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

Теория VSEPR помогает определять геометрию электронных пар и молекулярную геометрию.
Ряд этапов используется для прогнозирования геометрии и углов связи молекул, таких как трихлорид фосфора.
Первым шагом является рисование структуры молекулы Льюиса.
Далее подсчитайте общее количество электронных групп на центральном атоме. Вокруг фосфора существуют четыре электронные группы: три связующие пары и одна одинокая пара.
Теперь определим геометрию электронной пары. Геометрия электронных пар тетраэдрическая. Однако из-за одиночной пары молекулярная геометрия является тригональной пирамидальной. Одинокая пара уменьшает угол соединения до менее чем 109,5°.
Связывающие электронные пары не всегда поровну распределяются между двумя связывающими атомами.
В ковалентной связи, подобной той, что есть у фтористоводородной кислоты, электроны притягиваются к более электроотрицательному атому, на что указывает частичный заряд. Такие связи называются полярными связями.
Разделение зарядов создает вектор, называемый дипольным моментом связи, который обозначается греческой буквой μ. Его величина является произведением величины частичных зарядов на расстояние между ними.
Дипольные моменты обычно выражаются в дебаях. Один дебай равен 3,336 × 10−30 кулонов-метров.
Вектор указывает от меньшего к более электроотрицательному атому и указывает на дипольный момент связи. Его длина пропорциональна величине разности электроотрицательностей между двумя атомами. Большинство двухатомных молекул, содержащих атомы различных элементов, обладают дипольными моментами и поэтому являются полярными молекулами.
В многоатомных соединениях суммарный дипольный момент определяется индивидуальными дипольными моментами связи и геометрией соединения.
Рассмотрим молекулу воды с двумя полярными связями. Он имеет изогнутую форму и представляет собой полярную молекулу.
В отличие от этого, молекула углекислого газа является линейной. Две углерод-кислородные связи полярны, но ориентированы в противоположных направлениях, компенсируя дипольный момент друг друга и делая общую молекулу неполярной.

Молекулярная геометрия и дипольные моменты

Теорию ОЭПВО можно использовать для определения геометрии электронных пар и молекулярных структур следующим образом:


<ol style="display: flex; flex-direction: column; gap: 10px;">
 <li>Изобразим структуру Льюиса молекулы или многоатомного иона.
 </li>
 <li>Подсчитайте количество электронных групп (неподеленных пар и связей) вокруг центрального атома. Одинарная, двойная или тройная связь считается одной областью электронной плотности.
 </li>
 <li>Определите геометрию электронной пары по числу электронных групп.
 </li>
 <li>Используйте количество неподеленных пар, чтобы определить молекулярную структуру. Если возможно более одного расположения неподеленных пар и химических связей, выберите то, которое сведет к минимуму отталкивания.
 </li>
</ol>

Дипольный момент молекулы


Когда атомы с разной электроотрицательностью образуют связь, электроны притягиваются к более электроотрицательному атому, оставляя один атом с частичным положительным зарядом (δ+), а другой атом с частичным отрицательным зарядом (δ–). Такие связи называются полярными ковалентными связями, и разделение заряда приводит к возникновению дипольного момента связи. Величина дипольного момента связи обозначается греческой буквой µ и определяется как:


µ = Qr

где Q — величина частичных зарядов (определяемая разностью электроотрицательностей), а r — расстояние между ними. Дипольные моменты обычно выражаются в дебаях, где один дебай равен 3,336 × 10−30 C·m.


Дипольный момент связи представляет собой вектор, представленный стрелкой, направленной вдоль связи от менее электроотрицательного к более электроотрицательному атому, с небольшим знаком плюс на менее электроотрицательном конце.


Целая молекула также может иметь разделение заряда в зависимости от ее молекулярной структуры и полярности каждой из ее связей. Такие молекулы называются полярными. Дипольный момент измеряет степень общего разделения зарядов в молекуле в целом. В двухатомных молекулах дипольный момент связи определяет полярность молекулы.


Когда молекула содержит более одной связи, необходимо учитывать геометрию. Если связи в молекуле расположены так, что векторная сумма их моментов связей равна нулю, то молекула неполярна (например, CO2). Молекула воды имеет изогнутую молекулярную структуру, и два момента связи не компенсируются. Следовательно, вода представляет собой полярную молекулу с чистым дипольным

Теория VSEPR может быть использована для определения геометрии электронных пар и молекулярных структур следующим образом:

Напишите структуру молекулы Льюиса или...

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Covalent Bonding and Structure

Ковалентная связь и структура

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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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...

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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...

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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

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polar covalent bonds

Electronegativity in Polar Covalent Bonds

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Lewis Structures, Bonds, and Formal Charge

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vsepr theory

Molecular Shapes from Electron Pairs

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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...

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intermolecular forces and physical properties

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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

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Predicting Shape and Molecular Polarity

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