The following procedure can be provided to construct Lewis electron frameworks for more complex molecules and also ions:

How-to: building Lewis electron structures

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

You are watching: The ion no- has ________ valence electrons.

include together the valence electrons from every atom. (Recall that the number of valence electron is indicated by the place of the element in the regular table.) If the species is a polyatomic ion, mental to include or subtract the number of electrons necessary to provide the total charge on the ion.

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

2. Species the atoms to show details connections.

once there is a central atom, that is commonly the the very least electronegative facet in the compound. yellowcomic.comists generally list this main atom first in the yellowcomic.comistry formula (as in CCl4 and also CO32−, i beg your pardon both have C as the central atom), i beg your pardon is one more clue to the compound’s structure. Hydrogen and also the halogens are practically always connected to just one other atom, for this reason they space usually terminal fairly than central.

3. Ar a bonding pair that electrons in between each pair of adjacent atoms to give a single bond.

In H2O, because that example, there is a bonding pair that electrons between oxygen and also each hydrogen.

4. Beginning through the terminal atoms, include enough electron to each atom to provide each atom one octet (two for hydrogen).

these electrons will commonly be lone pairs.

5. If any electrons room left over, ar them ~ above the central atom.

we will explain later that some atoms space able come accommodate an ext than eight electrons.

6. If the central atom has fewer electrons 보다 an octet, usage lone pairs from terminal atoms to kind multiple (double or triple) bonds to the main atom to achieve an octet.

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

7. Last check

always make certain all valence electrons room accounted for and also each atom has actually an octet that electrons except for hydrogen (with two electrons).

The main atom is normally the the very least electronegative aspect in the molecule or ion; hydrogen and also the halogens are usually terminal.

Now let’s apply this procedure to some particular compounds, beginning with one we have already discussed.

Example (PageIndex1): Water

Write the Lewis structure for H2O.


Steps for creating Lewis Structures

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

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

2. Arrange the atom to show particular connections.


Because H atom are virtually always terminal, the plan within the molecule must be HOH.

3. Location a bonding pair the electrons in between each pair of nearby atoms to give a single bond.

4. Start with the terminal atoms, include enough electrons to every atom to give each atom an octet (two for hydrogen).

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

H -O- H

with 4 electron left over.

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

5. If any type of electrons space left over, place them on the main atom.

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

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because carbon is much less electronegative than oxygen and hydrogen is normally terminal, C have to be the main atom.

3. Ar a bonding pair of electrons in between each pair of nearby atoms to give a solitary bond.

Placing a bonding pair the electrons in between each pair of external inspection atoms gives the following:


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7. Last check

Both the oxygen and also the carbon now have actually an octet of electrons, for this reason this is an agree Lewis electron structure. The O has actually two bonding pairs and two lone pairs, and C has 4 bonding pairs. This is the framework of formaldehyde, i beg your pardon is provided in embalming fluid.

Exercise (PageIndex1)

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

Answer CO2




Answer SCl2




Writing Lewis frameworks for Polyatomic Ions

Recall the a polyatomic ion is a group of atom that are covalently external inspection together and also which carry an overall electrical charge. The ammonium ion, (ceNH_4^+), is formed when a hydrogen ion (left( ceH^+ ight)) attaches to the lone pair of an ammonia (left( ceNH_3 ight)) molecule in a name: coordinates covalent bond.


Exceptions come the Octet Rule

As important and also useful as the octet dominance is in yellowcomic.comical bonding, there room some well-known violations. This does not average that the octet ascendancy is useless—quite the contrary. Similar to many rules, there space exceptions, or violations.

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There are three violations come the octet rule. Odd-electron molecules represent the first violation come the octet rule. Although they space few, some secure compounds have an odd number of electrons in their valence shells. Through an odd variety of electrons, at least one atom in the molecule will have to violate the octet rule. Examples of steady odd-electron molecules space NO, NO2, and also ClO2. The Lewis electron period diagram because that NO is as follows:


Although the O atom has actually an octet the electrons, the N atom has actually only 7 electrons in the valence shell. Return NO is a stable compound, that is very yellowcomic.comically reactive, together are most other odd-electron compounds.

Electron-deficient molecules represent the second violation to the octet rule. These steady compounds have actually less 보다 eight electrons about an atom in the molecule. The most common examples space the covalent compound of beryllium and also boron. For example, beryllium can kind two covalent bonds, bring about only 4 electrons in that valence shell:

the ion no- has ________ valence electrons.