Name: predicting shape and molecular polarity
Uploaded: 2023-04-30T18:28:31.000Z
Duration: 2 min 36 s
Channel: JoVE

VSEPR理論は、電子対の形状と分子の形状を決定するのに役立ちます。
一連のステップを使用して、三塩化リンなどの分子の形状と結合角を予測します。
最初のステップは、分子のルイス構造を描くことです。
次に、中心原子の電子団の総数を数えます。リンの周りには、3つの結合対と1つの孤立電子対の4つの電子基があります。
次に、電子対の形状を決定します。電子対の形状は四面体です。ただし、孤立電子対のため、分子形状は三角錐形です。孤立電子ペアは、ボンディング角度を109.5°未満に縮小します。
結合電子対は、2つの結合原子間で常に等しく共有されるわけではありません。
フッ化水素酸のような共有結合では、電子はより電気陰性な原子に向かって引っ張られ、部分的な電荷で示されます。このような結合は極性結合と呼ばれます。
電荷分離により、結合双極子モーメントと呼ばれるベクトルが生成され、ギリシャ文字のμで示されます。その値は、部分電荷の大きさとそれらの間の距離の積です。
双極子モーメントは、通常、デバイで表されます。1回のデバイは、10-30クーロンメートル×3.336に相当します。
ベクトルは、電気陰性度の低い原子から電気陰性度の高い原子を指し、結合双極子モーメントを示します。その長さは、2つの原子間の電気陰性度の差の大きさに比例します。異なる元素の原子を含むほとんどの二原子分子は双極子モーメントを持っているため、極性分子です。
多原子化合物では、正味の双極子モーメントは、化合物の個々の結合双極子モーメントと形状によって決まります。
2つの極性結合を持つ水分子を考えてみましょう。曲がった形状をしており、極性分子です。
対照的に、二酸化炭素分子は線形です。2つの炭素-酸素結合は極性がありますが、反対方向に配向しているため、互いの双極子モーメントが打ち消され、分子全体が無極性になります。

分子の幾何学と双極子モーメント

VSEPR 理論は、次のように電子対の幾何学的配置と分子構造を決定するために使用できます。
1. 分子または多原子イオンのルイス構造を書きます。
2. 中心原子の周囲の電子グループ (孤立電子対と結合) の数を数えます。 単結合、二重結合、または三重結合は、電子密度の 1 つの領域としてカウントされます。
3. 電子群の数に基づいて電子対の幾何学的配置を特定します。
4. 孤立電子対の数を利用して分子構造を決定します。 孤立電子対と化学結合の配置が複数ある可能性がある場合は、反発を最小限に抑えるものを選択します。
分子の双極子モーメント
異なる電気陰性度を持つ原子が結合を形成すると、電子はより電気陰性度の高い原子に引き寄せられ、一方の原子は部分的に正電荷 (δ+) を持ち、もう一方の原子は部分的に負電荷 (δ-) を持ちます。 このような結合は極性共有結合と呼ばれ、電荷の分離により結合双極子モーメントが生じます。 結合双極子モーメントの大きさはギリシャ文字 µ で表され、次の式で与えられます。
μ = Qr
ここで、Q は部分電荷の大きさ (電気陰性度の差によって決まります)、r は部分電荷間の距離です。 双極子モーメントは通常デバイで表され、1 デバイは 3.336 × 10^−30 C·m に等しくなります。
結合双極子モーメントは、結合に沿って電気陰性度の低い原子から電気陰性度の高い原子に向かう矢印で表されるベクトルで、電気陰性度の低い原子側に小さなプラス記号が付いています。
分子構造と各結合の極性に応じて、分子全体でも電荷が分離される場合があります。 このような分子は極性があると言われます。 双極子モーメントは、分子全体における正味の電荷分離の程度を測定します。 二原子分子では、結合双極子モーメントが分子の極性を決定します。
分子に複数の結合が含まれる場合、幾何学的配置を考慮する必要があります。 分子内の結合が、その結合モーメントのベクトル和がゼロに等しくなるように配置されている場合、その分子は無極性です (例: CO_2)。 水分子は折れ線形の分子構造をしており、2 つの結合モーメントは相殺されません。 したがって、水は正味の双極子モーメントを持つ極性分子です。

VSEPR理論は、次のように電子ペアの形状と分子構造を決定するために使用できます。

分子のルイス構造を書くか...

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

Molecular Geometry and Dipole Moments

predicting shape and molecular polarity

Molecular shape and molecular polarity can be predicted with VSEPR theory and dipole moments. Start by writing the Lewis structure of the molecule or polyatomic ion. Then count the electron groups around the central atom. Lone pairs and bonds both count as electron groups, and a single, double, or triple bond each counts as one region of electron density.

Use that count to identify the electron-pair geometry. Next, use the number of lone pairs to determine the molecular structure. If more than one arrangement of lone pairs and bonds is possible, choose the one with the fewest repulsions. This step helps show how atoms arrange themselves in space.

Bond polarity depends on electronegativity, which is how strongly an atom pulls shared electrons. When atoms with different electronegativities form a bond, the electrons are pulled toward the more electronegative atom. That leaves one atom with a partial positive charge, written as δ+, and the other with a partial negative charge, written as δ−. These are polar covalent bonds.

The charge separation in a polar bond creates a bond dipole moment. Its magnitude is written with the Greek letter μ and follows the relation μ = Qr, where Q is the size of the partial charges and r is the distance between them. Dipole moments are usually measured in debyes. One debye equals 3.336 × 10^−30 C·m. A bond dipole is shown as an arrow pointing from the less electronegative atom toward the more electronegative atom, with a small plus sign on the less electronegative end.

A whole molecule can also have a dipole moment. That depends on both the polarity of its bonds and its molecular shape. In diatomic molecules, the bond dipole determines the molecular polarity. In molecules with more than one bond, the bond dipoles must be added as vectors. If the vector sum is zero, the molecule is nonpolar, as in CO2. Water has a bent molecular structure, so its bond moments do not cancel. As a result, water is a polar molecule with a net dipole moment.

Predicting Shape and Molecular Polarity

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

How Electrons Fill Atomic Orbitals

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 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...

molecular geometry and dipole moments

Molecular shape and molecular polarity can be predicted with VSEPR theory and dipole moments. Start by writing the Lewis structure of the molecule or polyatomic ion. Then count the electron groups around the central atom. Lone pairs and bonds both count as electron groups, and a single, double, or triple bond each counts as one region of electron density. Use that count to identify the electron-pair geometry. Next, use the number of lone pairs to determine the molecular structure. If more than...

resonance and hybrid structures

Resonance in Nitrite and Carbonate Ions

Resonance explains why some Lewis structures do not match the measured properties of a molecule or ion. When two or more Lewis structures have the same atom arrangement, the real electron distribution is an average of the structures shown. The nitrite anion, NO2−, is one example. It can be drawn in two ways, with the N–O and N=O bonds in different places. If nitrite had one single bond and one double bond, the two bond lengths should be different, because a double bond is shorter and stronger...

valence bond theory and hybridized orbitals

valence-bond-theory-and-hybridized-orbitals

Covalent Bond Overlap and Orbital Hybridization

Covalent bonds form when orbitals on two atoms overlap and their single electrons pair up. The bond is stronger when the overlap is greater. Maximum overlap happens when the orbitals meet along a direct line between the two nuclei. A sigma bond, or σ bond, is the kind of covalent bond found in a single bond in a Lewis structure. In a σ bond, electron density is concentrated along the internuclear axis. A pi bond, or π bond, forms when two p orbitals overlap side by side. In a π bond, the...

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

intermolecular-forces-and-physical-properties

How Intermolecular Forces Affect Matter

Intermolecular forces are the attractions between molecules, and they help explain how matter behaves. These forces influence melting points, boiling points, and whether substances mix with one another. Water has a much higher boiling point than hexane, which shows that water has stronger intermolecular forces. Three main kinds of intermolecular interactions are covered here. They include ion-dipole forces, dipole-dipole interactions, and van der Waals forces, which include London dispersion...

solubility

Why Some Substances Dissolve in Water

Solution formation depends on how solutes and solvents interact. A solution is a homogeneous mixture made of a solvent, which is the larger part, and a solute, which is the smaller part. The physical state of a solution is usually the same as the solvent. Concentration is often described as dilute for a low amount of solute or concentrated for a high amount of solute. When a solute dissolves, its particles are surrounded by solvent species. This process is called solvation. If water is the...

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 predicts molecular shape, how bond polarity forms, and why some molecules like CO2 are nonpolar while water is polar.

Predicting Shape and Molecular Polarity | JoVE K12

organic chemistry

JoVE Core 有機化学では、立体化学、反応、メカニズムの基本概念を、簡潔でわかりやすいアニメーション ビデオ レッスンを使用して説明します。さらに、科学者の実際のビデオでは、世界中の今日の研究室で行われている古典的および独自の研究実験における関連概念の適用を示しています。

Predicting Shape and Molecular Polarity

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