Isotopes of hydrogen — protium, deuterium, tritium: Foundations

Before you can read a single line of the parent note on isotopes of hydrogen, you need to earn every symbol it throws at you. This page builds them one at a time, from nothing. Each entry: what it means in plain words → the picture it stands for → why the topic needs it.

1. The atom: what are we even drawing?

Picture a tiny dense clump in the middle (the nucleus, made of protons + neutrons) with even tinier electrons buzzing around it like bees around a hive.

Figure — Isotopes of hydrogen — protium, deuterium, tritium

Why the topic needs this: the whole story of isotopes is "keep the protons and electrons fixed, change only the neutrons." You literally cannot say that sentence until you know these three particles exist and where each one lives.

2. $Z$ — the atomic number (who you are)

The picture: count the positive dots in the central clump. That count is $Z$ .

Why it matters: $Z$ is an atom's identity card. $Z = 1$ means hydrogen, always. $Z = 2$ means helium. Change $Z$ and you have literally changed which element you are holding. This is the reason all three hydrogen isotopes are still hydrogen — every one of them has $Z = 1$ .

3. $N$ — the neutron number (the free weight)

The picture: count the grey (neutral) dots in the central clump.

Why it matters: neutrons add mass but carry no charge, so they do not pull on electrons and do not change bonding. Changing $N$ while keeping $Z$ fixed is exactly what makes an isotope. For hydrogen:

Name	$Z$	$N$
Protium	1	0
Deuterium	1	1
Tritium	1	2

4. $A$ — the mass number (total heavy particles)

We have counted protons ( $Z$ ) and neutrons ( $N$ ) separately. The nucleus's weight comes from both together, so we add them:

$A = Z + N$

WHAT we did: added the two nucleon counts. WHY: because both proton and neutron weigh ~1 unit, so the total count is the weight (in nucleon units). WHAT IT LOOKS LIKE: just tally every dot in the central clump, ignoring colour.

Check it against hydrogen:

Protium: $A = 1 + 0 = 1$
Deuterium: $A = 1 + 1 = 2$
Tritium: $A = 1 + 2 = 3$

This is why deuterium is called "hydrogen-2" and tritium "hydrogen-3."

5. Reading the notation $_{Z}^{A} X$

You now have every piece to decode the strange stacked symbols the parent note uses.

The picture: a label taped to the atom, top number = "how heavy," bottom number = "who I am."

So:

$_{1}^{1} H$ = protium (1 nucleon, 1 proton).
$_{1}^{2} H$ , also written D = deuterium.
$_{1}^{3} H$ , also written T = tritium.

6. Percentage and abundance ( $%$ )

The picture: a jar of a million hydrogen atoms — nearly all are protium, a tiny sliver are deuterium, and you would need a microscope-of-a-microscope to spot a single tritium.

Why the topic needs it: abundance tells you what you actually have. Deuterium at $0.015%$ means 15 atoms in every 100,000 — rare but real and extractable (that is where heavy water comes from).

7. Radioactivity words: decay, half-life, $\overset{ν}{ˉ}_{e}$

Two of hydrogen's isotopes are stable rocks; one is a slowly-ticking clock. To read that clock you need three plain-word ideas.

The picture: a grey neutron dot flips to a positive proton dot, and a tiny electron shoots away like a spark. See Beta Decay for the full mechanism.

Why the topic needs these: tritium ( $_{1}^{3} H$ ) is radioactive with $t_{1/2} = 12.32$ years. Without "decay," " $β^{-}$ ," and "half-life," the tritium equation $_{1}^{3} H \to_{2}^{3} He + e^{-} + \overset{ν}{ˉ}_{e}$ is unreadable gibberish. Notice $Z$ climbs $1 \to 2$ (neutron became proton) but $A$ stays $3$ (a nucleon just changed type, none left).

8. Motion words: temperature, $v_{r m s}$ , and mass

The parent note claims heavier isotopes boil at higher temperatures and react slower. Both rest on one idea about heat and motion.

WHY this tool and not another: we want to compare how fast protium vs deuterium molecules move at the same temperature. This formula isolates exactly that: fix $T$ , and speed depends only on mass $M$ . See Kinetic Theory of Gases.

WHAT IT LOOKS LIKE: at the same temperature, the heavy molecule ambles; the light one zips. Because $M$ is under a square-root in the denominator, doubling the mass slows the molecule by a factor $2 \approx 1.41$ — i.e. it moves to about $0.707$ of the light one's speed. Slower molecules linger near each other, stick more, and so need more heat to boil off.

How the foundations feed the topic

Read it top to bottom: the three particles give you $Z$ and $N$ ; those add to $A$ ; $A$ and $Z$ make the isotope label; and three side-branches (how much exists, how it decays, how fast it moves) complete the picture the parent note paints.

Equipment checklist

Cover the right side and see if you can answer each before revealing.

What does the atomic number

Z

count, and why does it fix an element's identity?

Z

counts protons; the proton count defines which element you have, so changing it changes the element itself.

What does

N

count, and why can it change without changing the element?

N

counts neutrons; neutrons carry no charge and don't affect bonding, so changing

N

only changes mass, not chemistry.

Write the mass-number formula and say what a nucleon is.

A = Z + N

; a nucleon is any particle in the nucleus (a proton or a neutron).

_{Z}^{A} X

, which number sits on top and what is it?

The top-left number is

A

, the mass number (total nucleons); the bottom-left is

Z

, the proton count.

What are the

(Z, N, A)

triples for protium, deuterium, tritium?

Protium

(1, 0, 1)

; deuterium

(1, 1, 2)

; tritium

(1, 2, 3)

What does "natural abundance" mean?

The fraction of atoms of that isotope out of all atoms of the element, given as a percentage.

In beta-minus decay, what turns into what, and what flies out?

A neutron becomes a proton; an electron

e^{-}

and an antineutrino

\overset{ν}{ˉ}_{e}

are emitted.

Define half-life in one sentence.

The time for half of a sample of radioactive atoms to decay.

Why does a heavier molecule move slower at the same temperature?

Because

v_{r m s} = 3 R T / M

has mass

M

in the denominator under a square root, so larger

M

gives smaller speed.