Total Hydrogen (H)

Total number of hydrogen substituents.

.H+

H0-H4

.H-

H4-H0

Implicit Hydrogen (h)

Number of implicit hydrogen substituents.

.h+

h0-h4

.h-

h4-h0

Bond order (v)

Total bond order. (Some cases it is referred as valence.)

.v+

v0-v8

.v-

v8-v0

Connections (X)

Number of substituents including hydrogens.

.X+

X0-X3

.X-

X3-X0

Ring count (R)

Number of rings the atom is a member of.

.R+

R0-R6

.R-

R6-R0

Smallest ring size (r)

Size of the smallest ring the atom is a member of.

.r+

r0-r30

.r-

r30-r0

Ring bond (rb)

Number of ring bonds next to the atom.

.rb+

rb0-rb4

.rb-

rb4-rb0

.rb*

as drawn

Substitutions (s)

Substitution count: the number of non-hydrogen substituents plus number of isotopic hydrogen substituents.

.s+

s0-s6

.s-

s6-s0

.s*

as drawn

Unsaturated (u)

Unsaturated atom: atom that has double, triple or aromatic bond.

.u

u

Aromaticity (a/A)

It defines whether the atom has at least one aromatic bond (a) or only aliphatic (A) bonds.

.a/A

a, A