Alh3 Lewis Structure,Geometry:9 Facts You Should Know

In the present article we are going to discuss about the fascinating structure of Aluminium Hydride . AlH3 is among those molecules which do not follow the octet rule.

The AlH3 Lewis structure consist of Aluminium atom as the central atom and three Hydrogen atoms are present around the aluminium atom. The Lewis structure of AlH3 molecule contains 6 bonding electrons and no non-bonding electrons are present on it.

                      Table: Details of the AlH3 Structure

Molecular NameAluminium HydrideChemical FormulaAlH3Central atom of the lewis structureAluminiumNumber of lone pairs on central atom 0Molecular Geometry of AlH3Trigonal PlanarElectron Geometry of AlH3Trigonal PlanarBond Angle H-Al-H120 degreesNo. of valence electron for AlH36
The formal charge of AlH3 molecule0
Hybridization of AlH3 moleculeSp2

How to draw Lewis dot structure for AlH3?

Following are the steps to follow to draw the Lewis structure of AlH3 molecule –

 Step 1: Count total valence electrons present in AlH3 molecule

First step is to find the no. of valence electrons present in AlH3 molecule.

Aluminium belongs to group 13th and Hydrogen belongs to group 1st of the periodic table.

So,

Valence electron of Aluminium =3

Valence electron of Hydrogen = 1

       Valence ElectronsAtom In AlH3Total Valence Electrons 
Al            3          1            3*1 =3
H            1          3            1*3 =3
                         6

Step 2: Find central atom in AlH3

Always keep the least electronegative atom in the center. When comparing aluminium and hydrogen,hydrogen is less electronegative atom than aluminium But in accordance with the rule, we will have to hydrogen outside. Now draw the skeletal structure of AlH3 molecule

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Description automatically generatedskeletal structure of AlH3

Step 3: Put two electrons or a bond between Al and H atom

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Description automatically generatedBonds between Al and H atoms

Step 4: Complete octet of the Outer atoms (or duplet) in case of hydrogen atom.

Main group elements have tendency to attain octet configuration of their nearest noble gas element present in the 18 group. This is known as Octet Rule. Except hydrogen which completes duplet configuration like He

Complete the Duplet on outside hydrogen atoms then move on to central atom.

Now, all the six electrons of AlH3 are used in the structure and no more electron pairs remained.

Hence no change in the above Lewis Dot structure of AlH3

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Description automatically generatedLewis dot structure Of AlH3

                              Step 5: Check formal charge value

Before we confirm that this is a perfect Lewis structure, we need to check the formal charge values

Formal charge = Valence electrons – (1/2) * bonding electrons – non-bonding electrons

Valence electrons of Aluminium = 3

Non-bonding electrons of Aluminium = 0

Bonding electrons of Oxygen = 6

Formal charge for Aluminium atom = 3 – 1/2*6 –0 = 0

Valence electrons of hydrogen = 1

Non-bonding electrons of hydrogen = 0

Bonding electrons of hydrogen = 2

Formal charge for hydrogen atom = 1 – 1/2*2 – 0 = 0

As the Al and H elements have least possible formal charges, we have obtained suitable Lewis structure

                            

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Description automatically generatedAlH3 Lewis structure

                                              

What is the formal charge in AlH3 and how it is calculated?

Formal charge is the charge present on an atom in a molecule when all the bonding electrons are shared equally or when the electronegativity difference of atoms is ignored.

Formal charge is calculated by using the formula –

Formal charge = Valence electrons – (1/2) * bonding electrons – non-bonding electrons

Valence electrons of Aluminium = 3

Non-bonding electrons of Aluminium = 0

Bonding electrons of Aluminium =6

Formal charge for Aluminium atom = 3 – 1/2*6 – 0 = 0

Valence electrons of hydrogen = 1

Non-bonding electrons of Hydrogen = 0

Bonding electrons of hydrogen = 2

Formal charge for hydrogen atom =1 – 1/2*2 – 0 = 0

Thus, formal charge on Aluminium and hydrogen atom in AlH3 Lewis dot structure is zero.

Does AlH3 molecule follow the octet rule?

AlH3 has only 6 valence electrons as can be seen from the Lewis structure of AlH3 molecule. This proves that octet rule is not followed by the AlH3 molecule.

What is the molecular geometry and electron geometry of AlH3 molecule?

In order to find the geometry (molecular/electron) of AlH3 molecule, the three steps have to be followed. They are –

1)Find the number of lone pairs present on the central atom in the Lewis structure of AlH3 molecule

Lone Pair =1/2*(V.E. –N.A.)

Where –

V.E.= valence electron on the central atom

N.A.= Number of atoms attached to that central atom

In AlH3 molecule, the valence electrons of central Aluminium atom is 3 and three outer atoms are attached to it.

Lone Pairs = 1/2* (3 – 3) = 0

2) Find the hybridization number of AlH3 molecule

Hybridization of central atom can be found out by using the formula.

Hybridization number = N.A. + L.P

 Where N.A.= number of atoms attached to the central atom

L.P. = number of lone pairs on the central atom

There are 3 atoms attached to the central atom Aluminium and there are no lone pairs present on it.

 Hybridization number = 3 + 0 = 3

Hybridization number of AlH3 molecule is 3

hence hybridization of AlH3 molecule is Sp2

3) Use VSEPR Theory to determine molecular/electron geometry of AlH3

We have determined hybridization of AlH3 as Sp3 and lone pairs 0

Now, according to AXnEx notation of the VSEPR theory we are going to find VSEPR notation for AlH3 molecule

AXnEx notation

where,

 A: central atom

  X: number of atoms attached to the central atom

  E: number of lone pairs of electrons on the central atom

As per Lewis dot structure of Oxygen difluoride, Oxygen is the central atom which has 2 lone pairs of electrons and 2 fluorine atoms are connected to it.

So AlH3 formula becomes AX3

According to VSEPR chart, the molecule with AX3 formula has molecular shape as Trigonal planar and electron geometry Trigonal planar.

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Description automatically generatedTrigonal Planar geometry of AlH3
Total DomainsGeneral FormulaBonded atomsLone PairsMolecular ShapeElectron Geometry
1AX10LinearLinear
2AX220LinearLinear
 AXE11LinearLinear
3AX330Trigonal PlanarTrigonal Planar
 AX2E21BentTrigonal Planar
 AXE212LinearTrigonal Planar
4AX440TetrahedralTetrahedral
 AX3E31Trigonal PyramidTetrahedral
 AX2E222BentTetrahedral
 AXE313LinearTetrahedral

                                             VSEPR CHART

What is the Bond Angle of AlH3 molecule?

The Bond Angle of AlH3 is 120 degrees as there are no lone pairs present on the

 central atom Aluminium. The bonds are equally placed from each other.

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Description automatically generatedBond angle of AlH3

                                           

What is the Hybridization for AlH3 molecule?

The central atom Aluminium atom shares 3 electrons with 3 hydrogen atoms.

To share the electrons with the hydrogen atoms, the orbitals of Aluminium atom undergo hybridization to accommodate the electrons.

The electronic configuration of Aluminium is –

Aluminium in ground state: [Ne] 3s2 3p1

Aluminium in excited state: [Ne] 3s1 3px1 3py1

Steric Number = Number of atoms bonded to central atom + Number of lone pairs of electrons attached to central atom

Steric Number = 3 + 0 = 3

So, hybridization for the central atom oxygen in AlH3 is Sp2

Does AlH3 molecule shows resonance?

A molecule can have resonance if –

The molecule has alternate single and double bond.The molecule has presence of lone pair in conjugation with the double bond.

Aluminium Hydride does not have a double bond so it does not show resonance.

Solubility of Aluminium Hydride:

Aluminium hydride reacts with water and form hydrogen gas. The hydrogen gas produced may catch fire due to the heat generated in the reaction.

AlH3 is soluble in ether and reacts with C2H5OH.

Conclusion:

The hybridisation of AlH3 molecule is Sp2.Both molecular geometry and electron geometry of AlH3 molecule is Trigonal Planar. The bond angle of AlH3 molecule is 120 degrees.