This is a supplementary post to gather together the most commonly used symbols on Chemistry.SE and the compose key sequences for a Windows user running WinCompose.
The following list includes a symbol, keyboard sequence (compose key is normally the Right Alt button), Unicode character and description, as well as examples on the next line:
− ⎄ Compose - - = U+2212 MINUS SIGN
−42;
C−C bond (when not using mhchem, esp. in conjunction with double bond C=C).
– ⎄ Compose - - . U+2013 EN DASH
acid–base titration;
acetonitrile–water mixture;
Ziegler–Natta catalyst;
1–2 kg.
— ⎄ Compose - - - U+2014 EM DASH
"All three experimental parameters—temperature, time, and concentration—were
strictly followed."
≡ ⎄ Compose = = U+2261 IDENTICAL TO
a ≡ b (also: $a ≡ b$ ($a ≡ b$));
C≡C bond.
≅ ⎄ Compose ~ = U+2245 APPROXIMATELY EQUAL TO
A ≅ B.
≈ ⎄ Compose ~ ~ U+2248 ALMOST EQUAL TO
e ≈ 2.72.
≠ ⎄ Compose / = U+2260 NOT EQUAL TO
MathJax ≠ $\mathrm\LaTeX$.
≤ ⎄ Compose < = U+2264 LESS-THAN OR EQUAL TO
x ≤ y (also: $x ≤ y$ ($x ≤ y$)).
≥ ⎄ Compose > = U+2265 GREATER-THAN OR EQUAL TO
x ≥ y (also: $x ≥ y$ ($x ≥ y$)).
≪ ⎄ Compose + < U+226A MUCH LESS-THAN
x ≪ y (also: $x ≪ y$ ($x ≪ y$)).
≫ ⎄ Compose + > U+226B MUCH GREATER-THAN
x ≫ y (also: $x ≫ y$ ($x ≫ y$)).
± ⎄ Compose + - U+00B1 PLUS-MINUS SIGN
(142 ± 13) pm (also: $\pu{(142 ± 13) pm}$ ($\pu{(142 ± 13) pm}$)).
· ⎄ Compose . - U+00B7 MIDDLE DOT
BF3 · THF (also: $\ce{BF3·THF}$ ($\ce{BF3·THF}$));
HO· radical;
n = c · V (also: $n = c · V$ ($n = c · V$));
3 · 106 m/s (also: $\pu{3e6 m/s}$ ($\pu{3e6 m/s}$)).
• ⎄ Compose * 1 U+2022 BULLET
HCO•+ radical (useful when superscripted for denoting a radical, esp. in conjunction with plus/minus (e.g. <sup>•+</sup>)).
× ⎄ Compose x x U+00D7 MULTIPLICATION SIGN
1.2 × 3 = 3.6 (also: $1.2 × 3 = 3.6$ ($1.2 × 3 = 3.6$));
6.022 × 1023 mol−1 (also: $\pu{6.022E23 mol-1}$ ($\pu{6.022E23 mol-1}$)).
° ⎄ Compose o o U+00B0 DEGREE SIGN
57.296° (also: $57.296°$ ($57.296°$));
298.15 °C (also: $\pu{298.15 °C}$ ($\pu{298.15 °C}$))
… ⎄ Compose . . U+2026 HORIZONTAL ELLIPSIS
1, 2, 3, ….
Å ⎄ Compose o A U+00C5 LATIN CAPITAL LETTER A WITH RING ABOVE (aka angstrom symbol)
Bond length is 1.23 Å.
à ⎄ Compose ~ A U+00C3 LATIN CAPITAL LETTER A WITH TILDE
Electronic state of polyatomic molecule Ã.
â ⎄ Compose ^ a U+00E2 LATIN SMALL LETTER A WITH CIRCUMFLEX
Le Châtelier's principle.
à ⎄ Compose ` a U+00E0 LATIN SMALL LETTER A WITH GRAVE
vis-à-vis.
ä ⎄ Compose " a U+00E4 LATIN SMALL LETTER A WITH DIAERESIS
Friedländer quinoline synthesis.
é ⎄ Compose ' e U+00E9 LATIN SMALL LETTER E WITH ACUTE
déjà vu
ö ⎄ Compose " o U+00F6 LATIN SMALL LETTER O WITH DIAERESIS
Wöhler Synthesis of urea.
ß ⎄ Compose s s U+00DF LATIN SMALL LETTER SHARP S
Mößbauer spectroscopy
ü ⎄ Compose " u U+00FC LATIN SMALL LETTER U WITH DIAERESIS
Büchner flask.
é ⎄ Compose ' e U+00E9 LATIN SMALL LETTER E WITH ACUTE
Kekulé
ø ⎄ Compose / o U+00F8 LATIN SMALL LETTER O WITH STROKE
Brønsted–Lowry theory.
ᴅ ⎄ Compose d ` U+1D05 LATIN LETTER SMALL CAPITAL D
ᴅ-phenylglycine.
ʟ ⎄ Compose l ` U+029F LATIN LETTER SMALL CAPITAL L
ʟ-lactic acid.
α ⎄ Compose * a U+03B1 GREEK SMALL LETTER ALPHA
α-helix.
β ⎄ Compose * b U+03B2 GREEK SMALL LETTER BETA
β-decay.
γ ⎄ Compose * g U+03B3 GREEK SMALL LETTER GAMMA
γ-hydroxybutyric acid.
δ ⎄ Compose * d U+03B4 GREEK SMALL LETTER DELTA
δ– charge.
Δ ⎄ Compose * D U+0394 GREEK CAPITAL LETTER DELTA
Δ-ruthenium-tris(bipyridine).
ε ⎄ Compose * e U+03B5 GREEK SMALL LETTER EPSILON
ε is the molar absorptivity.
ζ ⎄ Compose * z U+03B6 GREEK SMALL LETTER ZETA
ζ-potential.
θ ⎄ Compose * u U+03B8 GREEK SMALL LETTER THETA
θ is the angle of scattering.
κ ⎄ Compose * k U+03BA GREEK SMALL LETTER KAPPA
κ6-EDTA.
λ ⎄ Compose * l U+03BB GREEK SMALL LETTER LAMDA
wavelength λ.
Λ ⎄ Compose * L U+039B GREEK CAPITAL LETTER LAMDA
molar conductivity Λ.
μ ⎄ Compose * m U+03BC GREEK SMALL LETTER MU
4.2 μmol.
ν ⎄ Compose * n U+03BD GREEK SMALL LETTER NU
ν = c/λ.
ξ ⎄ Compose * j U+03BE GREEK SMALL LETTER XI
ξ is the extent of reaction.
π ⎄ Compose * p U+03C0 GREEK SMALL LETTER PI
π-electron.
ρ ⎄ Compose * r U+03C1 GREEK SMALL LETTER RHO
ρ is the density.
σ ⎄ Compose * s U+03C3 GREEK SMALL LETTER SIGMA
σ bond.
τ ⎄ Compose * t U+03C4 GREEK SMALL LETTER TAU
bent bond τ.
φ ⎄ Compose * f U+03C6 GREEK SMALL LETTER PHI
φ bond.
χ ⎄ Compose * x U+03C7 GREEK SMALL LETTER CHI
χ is the electronegativity.
ψ ⎄ Compose * c U+03C8 GREEK SMALL LETTER PSI
ψ is the wave function.
ω ⎄ Compose * v U+03C9 GREEK SMALL LETTER OMEGA
ω is the mass fraction.
† ⎄ Compose d a g U+2020 DAGGER
Footnote†.
‡ ⎄ Compose d d a g U+2021 DOUBLE DAGGER
Footnote‡;
denotes a [transition state]‡.
→ ⎄ Compose → → U+2192 RIGHTWARDS ARROW
A + B → C + D.
ʹ ⎄ Compose * ' U+02B9 MODIFIER LETTER PRIME
2,2′-bipyridine.
″ ⎄ Compose * . " U+2033 DOUBLE PRIME
47°8′23″;
hν = E′ − E″ (also: $hν = E′ − E″$ ($hν = E′ − E″$)).
❬ ⎄ Compose @ < U+276C MEDIUM LEFT-POINTING ANGLE BRACKET ORNAMENT
❭ ⎄ Compose @ < U+276D MEDIUM RIGHT-POINTING ANGLE BRACKET ORNAMENT
❬+|−❭ (bra–ket notation);
❬x❭ (average/mean value).
‘ ⎄ Compose 6 ' U+2018 LEFT SINGLE QUOTATION MARK
’ ⎄ Compose 9 ' U+2019 RIGHT SINGLE QUOTATION MARK
‘high-6 high-9’ (British English).
“ ⎄ Compose 6 " U+201C LEFT DOUBLE QUOTATION MARK
” ⎄ Compose 9 " U+201D RIGHT DOUBLE QUOTATION MARK MARK
“high-66 high-99” (American English).
„ ⎄ Compose , " U+201E DOUBLE LOW-9 QUOTATION MARK
“ ⎄ Compose 6 " U+201C LEFT DOUBLE QUOTATION MARK
„low-99 high-66“ (German).
« ⎄ Compose < < U+00AB LEFT-POINTING DOUBLE ANGLE QUOTATION MARK MARK
» ⎄ Compose > > U+00BB RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
«guillemets» (French, Russian, Swiss German).
₀₁₂₃₄₅₆₇₈₉ ⎄ Compose _ <number 0–9> lowercase number 0–9 (also works with some other characters)
H₂O.
⁰¹²³⁴⁵⁶⁷⁸⁹ ⎄ Compose ^ <number 0–9> uppercase number 0–9 (also works with some other characters)
Cr³⁺.
(ﾉಥ益ಥ）ﾉ彡┻━┻ ⎄ Compose t f Table flip like a boss (not an actual Unicode symbol; can be found in chat)
— "hI... plz asap wat is number of moles in 18G h20?!, thx bai"
— *question becomes HNQ*
— (ﾉಥ益ಥ）ﾉ彡┻━┻
The majority of these combination will also work for Linux users out of the box.
One can also search for the key combinations in the dialogue opening via double-clicking on the tray icon 
.
Keep in mind: if you lack a combination for a symbol, you can always create/edit ~/.XCompose file.
I always append some commands to mine:
<Multi_key> <minus> <0> <minus>: "⦵" U29B5 # PLIMSOLL
<Multi_key> <a> <n> <g> <l> <e>: "∠" U2220 # ANGLE
<Multi_key> <e> <q> : "⇌" U21CC # RIGHTWARDS HARPOON OVER LEFTWARDS HARPOON
which allow me to type in the following way:
⦵ ⎄ Compose - 0 - U+29B5 CIRCLE WITH HORIZONTAL BAR
$Δ_\mathrm fH^⦵$ ($Δ_\mathrm fH^⦵$).
∠ ⎄ Compose a n g l e U+2220 ANGLE
∠H−O−H ≅ 104.5°.
⇌ ⎄ Compose e q U+21CC RIGHTWARDS HARPOON OVER LEFTWARDS HARPOON
2 H₂O ⇌ H₃O⁺ + OH⁻
Windows users can right-click on WinCompose tray icon , select Options… → Compositing → Sequences and click Edit button next to user-defined sequences:
