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Atomic number (Z)
= number of protons = number of electrons in a neutral atom
Mass Number (A)
= protons + neutrons
Isotopes
Same element, different neutron count. Average atomic mass on the periodic table is a weighted average of all naturally occurring isotopes.

Electron configuration
Fills sublevels in order (Aufbau principle)— 1s, 2s, 3s, 3p, 4s, 3d, 4p… Each orbital holds 2 electrons.

Noble gas shorthand
Replace the core electrons with the symbol of the previous noble gas, e.g. Na= [Ne] 3s1.
![<p>Replace the core electrons with the symbol of the previous noble gas, e.g. Na= [Ne] 3s1. </p>](https://assets.knowt.com/user-attachments/f78d98f9-bbf4-4363-8242-cfab6a272eac.jpg)