Periodic Table — Rows 1, 2, 3

The first three rows of the periodic table are 18 elements, from Hydrogen (H) to Argon (Ar).

For every element on this list you should be able to give:

• P = number of protons (this is the atomic number).
• N = number of neutrons in the most common isotope.
• e = number of electrons in a neutral atom (= number of protons).

In a neutral atom, P always equals e. Always. If you know the atomic number, you already know two of the three.

---

The three rules

1. P = atomic number. Look it up once and remember it.
2. e = P for a neutral atom.
3. N = (mass number rounded to nearest integer) − P. We use the most common natural isotope.

So really you only need to memorize P and N for each element. The e is a freebie.

---

The full table for our trial

Row 1 — 2 elements

Hydrogen is the only common element where N = 0. Its most common isotope (¹H, "protium") has just one proton and one electron, no neutron.

Row 2 — 8 elements

Row 3 — 8 elements

Note: Chlorine's average atomic mass is 35.45 because two isotopes (Cl-35 and Cl-37) are common. We use N = 18 (Cl-35, the more common one). Argon's most common isotope is Ar-40, so N = 22.

---

How to study this

There are 18 elements. There are three columns. That's 54 little facts. Don't memorize 54 facts. Memorize P (which is just position) and N (which has patterns) and let e = P take care of itself.

Patterns to lean on:

• For rows 1 and 2, N is roughly equal to P. Look at carbon, nitrogen, oxygen — they have N = P exactly.
• Once you cross into elements with P > 20ish, neutron counts start growing faster than protons. We don't see that here yet, but Ar (P=18, N=22) is a hint — argon is already neutron-heavy.
• Hydrogen is the weird one. N = 0. Don't forget.

---

Quick self-quiz

Cover the answers and try: