SeF4 Lewis Structure
Selenium tetrafluoride · A fluorinating agent, and the selenium analogue of sulfur tetrafluoride.
The SeF4 Lewis structure has 34 valence electrons, drawn as four Se–F bonds, with 13 lone pairs in total. The central Se is sp3d hybridized, which makes it a seesaw molecule with a bond angle of under 90 and under 120 degrees. SeF4 is polar: the Se-F bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
- 34 valence e−
- seesaw
- <90° and <120° bond angle
- sp3d
- polar
SeF4 molecular geometry
Seesaw, from 5 electron domains on the central Se: 4 bonded atoms and 1 lone pair (AX4E1). The electron geometry is trigonal bipyramidal; the shape is what is left when the lone pairs are not counted as atoms. Wedges point toward you, hashed wedges away. Every shape in the VSEPR chart →
Calculated properties
| Total valence electrons | 34 |
|---|---|
| Bonding electrons | 8 (4 shared pairs) |
| Nonbonding electrons | 26 (13 lone pairs) |
| Lone pairs on Se | 1 |
| Electron domains (steric number) | 5 |
| VSEPR notation | AX4E1 |
| Electron geometry | trigonal bipyramidal |
| Molecular geometry | seesaw |
| Bond angle | <90° and <120° |
| Hybridization | sp3d |
| Polarity | polar |
| Formal charges | all zero |
| Resonance structures | none |
| Molar mass | 154.963 g/mol |
Se Selenium 6 valence e− F Fluorine 4 atoms, 7 valence e− each
How to draw the SeF4 Lewis structure
Step through it. The bar shows how much of the 34-electron budget is spent at each stage - it never grows, which is the whole constraint.
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Skeleton. Join every connected pair with one bond. 4 bonds spends 8 electrons.
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Outer octets. Spend electrons as lone pairs on the outer atoms until each has eight. 2 are still unplaced.
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Centre. Everything still unplaced goes on Se, which now has 10 electrons around it.
The reasoning at each step
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Count the valence electrons
Add up the outer-shell electrons every atom brings. That total is the budget for the whole structure - every line and every dot has to come out of it, and nothing may be added.
6 (Se) + 4 x 7 (F) = 34 valence electrons -
Choose the central atom
Se is the least electronegative atom here, so it takes the centre. Hydrogen is never central: it can only form one bond.
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Join everything with single bonds first
Every connection starts as one shared pair. Draw them all before worrying about double bonds - the arithmetic in the next step is what tells you where the double bonds have to go.
4 bonds x 2 = 8 electrons used, 26 left -
Work out how far short you are
Add up what every atom still needs to fill its shell - eight electrons for most atoms, two for hydrogen - minus what its single bonds already give it. Compare that with the electrons you have left. The difference decides everything that follows.
needs 24, has 26, 2 to spare -
Park the spare electrons on the central atom
There are 2 electrons more than the outer atoms can use. They become 1 extra lone pair on Se, which is in period 4 and can hold more than eight electrons. A period-2 atom could not do this.
10 electrons around Se -
Fill in the lone pairs
Every electron not in a bond sits as a lone pair on an atom. Each atom takes exactly what it needs to finish its shell - there is no choice left at this point.
Se: 1, F1: 3, F2: 3, F3: 3, F4: 3 (26 electrons) -
Check the central atom
Se ends up with 10 electrons, an expanded octet. Only period 3 and below can do this.
Se: 8 bonding + 2 nonbonding = 10 -
Check the formal charges
Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. Every atom here comes out at zero, which is the sign of a good structure.
all formal charges = 0 -
Work out the shape
Count the electron domains on Se: 4 bonded groups plus 1 lone pair. A double or triple bond still counts as one domain, because it points in one direction. That gives trigonal bipyramidal electron geometry; ignore the lone pairs and the atoms themselves sit in a seesaw arrangement. The lone pair sits in an equatorial position, where it has the most room, and pushes the other bonds away.
steric number 5 -> sp3d -> seesaw, bond angle under 90 and under 120 -
Decide whether it is polar
The Se-F bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
dipoles do not cancel -> polar
Is SeF4 polar or nonpolar?
SeF4 is polar. The Se-F bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
| Bond | Electronegativity difference | Character |
|---|---|---|
| Se–F | 1.43 | polar covalent |
Common questions
How many valence electrons does SeF4 have?
SeF4 has 34 valence electrons. 8 of them are in bonds and 26 sit in lone pairs.
What is the molecular geometry of SeF4?
SeF4 is a seesaw molecule. The central Se has 4 bonded groups and one lone pair, a steric number of 5, which gives trigonal bipyramidal electron geometry and a seesaw molecule.
What is the bond angle in SeF4?
The bond angle in SeF4 is under 90 and under 120 degrees. The lone pair sits in an equatorial position, where it has the most room, and pushes the other bonds away.
What is the hybridization of SeF4?
The central Se in SeF4 is sp3d hybridized. Steric number 5 means 5 orbitals have to be mixed, which is exactly what sp3d gives you.
Is SeF4 polar or nonpolar?
SeF4 is polar. The Se-F bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
Why does SeF4 break the octet rule?
The central atom has more than four bonded groups, so it has to hold more than eight electrons. Elements from period 3 downward can do this; period-2 elements like carbon, nitrogen and oxygen cannot.