The adhering to procedure can be offered to construct Lewis electron frameworks for more complicated molecules and ions:

How-to: building Lewis electron structures

1. Identify the total number of valence electron in the molecule or ion.

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add together the valence electron from every atom. (Recall that the variety of valence electrons is indicated by the position of the aspect in the routine table.) If the varieties is a polyatomic ion, mental to include or subtract the variety of electrons crucial to provide the total charge on the ion.

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

2. Arrange the atoms to show details connections.

once there is a central atom, that is usually the the very least electronegative element in the compound. Lifwynnfoundation.orgists normally list this central atom first in the lifwynnfoundation.orgical formula (as in CCl4 and CO32−, i m sorry both have actually C as the central atom), i beg your pardon is an additional clue to the compound’s structure. Hydrogen and the halogens are virtually always connected to only one various other atom, therefore they are usually terminal rather than central.

3. Ar a bonding pair that electrons between each pair of adjacent atoms to provide a solitary bond.

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

4. Beginning with the terminal atoms, include enough electrons to every atom to provide each atom one octet (two because that hydrogen).

these electrons will generally be lone pairs.

5. If any type of electrons room left over, ar them top top the main atom.

we will describe later that part atoms are able to accommodate much more than eight electrons.

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

This will certainly not readjust the variety of electrons ~ above the terminal atoms.

7. Final check

always make sure all valence electrons room accounted for and each atom has actually an octet the electrons other than for hydrogen (with 2 electrons).

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

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

Example \(\PageIndex1\): Water

Write the Lewis framework for H2O.


Steps for composing Lewis Structures

example \(\PageIndex1\)
1. Determine the total number of valence electrons in the molecule or ion.

Each H atom (group 1) has actually 1 valence electron, and the O atom (group 16) has 6 valence electrons, because that a full of 8 valence electrons.

2. Arrange the atom to show certain connections.


Because H atoms are practically always terminal, the plan within the molecule should be HOH.

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

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

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

H -O- H

with 4 electron left over.

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

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

Adding the staying 4 electron to the oxygen (as 2 lone pairs) gives the adhering to structure:

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since carbon is much less electronegative than oxygen and hydrogen is typically terminal, C need to be the central atom.

3. Location a bonding pair that electrons between each pair of adjacent atoms to offer a solitary bond.

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


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7. Final check

Both the oxygen and the carbon now have an octet that electrons, therefore this is an acceptable Lewis electron structure. The O has two bonding pairs and two lone pairs, and also C has four bonding pairs. This is the framework of formaldehyde, which is supplied in embalming fluid.

Exercise \(\PageIndex1\)

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

Answer CO2




Answer SCl2




Writing Lewis structures for Polyatomic Ions

Recall that a polyatomic ion is a group of atoms that space covalently bonded together and which lug an overall electrical charge. The ammonium ion, \(\ceNH_4^+\), is created when a hydrogen ion \(\left( \ceH^+ \right)\) attaches to the lone pair of an ammonia \(\left( \ceNH_3 \right)\) molecule in a coordinate covalent bond.


Exceptions come the Octet Rule

As important and useful together the octet dominance is in lifwynnfoundation.orgical bonding, there room some renowned violations. This go not typical that the octet preeminence is useless—quite the contrary. As with many rules, there space exceptions, or violations.

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


Although the O atom has actually an octet that electrons, the N atom has actually only 7 electrons in that is valence shell. Back NO is a stable compound, it is an extremely lifwynnfoundation.orgically reactive, together are many other odd-electron compounds.

Electron-deficient molecules stand for the second violation come the octet rule. These steady compounds have actually less than eight electrons approximately an atom in the molecule. The most typical examples room the covalent link of beryllium and boron. Because that example, beryllium can type two covalent bonds, causing only 4 electrons in the valence shell:

how to find the total number of valence electrons in a compound