Foundations — Group 16 (Oxygen family) — allotropes of O (O₂, O₃); ozone chemistry, ozone layer

This page assumes you have seen nothing. We build the vocabulary the parent note (the topic note) leans on, one brick at a time.

1. What is an atom, and what does the symbol "O" mean?

What figure s01 shows: three groups drawn on the chalkboard — a single yellow ball labelled "O (1 atom)", a blue pair labelled "O₂", and a pink bent trio labelled "O₃". Look at the leftmost yellow ball: that lone ball is exactly what the bare letter $\text{O}$ means. Notice the trio on the right is drawn kinked, not straight — that kink is the whole point of Section 8.

Why the topic needs it: the whole chapter is about clipping these $\text{O}$ balls together in different counts and arrangements. If $\text{O}$ didn't mean "one atom", none of the formulas below would count anything.

2. The little subscript number — O₂ vs O₃

In figure s01: compare the middle blue pair (labelled O₂, two balls) with the right pink trio (labelled O₃, three balls). The subscript is simply how many balls you counted in that group — 2 versus 3.

Why the topic needs it: allotropy (the topic's opening word) literally means the same element wearing different atom-counts/arrangements. The subscript is the notation that tells ${\text{O}}_2$ apart from ${\text{O}}_3$.

3. The big number in front — coefficients and balancing

The picture: $3{\text{O}}_2$ is three dumbbells lying side by side — six balls in total, but grouped into three pairs.

Why the topic needs it: the preparation reaction is $3{\text{O}}_2⟶2{\text{O}}_3.$ Count the balls: left side $=3\times 2=6$ atoms; right side $=2\times 3=6$ atoms. Equal — this is what "balanced" means. Atoms are never created or destroyed, so both sides must show the same number of each ball.

4. The reaction arrow and what sits on it

Words written above the arrow are the condition — the push that makes it happen. In the parent note: $3{\text{O}}_2\stackrel{\text{silent electric discharge}}{\to }2{\text{O}}_3$ "silent electric discharge" above the arrow = a gentle sparkless high-voltage that supplies the energy.

Why the topic needs it: the ozone layer is not a fixed wall — it is a busy revolving door where formation and breakdown balance. The equilibrium idea is what makes "the layer stays there" true.

5. Bonds, lone pairs, and the dots/lines in Lewis drawings

What figure s02 shows: three sketches. On the left, two balls joined by one chalk line — that single line is a single bond (one shared pair). In the middle, two balls joined by two parallel lines — a double bond (two shared pairs). On the right, one ball with two yellow dots sitting alone on top — that pair of dots is a lone pair, kept to itself and shared with nobody.

Why the topic needs it: the whole shape-and-reactivity story rests on counting bonds and lone pairs. In ozone the central oxygen has 2 bonds + 1 lone pair — that count is exactly what bends the molecule (Section 8).

6. Bond order — a number for "how strong is this bond?"

The parent note computes bond order two ways. From Molecular Orbital Theory it uses $\text{Bond order}=\frac{N_b-N_a}{2},$ where $N_b$ = electrons in bonding orbitals (glue) and $N_a$ = electrons in antibonding orbitals (anti-glue). We meet these orbital names next.

7. Orbital labels: σ, π, and the star ${}^{\ast }$

This is the scariest-looking notation in the parent note. We disarm it symbol by symbol — but first we must say what an orbital even is, because the label ${\sigma }_{2p_z}$ is meaningless until we do.

• $\sigma$ ("sigma") = a room shaped like a sausage straight along the axis joining the two atoms (built by dumbbells pointing head-on at each other, i.e. the $p_z$ ones).
• $\pi$ ("pi") = a room shaped like two clouds above and below that axis (built by the sideways dumbbells, $p_x$ and $p_y$).
• The subscript (e.g. ${}_{2p_z}$) just records which atomic orbitals were merged to build the room — now you can read it: "2 = second shell, p = dumbbell, z = pointing along the bond".
• The star ${}^{\ast }$ = the antibonding version of that room (anti-glue).
• The superscript number (e.g. ${\pi }^{\ast 2}$) counts how many electrons currently sit in that room.

The picture: $\sigma$ is a single fat tube linking the two balls; $\pi$ is a pair of clouds hugging that tube top and bottom. A star turns the room into a "wall" that shoves the balls apart.

To go deeper on the rooms and filling order, see Molecular Orbital Theory.

8. Shape words: "bent", "V-shaped", and the angle ≈117°

What figure s03 shows: the central pink oxygen at the top with its two bonds going down to two blue oxygens, forming a wide V. The two yellow dots above the central atom are the pushy lone pair; the pink arc between the bonds is labelled ≈117°. Watch how the lone-pair dots sit on the outside of the V and press the two bonds downward — that pressing is the "120° → 117°" squeeze.

9. Resonance, formal charge, and the double-headed arrow $\leftrightarrow$

10. Charges, dots-as-radicals, and $[O]$

Why the topic needs all three: the reactivity of ozone (bleach, germicide, oxidiser) and the CFC ozone-destruction chain are entirely told in this dot/bracket/charge language.

11. Energy bookkeeping: the mole, $\Delta H$ and its sign

Why the topic needs it: ozone formation is written $3{\text{O}}_2\to 2{\text{O}}_3,\Delta H=+142{\text{kJ mol}}^{-1}$. Read that sign out loud: the $+$ means 142 kilojoules are absorbed for every mole of ozone made, so that energy is now locked inside the ozone molecule. A molecule holding extra stored energy is like a stretched spring — it is thermodynamically unstable and desperate to release that energy by falling apart into ${\text{O}}_2+[O]$. That single fact is the root of everything reactive about ozone: it bleaches, it kills germs, it oxidises metals and iodide — all because the positive $\Delta H$ makes breaking apart energetically downhill.

12. UV, the stratosphere, and "catalyst"

Prerequisite map

Equipment checklist

Cover the right side and test yourself — you are ready when every line is instant.