Draw the Lewis Dot structure for Aluminum hydride AlH3. The geometry is: trigonal planar trigonal bipyramidal trigonal pyramidal tetrahedral bent None of the above
The Correct Answer and Explanation is:
Lewis Dot Structure for Aluminum Hydride (AlH₃):
To draw the Lewis structure of AlH₃, follow these steps:
Step 1: Count the valence electrons
- Aluminum (Al) is in group 13 and has 3 valence electrons.
- Each Hydrogen (H) has 1 valence electron.
- Total valence electrons = 3 (from Al) + 3 × 1 (from H) = 6 valence electrons.
Step 2: Determine the central atom
Aluminum is less electronegative than hydrogen and is placed at the center.
Step 3: Distribute the electrons
Connect each hydrogen atom to aluminum using a single bond.
Each single bond uses 2 electrons, and we have 3 bonds, so 6 electrons are used.
This accounts for all available valence electrons.
Lewis Structure:
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H — Al — H
Each bond represents a pair of electrons. Aluminum forms three bonds with hydrogen atoms.
Step 4: Check the octet rule
- Hydrogen only needs 2 electrons to be stable. Each hydrogen has 2 electrons from its single bond.
- Aluminum has 6 electrons (3 bonds), not a full octet (8 electrons).
However, aluminum is an exception to the octet rule. In many of its compounds like AlH₃ and AlCl₃, aluminum is stable with only 6 electrons in its valence shell.
Geometry of AlH₃:
The molecular geometry is trigonal planar. This is because aluminum forms three regions of electron density (three Al–H bonds) with no lone pairs on the central atom. According to VSEPR theory, three electron domains arrange themselves in a planar triangle to minimize repulsion.
Explanation
Aluminum hydride (AlH₃) is a covalent compound composed of one aluminum atom bonded to three hydrogen atoms. To understand its bonding and structure, we analyze it using the Lewis dot structure and Valence Shell Electron Pair Repulsion (VSEPR) theory.
First, we determine the total number of valence electrons. Aluminum belongs to group 13 in the periodic table and thus has three valence electrons. Each hydrogen atom contributes one valence electron, giving a total of six electrons to work with. In the Lewis structure, aluminum is the central atom because it is less electronegative than hydrogen. We form three single bonds between aluminum and each hydrogen atom, using all six valence electrons.
In this structure, each hydrogen is surrounded by two electrons, fulfilling the duet rule, which is appropriate for hydrogen. Aluminum has only six electrons around it, which may seem insufficient by the octet rule. However, aluminum is one of the elements that frequently forms compounds in which it does not achieve a full octet. It is stable with six valence electrons in AlH₃.
With three bonding pairs and no lone pairs on the central aluminum atom, the molecule adopts a geometry that minimizes electron pair repulsion. According to VSEPR theory, three bonding pairs arrange themselves 120 degrees apart in a plane to minimize repulsion, resulting in a trigonal planar geometry.
AlH₃ does not have lone pairs that would push the bonds out of plane. Therefore, other geometries like trigonal pyramidal, tetrahedral, or bent are incorrect. Trigonal bipyramidal applies to molecules with five regions of electron density, which does not match AlH₃. Thus, the correct geometry for AlH₃ is trigonal planar.
