ABC graphs/Printable version

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ABC graphs

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Acronym and definitions[edit | edit source]

Letter Words Concept Notation Numerical value
A atom atom with bonding electrons symbol A
B bond bonding path or σ-binding pair line B
C charge residual charge characters (−,=,≡,+,⧺,ᚒ) C
D dot Non-σ-bonding electrons in pair(s) or as single(s) dots/dot D
E ellipse unoccupied orbital hyperellipse E

Atoms and number of bonding electrons[edit | edit source]

Atom symbols are given in the periodic table of elements. Number of bonding electrons equals to the group number for s, sd and sf elements and to the last digit in the group number of sp elements. Let us denote that number as A.

To arrange the atoms one may follow the rules: [comment: to be written].

Bonding paths and σ-bonds[edit | edit source]

Any bonds in the ABC-type graphs is drawn with a single line as it corresponds to a bonding path.

[Comment: add the definition of a bonding path and the difference from a σ-bond]

Let us denote the number of σ-bonds formed by a given atom as B.

Residual charge[edit | edit source]

Residual charge on the most electronegative atoms is a charge left after creating 'N' σ-bonds with the corresponding anions, e.g. N3−, O2−, F−.

All other residual charges are calculated using the electroneutrality principle.

Let us denote the residual charge value of a given atom as C.

Non-σ-bonding electrons[edit | edit source]

[Comment: there should be a better name. Obviously, these are not the typical lone electrons.]

Let us denote the number of non-σ-bonding electrons as D:

D = A − B − C

Unoccupied orbitals[edit | edit source]

Let us denote the number of unoccupied orbitals associated with a given atom as E:

E = N − B − D· − D:

where N is the number of bonding orbitals of the atom (s,p,d,f), D· is a number of singles, D: is a number of pairs.


AB graph[edit | edit source]

A: Arrange atom symbols (Atoms)

B: Connect symbols with lines (Bonding paths)

ABC graph[edit | edit source]

A: Arrange atom symbols (Atoms)

B: Connect symbols with lines (σ-Bonds)


  1. Find the most electronegative atoms evaluate its residual charge.
  2. Calculate the residual charges for the rest of atoms accounting for the total charge value.

ABCD graph[edit | edit source]

A. Arrange Atoms.
B. Allocate Bonds.
C. Assign Charges.
D. Add Dots.
A. Hydrogen as well as more electronegative atoms tend to be terminal or (less commonly) bridging.
B. Few-member cycles are less favorable than linear arrangements.
C. Residual charge of terminal H/F/Cl/Br/I is 0 (not written), O/S is −, N is =, C is ≡.
Residual charge of bridging H/F/Cl/Br/I is +, O/S is 0 (not written), N is −.
Residual charge of the central atom is found using eq ΣC = q (total charge).
D. Number of dots equals D = A − B − C, where A is the number of the atoms' bonding electrons and B is the number of bonds around that atom.
A equals to the last digit of the group numbers.
A. Use atom symbols.
B. Draw bonds with lines.
C. Write non-zero charges with characters: −, =, ≡, +, ⧺, ᚒ.
D. Draw dots as pairs or singles.

ABD graph[edit | edit source]

A and B steps the same as for ABC graph.

D: For each atom of interest, find the number of bonding electrons (A), evaluate the oxidation number (O), but do not write it, then calculate D as D = A − O. Draw D dots around the atom in pairs of singles (Hund rule applies).

ABDE and ABCDE graph[edit | edit source]

A–D steps the same as for ABD or ABCD graphs.

E: For each sp-atom of interest, calculate E as E = 4 − B − D· − D: Draw E hyper-ellipses around the atom.

Control[edit | edit source]

Ensure that ΣC = q.

For geometrically similar arrangements prefer one with minimal sum of absolute residual charges Σ|C|. For example, O–O–C vs O–C–O and N–N–O vs N–O–N.

D can not be negative.


1st Example[edit | edit source]

Compounds: H, O, H2, O2, OH, OH−, H2O, H3O+, H2O2.

Topic: combustion of H2 in O2

Media: explosion!

2nd Example[edit | edit source]

Compounds: Ca2C, Ca3N2, CaO, CaF2 -> CH4, NH3, H2O, HF

Topic: hydrolysis

Media: reaction of some oxide producing a basic solution visualised with phenolphthalein

3rd Example[edit | edit source]

Compounds: CO2, NO2, SO2 -> CO32-, NO3-, SO32- & SO3

Topic: atmospheric gases, redox

Media: acid rain from volcano and cars

4th Example[edit | edit source]

Compounds: PF5, -> H3PO4, Al3+ -> Al(OH)4-

Topic: hydrolysis again

Media: dissolution of aluminium foil in a strong vase