Atomic number = number of protons # protons = # electrons IN NEUTRAL ATOMS

So, how are the electrons arranged around the nucleus?

The electrons in an atom have different energies depending upon how close to the nucleus they are. The energy levels are spaced apart like steps on a flight of stairs. nucleus 2 e- 8 e- 18 e- 32 e- Energy Level 1 Level 2 Level 3 Level 4 #electrons = 2n 2

Let’s build some atoms! AKA:

1 p 0 n e Hydrogen H 1 1 Atomic number: the number of protons in the nucleus Mass number: the number of protons + the number of neutrons in the nucleus 0 n In neutral atoms, #p = #e

2 p 2 n e e Helium-4 He 2 4

3 p 4 n e e e Lithium-7 Li 3 7

4 p 6 n e e e Beryllium-10 Be 4 10 e

5 p 5 n e e e Boron-10 B 5 10 e e

6 p 6 n e e e Carbon-12 C 6 12 e e e

7 p 8 n e e e Nitrogen-15 N 7 15 e e e e

8 p 8 n e e e Oxygen-16 O 8 16 e e e e e

9 p 10 n e e e Fluorine-19 F 9 19 e e e e e e

10 p 12 n e e e Neon-22 Ne e e e e e e e Notice that when we have to add another ring, we move down one row on the periodic table.