ABC graphs/Printable version

ABC graphs

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Definitions

Acronym and definitions

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

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

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

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

[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

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.

Rules

AB graph

A: Arrange atom symbols (Atoms)

B: Connect symbols with lines (Bonding paths)

ABC graph

A: Arrange atom symbols (Atoms)

B: Connect symbols with lines (σ-Bonds)

C:

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

Rules

A. Arrange Atoms.

B. Allocate Bonds.

C. Assign Charges.

D. Add Dots.

Hints

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.

Style

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

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

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

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.

ABCD

1st Example

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

Topic: combustion of H2 in O2

Media: explosion!

2nd Example

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

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

Topic: atmospheric gases, redox

Media: acid rain from volcano and cars

4th Example

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

Topic: hydrolysis again

Media: dissolution of aluminium foil in a strong vase