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.

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What these particles actually are

An atom has a tiny, heavy nucleus in the middle and a cloud of electrons around it. Three particles matter:

• Protons — positive charge (+), sit in the nucleus. Heavy.
• Neutrons — no charge (neutral, hence the name), also in the nucleus. Almost the same weight as a proton.
• Electrons — negative charge (−), zoom around the outside. Almost weightless (about 1/1800 of a proton).

The nucleus (protons + neutrons) is basically all the mass. The electrons take up all the space and do all the chemistry.

Atomic number = identity

The atomic number is just the number of protons. That single number is the element's identity:

• 1 proton → it's hydrogen. Always. No exceptions.
• 6 protons → it's carbon. Add a proton and it stops being carbon and becomes nitrogen (7).

You can change how many neutrons or electrons an atom has and it's still the same element. Change the proton count and it's a different element. That's why the periodic table is ordered by atomic number, 1, 2, 3, …

Charge: why neutral atoms have P = e

Protons are +, electrons are −, and they're equal and opposite. A neutral atom has the same number of each, so the charges cancel: P = e.

• Lose an electron → more + than − → a positive ion (e.g. Na⁺).
• Gain an electron → more − than + → a negative ion (e.g. Cl⁻).

(That's the valence story from the other lesson — atoms gain/lose electrons to fill their outer shell.)

Isotopes: same element, different neutron count

Atoms of the same element always have the same number of protons, but can have different numbers of neutrons. Those versions are called isotopes.

• Carbon-12 has 6 protons + 6 neutrons. Carbon-14 has 6 protons + 8 neutrons. Both are carbon (6 protons) — carbon-14 is just heavier, and it's the radioactive one used for carbon dating.
• Hydrogen has three: ¹H (0 neutrons, normal), ²H (1 neutron, "deuterium"), ³H (2 neutrons, "tritium").
• The mass number is protons + neutrons (the "12" in carbon-12).

On the exam we use the most common isotope for the neutron count — so N for chlorine is 18 (Cl-35), even though the average atomic mass on a periodic table reads 35.45 because Cl-37 is also out there.

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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.

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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.

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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.

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Quick self-quiz

Cover the answers and try: