Name: molecular shapes from electron pairs
Uploaded: 2025-05-22T09:05:32.000Z
Duration: 2 min 37 s
Channel: JoVE

価電子殻電子対反発(VSEPR)理論は、単結合、多重結合、または孤立電子対に関与する電子団が互いに反発し、互いに可能な限り最大の距離に留まろうとすると仮定しています。 
分子形状は、中心原子の周りのさまざまな電子団の配置によって決定されます。
分子形状には、2つの結合電子群に線形、3つに三方平面、4つに四面体、5つに三角錐形、6つに八面体という5つの基本的な分子形状があります。
分子形状を予測する際には、孤立電子対-孤立電子対の反発力が、孤立電子結合対および結合対-結合対の反発力よりも大きいことを覚えておいてください。
メタン、アンモニア、水のルイス構造を考えてみましょう。それぞれにおいて、中心原子は4つの電子基に囲まれています。
メタンでは、4つの結合電子対が4面体に配置され、理想的なH-C-H角は109.5°です。
アンモニアでは、窒素原子には3つの結合対と1つの孤立電子対があります。
電子の孤立電子対は、結合対よりも大きなスペースを占めます。これは、孤立電子対が1つの原子核にのみ結合しているのに対し、結合電子グループは2つの原子核によって共有されているためです。
H-N-H結合角は、メタンで観察される予想される四面体角の109.5°よりも小さくなります。この結合角の圧縮は、隣接する結合電子基に孤立電子対が及ぼす反発力に起因します。
電子対の配置は、電子対幾何学と呼ばれます。分子形状は原子の配置を表し、電子対の形状とは異なります。アンモニアの電子対形状は四面体ですが、分子形状は三角錐形です。
水分子は、中心原子の周りにも4つの電子団を持っています。電子対の形状も四面体であり、2つの結合電子基と2つの孤立電子対があります。
2つの孤立電子対によって加えられるより大きな反発力は、水分子のH-O-H結合角をさらに圧縮します。理想的な四面体結合角よりもはるかに小さく、分子形状が曲がっています。

vsepr理論

価電子殻電子対反発理論 (VSEPR 理論) を活用すると、ルイス構造式内の結合電子対と孤立電子対の数を調べることで、中心原子の周囲の分子構造を予測できます。 VSEPR モデルは、中心原子の価電子殻内の電子対が、電子対間の距離を最大化することで電子対間の反発を最小限に抑える配置を採用すると仮定しています。 中心原子の価殻内の電子は、主に結合原子間に位置する結合電子対、または孤立電子対のいずれかを形成します。
分子内の 2 つの電子密度領域は、反発を最小限に抑えるために、中心原子の反対側で直線的に配向されています。 同様に、3 つの電子グループは平面三角形に配置され、4 つの電子グループは四面体を形成し、5 つの電子グループは平面三角形をとり、6 つの電子グループは八面体に配向します。
原子中心部の周囲におけるの電子対の幾何学配置は、必ずしも分子構造と一致するとは限らない点にに注意することが重要です。 電子対の幾何学的配置は、分子内、結合内、および孤立電子対内に電子が位置するすべての領域を記述します。一方、分子構造は電子ではなく、分子内の原子の位置を表します。 したがって、電子対の幾何学形状は、中心原子の周囲に孤立電子対がない場合にのみ分子構造と同じになります。
孤立電子対は結合電子対よりも大きな空間を占めます。 これは、孤立電子対は 1 つの原子核のみに結合するのに対し、結合電子対は 2 つの原子核で共有されるためです。 したがって、孤立電子対-孤立電子対の反発は、孤立電子対および結合電子対間の反発、または結合電子対同士の反発よりも大きくなります。
VSEPR 理論によれば、末端原子の位置は、線形、三角平面、四面体、および八面体の電子対構造において同等です。 したがって、どの位置も単一の孤立電子対によって占有される可能性があります。 しかし、三方両錐形では、2 つの軸方向の位置は 3 つの赤道方向の位置とは異なります。 ここで、赤道位置は結合角が 120° であるため利用可能なスペースが多く、より大きな孤立電子対はこの位置を優先的に占有します。 同様に、2 つの孤立電子対と 4 つの結合電子対が中心原子の周りに八面体状に配置されている場合、2 つの孤立電子対は 180°離れており、正方形の平面分子構造になります。
このテキストは下記から翻案されます<a href="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">Openstax, Chemistry 2e, Section 7.6 Molecular Structure and Polarity</a>.

VSEPR理論

価電子殻電子対反発理論(VSEPR理論)は、中心原子の周りの分子構造を予測できる理論です。

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Covalent Bonding and Structure

共有結合と構造

KEY TERMS AND CONCEPTS

主要用語と概念

VSEPR Theory

molecular shapes from electron pairs

Molecular shape can be predicted from the way electron pairs arrange around a central atom. Valence shell electron-pair repulsion theory, or VSEPR theory, uses a Lewis structure to count bonding pairs and lone electron pairs. The main idea is simple: electron pairs spread out as far as possible to reduce repulsion.

In a molecule, the valence electrons around the central atom can be bonding pairs or lone pairs. Bonding pairs are found mainly between bonded atoms. Lone pairs stay on one atom and take up more space than bonding pairs because they are held by only one nucleus.

The number of electron groups helps determine the electron-pair geometry. Two regions of electron density arrange in a linear shape. Three electron groups form a trigonal planar geometry, four form a tetrahedron, five prefer a trigonal bipyramidal geometry, and six arrange in an octahedral pattern.

Electron-pair geometry is not always the same as molecular structure. Electron-pair geometry includes both bonds and lone pairs. Molecular structure shows only the positions of the atoms, so the two match only when the central atom has no lone pairs.

Lone pair repulsions are not equal to bonding pair repulsions. Lone pair–lone pair repulsions are strongest. Lone pair–bonding pair repulsions are next, and bonding pair–bonding pair repulsions are weakest.

In some geometries, the terminal positions are equivalent. This is true for linear, trigonal planar, tetrahedral, and octahedral electron-pair geometries, so a lone pair can occupy any one of those positions. In a trigonal bipyramidal geometry, the two axial positions are different from the three equatorial positions. The equatorial position has more space because of the 120° bond angles, so larger lone pairs prefer it.

A square planar molecular structure can also form in an octahedral arrangement. When two lone pairs and four bonding pairs are placed octahedrally around a central atom, the lone pairs sit 180° apart. That arrangement leaves the atoms in a square planar shape.

Molecular Shapes from Electron Pairs

what is organic chemistry?

Carbon Compounds in Everyday Life

Organic chemistry focuses on carbon compounds called organic compounds. These compounds may come from living things or be made by chemists. Carbon is the main element in them, and it bonds with other carbon atoms as well as hydrogen, oxygen, nitrogen, and sulfur. Carbon can form up to four strong bonds. That ability lets carbon atoms build chains and rings of many sizes, shapes, and levels of complexity. Not every carbon-containing substance is classified as organic. Carbonates, cyanides, and...

electronic structure of atoms

Atomic Orbitals and Quantum Numbers

Atomic orbitals and quantum numbers describe how electrons are arranged around the nucleus. An atom has a small, positively charged nucleus with electrons surrounding it. The nucleus contains protons and neutrons, and the quantum mechanical model gives a better picture of this structure than older models. Protons carry a +1 charge and have a mass of 1.0073 amu. Neutrons have no charge and have a mass of 1.0087 amu. Electrons carry a −1 charge and have a mass of 0.00055 amu. The number of...

electron configurations

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Electron filling in atoms follows a set order based on orbital energy. The Aufbau principle says each new electron enters the lowest-energy subshell available. The Pauli exclusion principle says no two electrons can share the same set of four quantum numbers. Hund’s rule of maximum multiplicity says electrons stay unpaired in equal-energy orbitals when possible. The order of filling follows the relative energies of the subshells. A common sequence is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.

chemical bonds

Chemical Bonds: Types and Strength

Chemical bonds help atoms reach a full valence shell like the nearest noble gas. The main bond types are ionic, covalent, and metallic. Bond energy and bond length also help explain how strong a bond is. An ionic bond forms from electrostatic attraction between cations and anions. In many cases, one atom transfers electrons to another atom, and the ions form from that transfer. Ionic bonds have no defined direction because the attraction spreads evenly through three-dimensional space. A...

polar covalent bonds

Electronegativity in Polar Covalent Bonds

Polar covalent bonds form when atoms share electrons unevenly. This happens in covalent bonds between atoms that have similar, but not identical, tendencies to attract electrons. Nonmetal atoms often make these bonds with other nonmetals. Hydrogen gas, H2, is a simple example of a covalent bond between two hydrogen atoms. A covalent bond begins when two atoms move close enough for their valence orbitals to overlap. In hydrogen, the 1s orbitals overlap and the single electrons on each atom...

lewis structures and formal charges

Lewis Structures, Bonds, and Formal Charge

Lewis structures show how atoms share electrons in covalent bonds and in polyatomic ions. Lewis symbols help draw these structures, and the periodic table helps predict how many valence electrons an atom has and how many bonds it can form to reach an octet. Group 18 elements, such as argon and helium, already have filled electron configurations. They rarely take part in chemical bonding. Atoms in group 17, such as bromine and iodine, need one more electron to reach an octet, so they can form...

vsepr theory

Molecular shape can be predicted from the way electron pairs arrange around a central atom. Valence shell electron-pair repulsion theory, or VSEPR theory, uses a Lewis structure to count bonding pairs and lone electron pairs. The main idea is simple: electron pairs spread out as far as possible to reduce repulsion. In a molecule, the valence electrons around the central atom can be bonding pairs or lone pairs. Bonding pairs are found mainly between bonded atoms. Lone pairs stay on one atom and...

molecular geometry and dipole moments

Predicting Shape and Molecular Polarity

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resonance and hybrid structures

Resonance in Nitrite and Carbonate Ions

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valence bond theory and hybridized orbitals

valence-bond-theory-and-hybridized-orbitals

Covalent Bond Overlap and Orbital Hybridization

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mo theory and covalent bonding

Bond Order and Molecular Orbitals

Molecular orbital theory explains how electrons are arranged in molecules. It treats electron regions in a molecule much like atomic orbitals in an atom. A molecular orbital is the region of space where a valence electron is most likely to be found. Molecular orbitals form through the linear combination of atomic orbitals, or LCAO. When atomic wave functions combine in phase, they create regions with high electron density. When they combine out of phase, they create nodes, or regions with no...

intermolecular forces and physical properties

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How Intermolecular Forces Affect Matter

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solubility

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introduction to functional groups

Functional Groups and Organic Molecules

Functional groups are groups of atoms with specific chemical properties that appear within organic molecules. They are sometimes shown as "R." These groups sit along the carbon backbone of macromolecules, which can form chains or rings of carbon atoms. Functional groups help functionalize a compound. That means they can give the molecule different physical and chemical properties. In organic chemistry, this change in group structure can strongly affect how a compound behaves. Common...

overview of advanced functional groups

Advanced Functional Groups in Organic Chemistry

Advanced functional groups are atom clusters in organic molecules that give those molecules specific chemical properties. They are sometimes shown as "R" in formulas. By changing the atoms attached to a carbon skeleton, a functional group can make a compound behave differently in physical and chemical reactions. This topic covers several major classes of organic compounds with advanced functional groups. Aromatic hydrocarbons include benzene. Phenol is another example and is also called...

key terms and concepts

Learn how VSEPR theory uses bonding pairs and lone pairs to predict molecular shape, electron-pair geometry, and common arrangements like linear and octahedral.

Molecular Shapes from Electron Pairs | JoVE K12

organic chemistry

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Molecular Shapes from Electron Pairs

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