The adhering to procedure deserve to be supplied to construct Lewis electron structures for more complicated molecules and ions:

How-to: constructing Lewis electron structures

1. Determine the total variety of valence electron in the molecule or ion.

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add together the valence electrons from each atom. (Recall the the variety of valence electron is suggested by the position of the aspect in the regular table.) If the species is a polyatomic ion, remember to add or subtract the variety of electrons vital to provide the full charge on the ion.

For CO32−, for example, we add two electrons to the total because of the −2 charge.

2. Species the atoms to show particular connections.

as soon as there is a main atom, that is usually the the very least electronegative element in the compound. Rebab.netists typically list this main atom first in the rebab.netical formula (as in CCl4 and CO32−, which both have C together the main atom), i beg your pardon is an additional clue to the compound’s structure. Hydrogen and the halogens are almost always connected to just one various other atom, for this reason they room usually terminal quite than central.

3. Ar a bonding pair that electrons in between each pair of surrounding atoms to offer a solitary bond.

In H2O, for example, over there is a bonding pair of electrons between oxygen and each hydrogen.

4. Beginning with the terminal atoms, add enough electron to every atom to give each atom an octet (two because that hydrogen).

this electrons will generally be lone pairs.

5. If any kind of electrons space left over, ar them ~ above the central atom.

we will define later that some atoms room able to accommodate more than eight electrons.

6. If the central atom has fewer electrons than an octet, usage lone pairs from terminal atom to form multiple (double or triple) bonds to the central atom to attain an octet.

This will not adjust the variety of electrons on the terminal atoms.

7. Last check

constantly make certain all valence electrons are accounted for and each atom has an octet of electrons except for hydrogen (with two electrons).

The main atom is commonly the least electronegative element in the molecule or ion; hydrogen and also the halogens room usually terminal.

Now let’s use this procedure come some specific compounds, start with one us have already discussed.

Example \(\PageIndex1\): Water

Write the Lewis structure for H2O.


Steps for composing Lewis Structures

instance \(\PageIndex1\)
1. Identify the total variety of valence electrons in the molecule or ion.

Each H atom (group 1) has actually 1 valence electron, and also the O atom (group 16) has actually 6 valence electrons, for a complete of 8 valence electrons.

2. Kinds the atoms to show specific connections.


Because H atom are nearly always terminal, the plan within the molecule need to be HOH.

3. Place a bonding pair the electrons between each pair of nearby atoms to offer a solitary bond.

4. Beginning with the terminal atoms, add enough electron to every atom to offer each atom an octet (two for hydrogen).

Placing one bonding pair of electrons in between the O atom and also each H atom gives

H -O- H

with 4 electrons left over.

Each H atom has a full valence shell of 2 electrons.

5. If any kind of electrons space left over, place them top top the central atom.

Adding the continuing to be 4 electrons to the oxygen (as 2 lone pairs) provides the following structure:

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because carbon is less electronegative than oxygen and also hydrogen is usually terminal, C should be the central atom.

3. Place a bonding pair that electrons in between each pair of nearby atoms to provide a solitary bond.

Placing a bonding pair that electrons in between each pair of bonded atoms gives the following:


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7. Last check

Both the oxygen and the carbon now have actually an octet that electrons, therefore this is an agree Lewis electron structure. The O has two bonding pairs and also two lone pairs, and also C has four bonding pairs. This is the structure of formaldehyde, i m sorry is used in embalming fluid.

Exercise \(\PageIndex1\)

Write Lewis electron structures for CO2 and SCl2, a vile-smelling, stormy red liquid that is offered in the manufacture of rubber.

Answer CO2




Answer SCl2




Writing Lewis structures for Polyatomic Ions

Recall that a polyatomic ion is a team of atoms that space covalently bonded together and also which bring an as whole electrical charge. The ammonium ion, \(\ceNH_4^+\), is developed when a hydrogen ion \(\left( \ceH^+ \right)\) attaches come the lone pair of an ammonia \(\left( \ceNH_3 \right)\) molecule in a coordinate covalent bond.


Exceptions to the Octet Rule

As important and also useful as the octet ascendancy is in rebab.netical bonding, there space some well-known violations. This does not median that the octet ascendancy is useless—quite the contrary. Just like many rules, there room exceptions, or violations.

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There room three violations come the octet rule. Odd-electron molecules stand for the first violation come the octet rule. Back they are few, some stable compounds have an odd variety of electrons in their valence shells. With an odd variety of electrons, at the very least one atom in the molecule will need to violate the octet rule. Instances of stable odd-electron molecules room NO, NO2, and ClO2. The Lewis electron dot diagram for NO is as follows:


Although the O atom has an octet the electrons, the N atom has only seven electrons in its valence shell. Back NO is a stable compound, the is really rebab.netically reactive, as are many other odd-electron compounds.

Electron-deficient molecules stand for the 2nd violation come the octet rule. These stable compounds have less 보다 eight electrons about an atom in the molecule. The most common examples space the covalent link of beryllium and boron. For example, beryllium can form two covalent bonds, resulting in only 4 electrons in that valence shell:

how many valence electrons does hydrogen have