Intuition The ONE core ideaA giant plastic molecule is nothing more than thousands of tiny identical molecules holding hands in a long line, and the "handshake" is made by opening a double bond so each little molecule can grab the next. Once you can read the picture of that double bond opening — and every letter and symbol drawn around it — the whole chapter is just this one trick repeated with different decorations.
Before you can read a single line of the parent note, you must be able to read every mark on the page. Below is every symbol, letter, and idea the parent uses — built from nothing, each one leaning on the one before it.
Definition Chemical formulaA chemical formula is a shorthand list of which atoms are stuck together in one molecule. The big letters are element symbols (C = carbon, H = hydrogen, Cl = chlorine, F = fluorine, N = nitrogen, O = oxygen). A small number after and below a letter — a subscript — tells you how many of that atom.
So C H 2
Intuition Why we bother with formulasA molecule is a real 3-D cluster of atoms, too fiddly to draw every time. The formula is a label on that cluster — like writing "3 apples" instead of drawing three apples. Look at the figure: the picture on the left and the symbols on the right say the same thing .
Subscript the small low number saying how many of the atom just before it.
Definition Single bond, written "− bond is a shared pair of electrons that glues two atoms together. We draw it as a short straight line, a dash : H − H
Think of a bond as two kids each offering one hand and clasping — that clasp is one shared pair. One dash = one clasp = one shared pair.
A line is the perfect picture: it connects two things and it is one thing. One line, one shared pair. Two lines between the same atoms will mean something stronger — that comes next.
Definition Double bond, written "= double bond is two shared pairs of electrons between the same two atoms — drawn as two parallel lines: C = C π
Intuition Why the double bond matters more than anything else hereThis whole chapter lives on one event: the double bond opening up . Look at the figure — the tightly-held bottom line stays, but the loose top pair (π No double bond, no addition polymer. This is the C=C double bond you must know cold.
Why does the parent note keep saying "the double bond opens"? One of its two shared pairs breaks so each carbon can bond to a new neighbour instead.
Definition Radical (the dot∙ radical is an atom or group with one lonely, unpaired electron , written as a dot: R O ∙ pair ; a radical has a spare single electron desperate to find a partner, so it is very reactive.
Intuition Picture the dot as a single reaching handA pair of electrons = two clasped hands = stable. A single electron = one hand sticking out with nobody to hold. That hand will grab the first loose pair it meets — and the loosest pair around is the π C = C Free-radical mechanism .
Radical a species with one unpaired electron (a lone reaching "hand"), drawn as a dot.
R X R X placeholders : they stand for "some group of atoms whose exact identity doesn't matter right now." In R − O − O − R R C H 2 = C H X X side group (it might be H C l F C H 3 C 6 H 5 C N
Intuition Why use a letter for "anything"?The chain-building trick works identically no matter what X X X
X C H 2 = C H X the side group hanging off the double bond (H C l F C H 3
Definition Repeat unit and the subscriptn repeat unit . We wrap it in square brackets and write a subscript n [ − C H 2 − C H 2 − ] n n n keeps going in both directions.
Intuition Why the brackets, then × n n stops — but a polymer's middle never stops. Every mark earns a meaning.
What does the subscript n The repeat unit is copied a very large number of times to make one giant molecule.
Why do we draw dashes sticking out of the brackets? To show the chain continues — this is a fragment of a huge molecule, not a complete small one.
Definition Reaction arrow and leading numberThe arrow ⟶ turns into " — reactants on the left become products on the right. A number written in front of a formula (a coefficient ) says how many of that molecule take part: n C H 2 = C H 2 n
Common mistake Confusing the front-n n Why it feels wrong at first: the same letter n The fix: the front n subscript n because each monomer becomes exactly one repeat unit (no atoms lost). That equality is why Example 3 can say n = M p o l y m er / M m o n o m er
Definition Monomer and polymerMono- = one, poly- = many, -mer = part. A monomer is one small building-block molecule; a polymer is the many-parts giant made by joining monomers. Addition polymers join without losing atoms ; condensation polymers join while spitting out a small molecule like H 2 O
Break the Greek: monomer "one-part" — a single small building-block molecule (must contain a C = C
Break the Greek: polymer "many-parts" — the giant molecule made by joining many monomers.
Definition Relative molar massM M total mass of all atoms in one molecule , adding up each atom's mass. For ethene C H 2 = C H 2 = 24 + 4 = 28 the repeat unit weighs the same as its monomer , so dividing the polymer's M M n
Worked example Sanity-check Example 3Polymer mass 42000 28 n = 28 42000 = 1500 units. only because no atoms were expelled — that's why this formula belongs to addition, not condensation.
Why does n = M p o l y m er / M m o n o m er No atoms are lost, so each repeat unit has the same mass as its monomer.
Element symbols C H Cl F N O
Chemical formula with subscripts
General monomer CH2 equals CHX
Repeat unit in brackets sub n
Counting units n equals M over M
Addition polymers the topic
Test yourself — cover the right side and answer aloud.
I can read C H 2 One carbon, two hydrogens (subscript = how many).
I know what a single dash "− One shared pair of electrons — one bond.
I know what a double bond "= Two shared pairs; the loose one is the π
I can explain the dot "∙ R O ∙ A radical — one unpaired electron, very reactive.
I know why R X They're placeholders for "any group"; X
I can read [ − C H 2 − C H 2 − ] n A repeat unit copied n
I know why the front n n Each monomer becomes exactly one repeat unit; no atoms lost.
I can define monomer, polymer, and addition vs condensation Small block vs giant; addition loses no atoms, condensation expels a small molecule.
I can compute n n = M p o l y m er / M m o n o m er
Ready? Now the parent note — Addition polymers — reads like plain English.