What is Tetrahedral Shape?
The tetrahedral shape is formed when four atoms in their elemental form covalently bond together. The word "tetra" means "four," and the word "hedral" represents a solid face. When we combine the definitions of these two terms, we learn that tetrahedral refers to a solid with four faces.
The study of different atoms that combine with one primary atom at the centre by bonding and forming a specific physical structure is known as molecular geometry. A three-dimensional molecule's molecular structure does not change rapidly and is found in nature in the same orientations. Tetrahedral geometry is common in molecules, and it has different bond angles. This article will explain what the tetrahedral shape of a molecule is and what compounds exist in the tetrahedral shape.
Tetrahedral Shape
Tetrahedral Molecular Geometry
A central atom is located in the centre of a tetrahedral molecular geometry, with four substituents located at the corners of the tetrahedron. ${{109.5}^{o}{cos}^{-1}{}{(-⅓)}}$, are the bond angles. When all four substituents are the same and the tetrahedron is complete, it belongs to the point group ${{T}_{d}}$. Saturated carbon and silicon compounds exhibit this chemical geometry. Other molecules and ions with this geometry include the xenon tetroxide molecule ${{Xe}{O}_{4}}$, the perchlorate ion ${{Cl}{O}^{4-}}$, the sulphate ion ${{S}{O}_{4}^{2-}}$, the phosphate ion ${{P}{O}_{4}^{3-}}$, and tetrakis (triphenylphosphine) palladium.
Possible Shapes of Tetrahedron
0 Lone Pairs
This molecule is composed of four evenly spaced ${{sp}^{3}}$ hybrid orbitals with bond angles of ${{109.5}^{o}}$. The orbitals have a tetrahedral pattern. Since each orbital has an atom at the end, the molecule has a tetrahedral structure.
1 Lone Pairs
These have trigonal pyramidal molecular geometries and are of the form ${{A}{X}_{3}{E}}$. A trigonal pyramidal structure is formed when three bonds and one lone pair occur on the central atom of the molecule. ${{sp}^{3}}$ hybridisation occurs at the centre atom in molecules with tetrahedral electron pair geometries. Ammonia $\left( NH_3 \right)$ is a pyramidal trigonal molecule.
2 Lone Pairs
These have the shape ${{A}{X}_{2}{E}_{2}}$ and curved angles, as seen in the case of water. This molecule is composed of four uniformly spaced ${{sp}^{3}}$ hybrid orbitals that produce bond angles of approximately ${{109.5}^{o}}$, which is close to ${{104.5}^{o}}$. The orbitals have a tetrahedral pattern. In two of the orbitals, lone electron pairs exist. The formula for compounds with this molecular shape will be ${{A}{X}_{4}}$.
Tetrahedral Structure
The VSEPR theory heavily influences tetrahedral molecular geometry. VSEPR stands for valence-shell electron-pair repulsion; a theory that predicts molecule shape based on electron interactions in atoms' outer, or valence, shells. When the four substituent atoms are transition metals rather than the typical elements studied in organic chemistry (for example, hydrogen, oxygen, or nitrogen), the molecule takes on a square planar shape rather than the traditional tetrahedral molecular geometry. The central atom and four substituents are located on the same plane in the square planar shape, with the substituents representing each corner of the square.
Examples of Tetrahedral Molecular Geometry
Tetrahedral molecular structures can be found in a wide range of molecules, the most common of which are methane ${{(}{C}{H}_{4}{)}}$, silane ${{(}{Si}{H}_{4}{)}}$, and thiazyl trifluoride ${{(}{NS}{F}_{3}{)}}$. Tetrahedral structures are shared by the phosphate ion ${{(}{P}{O}_{4}{)}^{3-}}$, the sulphate ion ${{(}{S}{O}_{4}{)}^{2-}}$, and the perchlorate ion ${{(}{Cl}{O}_{4}{)}^{-}}$.
Important Questions
1. What is the symmetry of a tetrahedral molecule?
Ans. A regular tetrahedron has 12 rotational (or orientation-preserving) symmetries and a symmetry order of 24 when transformations that combine a reflection and a rotation are considered. Since there is exactly one such symmetry for each permutation of the vertices of the tetrahedron, the group of all (not necessarily orientation preserving) symmetries is isomorphic to the group ${{S}_{4}}$, the symmetric group of permutations of four objects. The set of orientation-preserving symmetries is known as the alternating subgroup ${{A}_{4}}$ of ${{S}_{4}}$.
2. Are tetrahedral and linear shaped molecules always nonpolar?
Ans. No. Polarity is caused by a difference in electronegativity between the ends/sides/points of a molecule. It may not be very polar in some cases, but it will be polar nonetheless. Consider the linear case, such as HCN or CO. Consider the tetrahedral case of ${{C}{H}_{3}{Cl}}$. Of course, there are many more examples in each category; in fact, there are far fewer cases of purely non-polar substances in each of these categories than polar substances.
Key Features
The tetrahedral shape is formed when four atoms in their elemental form covalently bond together.
The word "tetrahedral" gives us a good idea of what this term means. The word "tetra" means "four," and the word "hedral" represents a solid face.
A central atom is located in the centre of a tetrahedral molecular geometry, with four substituents located at the corners of the tetrahedron.
Multiple Choice Questions
1. Which of the following molecules has tetrahedral geometry?
(a) ${{Si}{H}_{2}{Br}_{2}}$
(b) ${{Kr}{Cl}_{2}{F}_{2}}$
(c) ${{P}{Cl}_{5}}$
(d) ${{S}{F}_{4}}$
Answer: (a)
2. The atoms in a molecule of water adopt what kind of geometry?
(a) Trigonal Planar
(b) Linear shape
(c) Tetrahedral
(d) Octahedral
Answer: (c)
FAQs on Tetrahedral Shape
1. What does the term 'tetrahedral' mean in the context of molecular geometry?
In chemistry, the term 'tetrahedral' describes a specific three-dimensional arrangement of atoms in a molecule. The name comes from a tetrahedron, which is a geometric solid with four faces. A tetrahedral molecular geometry features a central atom bonded to four other atoms (substituents), which are positioned at the corners of the tetrahedron to minimise repulsion between them.
2. What is the ideal bond angle in a perfect tetrahedral shape and why is it 109.5°?
The ideal bond angle in a perfect tetrahedral molecule is 109.5 degrees. This specific angle arises from the need to position four electron pairs (in bonding orbitals) around a central atom as far apart as possible in three-dimensional space. According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, this 109.5° angle represents the maximum separation, which minimises the electrostatic repulsion between the electron pairs, leading to the most stable arrangement.
3. What are some common examples of molecules that exhibit a tetrahedral shape?
Many common molecules have a tetrahedral geometry. The most classic example is methane (CH₄). Other important examples include:
- Silane (SiH₄)
- Carbon tetrachloride (CCl₄)
- Ions like the sulfate ion (SO₄²⁻) and the phosphate ion (PO₄³⁻)
4. What type of atomic orbital hybridization leads to a tetrahedral geometry?
A tetrahedral geometry is typically associated with sp³ hybridization. In this process, one 's' atomic orbital and three 'p' atomic orbitals of the central atom mix to form four new, equivalent hybrid orbitals. These four sp³ hybrid orbitals arrange themselves in a tetrahedral shape around the nucleus to be as far apart as possible, ready to form sigma bonds with other atoms.
5. Why is the shape of a water (H₂O) molecule described as bent, not tetrahedral, even though its central oxygen atom is sp³ hybridized?
This is a key distinction between electron geometry and molecular geometry. The oxygen atom in H₂O is indeed sp³ hybridized, leading to four electron pairs arranged in a tetrahedral electron geometry. However, two of these pairs are bonding pairs (with hydrogen atoms) and two are non-bonding lone pairs. Since molecular shape only describes the position of the atoms, we only 'see' the arrangement of the oxygen and two hydrogen atoms. The lone pairs exert a stronger repulsion than bonding pairs, pushing the two O-H bonds closer together and resulting in a bent or V-shaped molecular geometry with a bond angle of approximately 104.5°.
6. How does the presence of lone pairs on the central atom affect the ideal tetrahedral geometry?
The presence of lone pairs on the central atom significantly distorts the ideal tetrahedral shape. While the electron pairs maintain a roughly tetrahedral arrangement, the molecular shape changes. According to VSEPR theory:
- One lone pair: Results in a trigonal pyramidal shape, as seen in ammonia (NH₃). The lone pair repels the bonding pairs, compressing the bond angles to about 107°.
- Two lone pairs: Results in a bent or V-shape, as seen in water (H₂O). The two lone pairs create even greater repulsion, reducing the bond angle further to about 104.5°.
7. Are all molecules with a tetrahedral geometry nonpolar?
No, not all tetrahedral molecules are nonpolar. A molecule's polarity depends on both its shape and the polarity of its bonds. For a tetrahedral molecule to be nonpolar, all four substituents bonded to the central atom must be identical, as in methane (CH₄) or carbon tetrachloride (CCl₄). In these cases, the individual bond dipoles are symmetrical and cancel each other out. However, if the substituents are different, like in chloromethane (CH₃Cl), the bond dipoles do not cancel, and the molecule has an overall net dipole moment, making it polar.
8. What is the difference between electron geometry and molecular geometry for a molecule like ammonia (NH₃)?
For ammonia (NH₃), the central nitrogen atom has four electron pairs in its valence shell (three bonding pairs with hydrogen and one lone pair).
- The electron geometry describes the arrangement of all electron pairs (both bonding and lone pairs). Since there are four pairs, their arrangement is tetrahedral to minimise repulsion.
- The molecular geometry (or shape) describes the arrangement of only the atoms. Since one position is occupied by a non-bonding lone pair, the shape formed by the nitrogen and three hydrogen atoms is trigonal pyramidal.
