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Arrows that say where electrons go

Science · Chemistry · ages 18-19

Name ______________________ Date ____________

What does a curved arrow with a one-barbed fishhook head represent?
- Two electrons in a lone pair or bond
- A whole nucleus during a rearrangement
- One electron in a radical step
Where must the tail of an electron-pair curved arrow start?
- At any atom with a formal charge
- At a lone pair or a bond
- At the product side of the step
A curved electron-flow arrow shows the path of an atom migrating from one place to another.
Circle one: True False
A proposed product gives one carbon five ordinary two-electron bonds. What is the electron-count problem?
- A positive formal charge permits all five full bonds
- Five bonds are allowed whenever the product is reactive
- Ten bonding electrons exceed carbon's octet
An oxygen atom has three single bonds and one lone pair in a Lewis structure. Oxygen has six valence electrons. What is its formal charge?
- +1
- -1
- 0
An H-Cl bond breaks to give H+ and Cl-. Both bonding electrons go to chlorine. Where should the pair-arrow head land?
- On hydrogen, which loses its share of the pair
- On chlorine, which receives the bond pair
- At the midpoint, keeping the pair in the broken bond
A complete proposed step has total formal charge 0 before and -1 after. No electrons enter or leave the system. What does this tell you?
- Bond changes can create extra negative charge without electrons
- A neutral starting system may end with any total charge
- The proposed step or its charge labels need correction
Hydroxide attacks the carbon of H2C=O. A proposed step draws only an arrow from a hydroxide lone pair to carbon. Which additional pair arrow keeps carbon within its octet?
- From the C=O pi bond onto oxygen
- From a C-H sigma bond onto the same carbon
- From an oxygen lone pair into another C-O bond

Answer key

For grown-ups. Fold this page away before handing over the rest.

Arrows that say where electrons go W1-mt_C0bq2Mu8HT-s1

One electron in a radical step · A fishhook represents one electron. A two-barbed curved arrowhead represents an electron pair.
At a lone pair or a bond · The tail identifies the electron pair being reassigned. A formal-charge sign alone does not identify that pair.
False · The arrow represents electrons, not an atom's path. Atoms can still move and change bonds during a reaction.
Ten bonding electrons exceed carbon's octet · Five ordinary two-electron bonds put ten electrons around carbon. That product drawing exceeds carbon's octet.
+1 · The lone pair contains two electrons. Three single bonds contain six bonding electrons. Formal charge is 6 - 2 - 3 = +1.
On chlorine, which receives the bond pair · Chlorine receives both electrons from the H-Cl bond. The tail starts at that bond and the head points to chlorine.
The proposed step or its charge labels need correction · With all species included and no electron exchange, total charge must stay the same. The mismatch does not identify a particular faulty arrow.
From the C=O pi bond onto oxygen · Move the carbonyl pi pair onto oxygen as the new C-O bond forms. Carbon then has four single bonds and eight bonding electrons.