We have 3 rules to write the electronic configuration of atoms and ions using the Quantum Atomic Model. Aufbau Rule states that electrons are first added to the subshell of the lowest energy level, and when that subshell is filled, electrons are then added to the next most stable subshell. Hence the order of filling up electrons for the first 30 elements will therefore be: 1s 2s 2p 3s 3p 4s 3d ... Hund's Rule states that in a subshell, electrons are filled singly and with parallel spin before they are paired up. Pauli Exclusion Rule states that two electrons in the same orbital cannot have the same spin. With a combination of these rules we can then write out the electronic configuration of atoms and ions using the spdf configuration. To learn how this is done, watch this video tutorial now! Topic - Atomic Structure, Physical Chemistry, JC, H2, A Level Chemistry, Singapore Found this video useful? Please LIKE this video and SHARE it with your friends. SUBSCRIBE to my YouTube Channel for new A Level H2 Chemistry video lessons every week! Any feedback, comments or questions to clarify? Suggestions for new video lessons? Drop them in the COMMENTS Section, I would love to hear from you! Do you know you can learn Chemistry Concepts under a minute? Follow me on Instagram for my weekly one-minute video lessons at https://www.instagram.com/chemistryguru/ I am also conducting JC H2 Chemistry classes at Bishan Central, Singapore. With my years of experience tutoring hundreds of JC students since 2010, I am confident that I can make H2 Chemistry Simpler for you too! For more information please visit https://chemistryguru.com.sg/ -~-~~-~~~-~~-~- Please watch my latest video: "Rate Equation and Reaction Mechanism - Kinetics" https://www.youtube.com/watch?v=3GVqAq0ggp0 -~-~~-~~~-~~-~-