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.