or more electron is in an energy level other than the lowest possible one

Here are some other things that you should know:

Orbital: the actual amount of space that is being occupied by an electron

Shell: the set of all orbitals that have the name n-value

Subshell: a set of orbitals of the same type (like 2s and 2p)

s, p, d, and f refer to four types of orbitals.

An atom's electron shells are filled according to the following theoretical constraints:

• Each s subshell holds at most 2 electrons
• Each p subshell holds at most 6 electrons
• Each d subshell holds at most 10 electrons
• Each f subshell holds at most 14 electrons

There can only be a max of 2 electrons per orbit, so the p subshell would need 3 orbitals, each with 2 electrons. d subshell would need 5 orbitals, each with 2 electrons. f subshell would need 7 orbitals, each with 2 electrons. Sometimes an orbital will only have one electron is there is an odd number of electrons. 

 1s²2s² 2p⁶3s² 3p⁶4s² 3d¹⁰ 4p⁶5s² 4d¹⁰ 5p⁶6s² 4f¹⁴ 5d¹⁰ 6p⁶7s² 5f¹⁴ 6d¹⁰ 7p⁶

Above is the order in which electrons are filled. For example, a neutral oxygen atom has 8 electrons, so the electron configuration would be:

1s² 2s² 2p⁴ 

^{Notice that the p only has a 4, and not a 6. This is alright. The superscripts should add up to the number of electron in the atom. In this case, it adds up to 8, which is correct.} 

When writing electronic configurations for negative ions, simply add the appropriate number of electrons from where the neutral atom left off. 

When writing electronic configurations for positive ions, take away electrons from the outermost shell first. (aka. the higher energy shell/larger n-value) 

Another method of writiting configurations is Core Notation.

The core is the part of the configuration of the nearest noble gas that comes before the given element. (Ex/ Ar is the noble gas that comes before Br)

The outer part consists of all the electrons outside of the core. 

The core notation for Rubidium would be... [Kr]5s^1 

Do you see how all the numbers and letters that should've been before 5s^1 was replaced by Kr? 

Another example... the core notation for Chlorine would be... [Ne]3s^2 3p^5 

Electron configuration can also tell me the number of valence electrons that an atoms has. Valence electrons are electrons in the outermost open shell. By open, we mean that the shell contains less than its maximum number of electrons. Whereas, a closed shell is a shell containing exactly its max number of electrons.

In electron configuration, valence electrons are those that aren't in the core, or in d- and f- subshells.

Ex/ Determine number of valence electrons by looking at core notation of Selenium.

[Ar] 4s^2 3d^10 4p^4

There are 6 valence electrons because of the 4s^2 and the 4p^4 part of the configuration. 3d^10 doesn't count because its a d- subshell.