Electronic Structure

 Electronic Structure(configuration) is the arrangement of electrons in energy level  around an atomic nucleus. It tells us how many electrons are in each orbital.
 For example, Electronic structure for Na(sodium) is 1s2 2s2 2p6 3s1.
It looks complicated right? But as long as you get the rules, it will be really easy.

First there are four types of shell. There are s,p,d,and f. Like u can see from the periodic table below those shells are based on periodic table.

  To write electronic configuration, you just needed to find location of the element that you want, and write down orderly from left to right.
 Let's use Na for the example, Na has 11 electrons,and it's at 3rd period fist group. Fisrt we start with left top. there are two elements are in 1s(1 simply represent the  period). Thus, you write 1s2, and there are nothing on the 1period on the right side, so we go to period 2. They have two elements as well, thus you write 2s2, then there are 6 elements in 2p, thus you write 2p6, got it? So,over all we have 1s2 2s2 2p6 and finally where Na is 3s1. Thus,  answer is 1s2 2s2 2p6 3s1.
Just be aware of d d start wih row 3 instead of  4









Core Notation
 Some Scentists are really lazy thus, they found the easiler way to write elecronic configuration. It's easy you just need to find the near noble gas and start the electronic configuration from there. Thus, for Na, Ar is the closest noble gas, thus u write (Ar)3s1.

Valence Electrons
They are electrons that are located most outer shell, if we have core notation we just simply need to count the numbers of elecrons exept d and f. For example,Na has 1 valence eletrons
 Here is the video that will help