Foundations — Mendeleev's periodic table — based on atomic mass

Before you can follow the parent note, you need a toolbox. Below is every idea it quietly assumes. We build them one at a time, each on top of the last, from absolute zero.

1. What is an "element"?

Why the topic needs it: Mendeleev's table is nothing but a way of arranging the boxes. If you don't know what one box (one element) is, the arranging makes no sense.

2. The atom, and the two numbers hiding inside it

Every atom is a tiny clump. At its centre sits a nucleus (a dense core), and around it whizz electrons (tiny negative specks).

Look at the figure. The nucleus (yellow) holds two kinds of particle:

• Protons — positive charge, counted by the letter $Z$.
• Neutrons — no charge, they just add weight.

Why the topic needs it: The whole parent note is the story of Mendeleev ordering by weight ($A$-ish) before anyone knew about identity ($Z$). You can't feel the drama unless you hold both numbers in your hand.

3. Atomic mass — the ruler Mendeleev actually had

Here is the key subtlety that becomes a defect later:

Two atoms of the same element can have different mass numbers, because they carry different numbers of neutrons. In the figure both atoms have $Z=17$ (both are chlorine — same identity), but one has $A=35$ and the other $A=37$. These twins are called isotopes (see Isotopes). The atomic mass you look up is the average over these twins — which is why chlorine's mass is $35.5$, not a whole number.

Why the topic needs it: Atomic mass is the ONLY ruler Mendeleev could use in 1869 — the proton (and so $Z$) wasn't known yet. Every "line them up" step in the parent note is a line-up by this ruler. And isotopes are exactly why that ruler eventually fails.

4. Valency — the "number of hands" an atom has

We read valency off a formula. In an oxide ${\text{R}}_x{\text{O}}_y$, oxygen always brings valency $2$, so the metal's valency comes from balancing hands:

Why the topic needs it: Mendeleev grouped elements by their valency pattern (rising oxide valency $1\to 7$, falling hydride valency $4\to 1$). Valency is the "personality fingerprint" that decides which column an element joins. (More in Valency and oxide formulas.)

5. Reading a chemical formula like ${\text{R}}_2{\text{O}}_7$

Why the topic needs it: The parent note's central table of oxides and hydrides is written entirely in R-notation. Without knowing R is a stand-in, that series looks like gibberish.

6. Period and Group — the two directions of the table

The figure shows the idea that turns a plain list into a table: you write the sorted list left-to-right, and every time a personality returns, you wrap the line onto a new row so twins land in the same column. Reading across = increasing mass (a period); reading down = same family (a group).

Why the topic needs it: "Periodic" literally means "row-repeating." Period and group are the map coordinates for every element in the parent note.

7. Putting it together — the prerequisite map

Read the arrows as "feeds into." Notice the two great streams: weight (mass → sort → periods) and personality (valency → group). Mendeleev's table is where they meet.

Equipment checklist

Test yourself — cover the right side and answer before revealing.

Connections

• Parent topic — Mendeleev's table
• Atomic number and mass number (the $Z$ vs $A$ distinction built here)
• Isotopes (same $Z$, different $A$ — the neutron twins)
• Valency and oxide formulas (reading hands from formulas)
• Modern Periodic Law — based on atomic number (where $Z$ finally replaces mass)
• Döbereiner's Triads and Newlands' Law of Octaves (earlier attempts at the same repeating pattern)