GEOMETRIA MOLECULAR | Resumo de Química para o Enem

Summary

In this lesson on molecular geometry, the focus is on understanding the simplest and most common molecular shapes encountered in entrance exams. Key topics include the impact of ligands and electron pairs on geometry, with practical examples like ammonia (NH₃). The script explains various geometries such as linear, trigonal planar, pyramidal, and tetrahedral, as well as their associated bond angles. The lesson emphasizes the importance of counting ligands and considering electron pairs to determine molecular shape. A table is provided to simplify the process, and students are encouraged to explore further resources and simulations to deepen their understanding.

Takeaways

• 😀 The lesson covers molecular geometry, an important topic for entrance exams.
• 😀 Understanding covalent bonds is crucial before diving into molecular geometry.
• 😀 The number of ligands (atoms bonded to the central atom) is essential in determining the molecular geometry.
• 😀 A molecule with five atoms has four ligands: one central atom and the other bonded atoms.
• 😀 If there is only one ligand, the geometry will always be linear.
• 😀 With two ligands and no electron pairs, the geometry is also linear.
• 😀 If there are two ligands and one pair of electrons left on the central atom, the geometry will be bent due to electron pair repulsion.
• 😀 With three ligands and no electron pairs, the geometry is trigonal planar.
• 😀 If three ligands are present and there is one pair of electrons, the geometry becomes pyramidal.
• 😀 For four ligands and no electron pairs, the geometry will be tetrahedral. Bond angles and electron pair repulsion affect the shape.

Q & A

What is molecular geometry?

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, which helps predict the shape and reactivity of molecules.

How can we determine the geometry of a molecule?

The geometry of a molecule can be determined by counting the number of ligands (atoms bonded to the central atom) and the number of electron pairs left on the central atom.

What is the significance of electron pairs in determining molecular geometry?

Electron pairs on the central atom influence the geometry by pushing the bonds apart, affecting the shape of the molecule. They count as bonds but don’t appear in the final geometry.

What is the shape of a molecule with two ligands and no electron pairs?

A molecule with two ligands and no electron pairs will have a linear geometry with bond angles of 180°.

What is the geometry of a molecule with three ligands and one pair of electrons?

A molecule with three ligands and one pair of electrons will have a pyramidal geometry, where the electron pair pushes the bonds downward.

How does a lone pair affect molecular geometry?

A lone pair of electrons on the central atom will push the bonded atoms closer together, altering bond angles and changing the molecule's shape. This is particularly seen in geometries like pyramidal.

What is the geometry of a molecule with four ligands and no lone pairs?

A molecule with four ligands and no lone pairs will have a tetrahedral geometry, with bond angles of 109.5°.

How do bond angles vary based on the geometry of the molecule?

Bond angles vary depending on the geometry: linear (180°), trigonal planar (120°), pyramidal (109.5°), and tetrahedral (109.5°). The presence of lone pairs can cause these angles to decrease slightly.

What is the bond angle in a trigonal planar geometry?

In a trigonal planar geometry, the bond angles are typically 120°.

Why are tetrahedral and pyramidal geometries associated with bond angles of 109.5°?

Both tetrahedral and pyramidal geometries have bond angles of 109.5° because of the spatial arrangement of the ligands and electron pairs around the central atom, though lone pairs in pyramidal geometries can slightly alter this.