Types of reactions — combination, decomposition, displacement, double displacement, redox: Foundations

Before you can read $2 M g + O_{2} \to 2 M g O$ and know what it says, you need to earn every mark on the page. We go from the smallest scribble (a letter) up to the idea of electron transfer. Nothing is used before it is drawn.

Symbol 0 — the chemical formula (a "word" for a substance)

Look at the first figure. The formula $H_{2} O$ is not "H times 2 times O" — the little sinking 2 ("subscript") means "two hydrogen atoms are stuck inside one unit of this substance."

Figure — Types of reactions — combination, decomposition, displacement, double displacement, redox

A subscript after a symbol counts that atom: $O_{2}$ = two oxygen atoms bonded together.
A subscript after a bracket multiplies everything inside: $C a (O H)_{2}$ = one Ca, two O, two H.
No subscript means one (we never write $O_{1}$ ).

Symbol 1 — the coefficient (a "how many" counter)

Why does the topic need it? Because atoms are never lost, the left and right of the arrow must have equal atom-counts. The only dial we are allowed to turn is the coefficient — see Balancing Chemical Equations.

Symbol 2 — the reaction arrow and its friends

The arrow often carries little labels that tell you what energy pushed the change — vital for decomposition (see Exothermic and Endothermic Reactions and Electrolysis).

Mark	Plain meaning	Picture
$\to$	"becomes"	before → after
$Δ$	heat was supplied ( $Δ$ = "heat")	a flame under the arrow
$electricity$	electric current supplied	wires into the beaker
$↑$	product gas leaves upward	bubbles rising out
$↓$	product solid sinks out (precipitate)	powder dropping to the bottom

Symbol 3 — charge and the ion (a "+" or "−" riding on an atom)

Electrons are tiny negative particles. Take one away from a neutral atom and it's left positive ( $+$ ); add one and it becomes negative ( $-$ ). The superscript number says how many electrons' worth of charge.

Ions are the raw material of Acids, Bases and Salts and of every double-displacement swap, because in water ions drift apart and can re-pair with new partners.

Symbol 4 — oxidation number (a bookkeeping charge)

Why invent a pretend charge when real ions already have one? Because in molecules like $H_{2} O$ the electrons are shared, not fully handed over — there's no real ion. Oxidation number is a fair accounting trick so we can still track which way electrons leaned and thus spot electron transfer. Full rules live in Oxidation Number Rules; the starter kit:

Symbol 5 — reactivity, the referee for displacement

Why does the topic need a list and not a formula? Because "will A kick out B?" has no equation — it's decided by who is more reactive, and that order is measured, then memorised. Details in Reactivity Series of Metals.

$K > N a > C a > M g > A l > Z n > F e > P b > (H) > C u > A g > A u$

Read it like a ladder: whoever stands higher wins the electrons' company and shoves the lower one out of its compound.

H_{2} O

is called a

subscript — counts atoms inside one unit, never changed to balance

The big number in front like the 2 in

2 M g O

is called a

coefficient — counts whole units, the only thing you adjust to balance

What the arrow

\to

reads as in words

"becomes" / "gives" — before on the left, after on the right

Meaning of

↑

and

↓

on a product

gas escapes upward / solid precipitates downward — and this pulls the reaction forward

What a superscript like

C l^{-}

S O_{4}^{2 -}

shows

net electric charge (electrons lost or gained), not an atom count

An oxidation number is

a pretend charge from giving shared electrons to the greedier atom — used to track electron transfer

Oxidation number of any free uncombined element

zero (e.g.

M g

O_{2}

)

Rule: all oxidation numbers in a species add up to

the species' overall charge

What

(a q)

tells you and why it matters

dissolved in water → ions float free → partners can swap (double displacement)

Who wins in a displacement reaction

the metal higher in the reactivity series displaces the lower one

Foundations — Types of reactions — combination, decomposition, displacement, double displacement, redox

Symbol 0 — the chemical formula (a "word" for a substance)

Symbol 1 — the coefficient (a "how many" counter)

Symbol 2 — the reaction arrow and its friends

Symbol 3 — charge and the ion (a "+" or "−" riding on an atom)

Symbol 4 — oxidation number (a bookkeeping charge)

Symbol 5 — reactivity, the referee for displacement

Symbol 6 — states and the phase labels

How these feed the topic

Equipment checklist