Chemistry: Lewis Diagrams & the Octet Rule

What is a Lewis diagram?

a simple model showing an atom's valence electrons as dots around its symbol

Who created Lewis diagrams?

the chemist Gilbert N. Lewis

Which electrons does a Lewis diagram show?

only the valence electrons, the ones in the outermost shell

Does a Lewis diagram show core electrons?

no, though they are still there filling the lower energy levels

Why are Lewis diagrams useful?

they are a shorthand that highlights the valence electrons involved in reactions

What are the steps to draw a Lewis diagram?

write the symbol, count the valence electrons, then place them as dots around the four sides

How are the dots arranged in a Lewis diagram?

around the four sides of the symbol, pairing up so there are at most four pairs

What is the maximum number of dots on a Lewis diagram?

eight, since there are four sides and the dots pair up

Why is eight the maximum number of dots?

four sides each hold a pair, which gives four pairs or eight electrons

Which group is an exception with four unpaired dots?

group 14, whose four valence electrons stay unpaired

What does the number of dots in a Lewis diagram tell you?

the element's number of valence electrons, which matches its group number

How does the dot count change across a period?

each element has one more dot than the one before it

How does the dot count change down a group?

it stays the same

For groups 13 to 18, how do you find valence electrons from the group number?

subtract 10 from the group number

How many valence electrons does a group 16 element have?

six, which is 16 minus 10

What is the octet rule?

atoms tend to gain, lose, or share electrons to end up with eight valence electrons

Why do atoms follow the octet rule?

a full outer shell of eight is a stable, low-energy arrangement

Which group already has a full octet and so is unreactive?

group 18, the noble gases

What is the duet rule?

helium is stable with just two valence electrons, because its shell only holds two

Why is helium an exception to the octet rule?

its valence electrons are in the first shell, which holds only two

How does an atom decide whether to gain or lose electrons?

it takes whichever path to a full octet needs less energy

An atom with six valence electrons: does it gain or lose to reach an octet?

it gains two, since that is easier than losing six

An atom with two valence electrons: does it gain or lose to reach an octet?

it loses two, revealing the full shell below

How do you draw a Lewis diagram for an anion?

add the gained electrons, draw brackets around it, and put the negative charge outside

How do you draw a Lewis diagram for a cation?

show no dots, because the lost electrons leave a full shell below that is not redrawn

Why does a cation's Lewis diagram have no dots?

it lost its valence electrons, and the new full shell underneath is not redrawn

How many valence electrons do metals usually have?

one, two, or three

Do metals gain or lose electrons, and what ion do they form?

they lose electrons and form cations

How many valence electrons do nonmetals usually have?

five, six, or seven

Do nonmetals gain or lose electrons, and what ion do they form?

they gain electrons and form anions

What charge do group 1, 2, and 13 metals tend to form?

1 plus, 2 plus, and 3 plus

What charge do group 15, 16, and 17 nonmetals tend to form?

3 minus, 2 minus, and 1 minus

Why do group 14 elements usually not form ions?

gaining or losing four electrons takes too much energy, so they share instead

How do group 14 elements like carbon reach a full shell?

by sharing electrons in covalent bonds

Where do these ion-charge trends break down?

for transition metals, which have variable charges, and atoms with expanded octets