Intuition The one core ideaA chemical reaction is nothing more than electrons moving from where they are to where they're wanted , and a curved arrow is the little map that draws that motion. Master what a bond, a lone pair, a charge, and an arrowhead each mean as a picture, and every mechanism in this chapter becomes readable like a sentence.
Before you can push a single arrow, you must know exactly what every mark on the page is . This page builds them one at a time, from nothing, each one earning its place before the next arrives.
Definition Electron (the dot·)
An electron is a tiny particle of negative charge that lives around an atom. On paper we draw it as a single dot : ·.
Picture: a lone dot floating near an atom's letter, like H·.Why the topic needs it: every arrow in this whole chapter is really "an electron went from here to there." If the dot has no meaning, arrows have no meaning.
Everything else on this page is built from counting and moving these dots.
Definition Covalent bond (the pair: or the line —)
When two atoms share two electrons , the shared pair holds them together — that is a covalent bond . We draw it two ways that mean exactly the same thing :
as two dots between the atoms: A:B
as a line between the atoms: A—B
Picture: two dots (or one line) sitting in the middle , belonging to both atoms at once.
Why the topic needs it: the parent note says "a covalent bond is a shared pair of electrons." Breaking a bond means deciding who keeps those two shared dots — the choice we will name heterolysis vs homolysis in section 6.
What to look for in the figure: on the left , the bond is drawn as two amber dots sitting in the gap between letters A and B — those two dots are the shared pair. On the right , the same pair is drawn as a single cyan line. The big = in the middle says these two pictures mean the identical thing: a bond is not the line — the line is shorthand for the two shared electrons. Train your eye to see the hidden pair inside every line.
See Bond dissociation energy for how much energy it takes to pull that pair apart.
: sitting on one atom)
A lone pair is two electrons that belong to one atom only — not shared with anyone. We draw them as two dots sitting on the atom.
How to draw them (plain ASCII): oxygen with its two lone pairs looks like :Ö: — a pair of dots on the left , a pair on the right , and (the two dots the Ö accent stands for) a pair on top . Each : or pair of dots = two electrons on that one side.
In tidy textbook notation the same oxygen is written : O ¨ : \ddot (two dots over the O) is one lone pair sitting on top, and each : beside the O is another lone pair. Three pairs of dots = three lone pairs = six lone-pair electrons.
Picture: pairs of dots hugging a single letter, off to its sides, not in the gap between two atoms.
Why the topic needs it: an arrow's tail can start from a lone pair (see the arrow-pushing rules in section 8). In Worked Example 2 of the parent, the arrow starts from the lone pair on O H −
Common mistake Confusing a bonding pair with a lone pairWhy it feels right: both are "two dots."
Fix: ask where the dots sit. In the gap between two atoms → bonding pair (a bond). Hugging one atom → lone pair. Position is everything.
Definition Formal charge (+ − neutral when it "owns" its fair share of electrons. If it loses electrons it becomes positive (+ gains electrons it becomes negative (−
A + short of electrons → electron-poor.B − extra electrons → electron-rich.
Picture: a small superscript sign floating at an atom's top-right corner.
Why the topic needs it: heterolysis (section 6) makes ions (A + : B − net charge is conserved — only makes sense once you can add these signs up. ( + 1 ) + ( − 1 ) = 0
Definition Free radical (A•)
A free radical is a neutral atom or group carrying one unpaired electron — a lone dot with no partner. We write it A ∙ Picture: a single dot on an otherwise-neutral atom, like Cl ∙ Why the topic needs it: homolysis (section 6) splits a bond evenly , giving each atom one of the two shared dots — so each product is a radical. You cannot describe Free radical substitution (halogenation of alkanes) without this dot.
Intuition Charge dot vs radical dot — don't mix them upA charge (+ / − number of electrons overall." A radical dot (∙ unpaired ." A radical is usually neutral — it just has a lonely electron.
We now have every piece needed to state the two outcomes when a shared pair splits. A bond holds two electrons; when it breaks, those two dots must be divided — and there are only two ways to divide two things.
HETERO = different → one greedy atom takes both → ions . HOMO = same/even → each takes one → radicals .
Definition ElectronegativityElectronegativity measures how strongly an atom pulls shared electrons toward itself in a bond. Bigger number = greedier atom.
Picture: a rope (the bond) with two hands (the atoms) pulling. The stronger hand drags the two dots to its side.
Why the topic needs it: it decides which way a bond breaks. In C–Br, Br is more electronegative, so it wins the tug-of-war and grabs both electrons → heterolysis to : B r − homolysis . Deepen this in Inductive effect and electronegativity .
What to look for in the figure: two tug-of-war scenes. On the left (Cl–Cl) both white arrows pulling on the amber dot-pair are the same length — equal pull — so the pair sits dead-centre and the split is even (homolysis → two radicals) . On the right (C–Br) the amber arrow toward Br is thicker and longer — Br out-pulls C — so the dot-pair is dragged fully onto Br and the split is uneven (heterolysis → C⁺ and :Br⁻) . The length/thickness of the pulling arrow is the visual code for "who is greedier."
Definition Single, double, triple bondsA single bond shares one pair (2 electrons) → drawn as one line A—B.
A double bond shares two pairs (4 electrons) → drawn as two lines A=B.
A triple bond shares three pairs (6 electrons) → drawn as three lines A≡B.
Picture: each extra line = one more shared pair of dots stacked alongside the first.
Definition π (pi) bond — the "reactive extra" lineIn a double bond A=B, the first line is a tight, head-on bond called a σ (sigma) bond . The second line is a looser, side-by-side bond called a π (pi) bond — the Greek letter π π.
Picture: the σ bond is the strong core line between the atoms; the π bond is an extra electron pair hovering above and below that line, less tightly held.Why the topic needs it: because π electrons are loosely held and exposed, they are electron-rich and easy to donate — a π bond is a classic nucleophile (section 9). In the parent's Worked Example 4, the arrow tail sits on the C=C π bond and attacks the δ + Electrophilic addition to alkenes .
Before the arrows can mean anything, here are the rules they obey — the same three the parent lists.
Definition Curved arrows — full vs fishhook
A double-barbed (full) arrow (↷, a complete two-barb head) moves an electron pair — two dots together.
A single-barbed (fishhook) arrow (⇀, a half one-barb head) moves one dot only.
Both curve from source to destination, both start at electrons and point where the electrons land .
Why the topic needs it: these two arrows are the grammar of the topic. One full arrow → heterolysis and ionic reactions. Two fishhooks → homolysis (radicals).
What to look for in the figure: both arrows curve from a starting pair of dots to their target — that curvature is deliberate, showing electrons sweeping over. On the left , the cyan full arrow has a complete (two-barbed) head and starts on two amber dots → it carries a whole pair. On the right , the amber fishhook arrow has only half a head (one barb) and starts on a single amber dot → it carries just one electron. Compare the heads side by side: two barbs vs one barb is the only difference that tells pair-movement from single-electron movement.
Definition Partial charge (delta,δ δ δ) means "a little bit of ." So δ + δ − Picture: faint charge signs — the electrons aren't fully handed over, just pulled partway toward the greedier atom.
Why the topic needs it: in the parent's Worked Example 4, the H of H–Br is δ +
δ + + Why it feels right: both are plus signs.
Fix: δ + lean , not a loss . The atom still shares the pair; it's merely being out-pulled. A full +
Definition Nucleophile & Electrophile
A nucleophile is electron-rich — it has a pair to give (a lone pair, a − tail sits (Rule 1).
An electrophile is electron-poor — it wants a pair (a + δ + head points (Rule 2).
Picture: electrons always flow rich → poor , like water running downhill.
Why the topic needs it: the first two golden rules (section 9) are literally "tail on nucleophile, head on electrophile." Full detail in Nucleophiles and Electrophiles .
h ν h ν the energy carried by one particle of light (UV, say). Read it as "shine light on it."
Picture: a squiggly arrow of sunlight hitting the bond and knocking it apart.
Why the topic needs it: homolysis (section 6) usually needs an energy push with no ions to help. Written over the reaction arrow, h ν
Covalent bond shared pair
Double and triple bonds pi electrons
Every arrow into MECH is a symbol you now own. Carry them into the parent topic and into Resonance and arrow pushing , Carbocations — stability and structure , and Electrophilic addition to alkenes .
Cover the right side and test yourself — you are ready when every line is instant.
What a single dot · represents One electron.
What a line — (or :) between two atoms represents A covalent bond = two shared electrons.
Where a lone pair sits vs a bonding pair Lone pair hugs ONE atom; bonding pair sits in the gap BETWEEN two atoms.
What A=B and A≡B mean A double bond (two shared pairs, 4 e⁻) and a triple bond (three shared pairs, 6 e⁻).
What a π (pi) bond is The looser second (or third) bond in a multiple bond, an electron pair above/below the σ core — electron-rich and easy to donate (a nucleophile).
What A + The atom is short of electrons (electron-poor).
What A ∙ Neutral atom/group with one unpaired electron — a free radical.
Define heterolysis in electron language Bond breaks unevenly — both electrons go to one atom → a cation and an anion (ions); drawn with one full arrow.
Define homolysis in electron language Bond breaks evenly — each atom keeps one electron → two neutral radicals; drawn with two fishhook arrows.
What electronegativity decides in bond breaking Which atom wins the tug-of-war for the shared pair (drives heterolysis vs homolysis).
Meaning of δ + "Slightly positive" — electrons pulled partway away, not fully lost.
Full arrow vs fishhook arrow Full (double-barbed) = moves an electron pair (2 e⁻); fishhook (half) = moves one electron.
The three golden rules of arrow-pushing (1) tail starts on electrons; (2) head points to an electron-poor site; (3) conserve electron count and net charge.
Where an arrow's tail must start On electrons — a bond or a lone pair (a nucleophile / electron-rich site).
Where an arrow's head points To an electron-poor site — a + δ +
What h ν Light energy input — usually the homolytic (radical) pathway.
The conservation rule you run every step Net charge before = net charge after (e.g. ( + 1 ) + ( − 1 ) = 0
