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

تساعد نظرية VSEPR في تحديد هندسة زوج الإلكترون والهندسة الجزيئية.
يتم استخدام سلسلة من الخطوات للتنبؤ بالهندسة وزوايا الرابطة للجزيئات ، مثل ثلاثي كلوريد الفوسفور.
الخطوة الأولى هي رسم بنية لويس للجزيء.
بعد ذلك ، احسب العدد الإجمالي لمجموعات الإلكترونات على الذرة المركزية. حول الفوسفور ، هناك أربع مجموعات إلكترونية: ثلاثة أزواج رابطة وزوج واحد وحيد.
الآن ، حدد هندسة زوج الإلكترون. هندسة زوج الإلكترون رباعية السطوح. ومع ذلك ، بسبب الزوج الوحيد ، فإن الهندسة الجزيئية هرمية مثلثية. يقلل الزوج الوحيد من زاوية الرابطة إلى أقل من 109.5 درجة.
لا يتم دائما مشاركة أزواج الإلكترونات المترابطة بالتساوي بين ذرتي الترابط.
في رابطة تساهمية مثل رابطة حمض الهيدروفلوريك ، يتم سحب الإلكترونات نحو الذرة الأكثر كهربية ، ويشار إليها بشحنة جزئية. تسمى هذه الروابط بالروابط القطبية.
يخلق فصل الشحنة متجها يسمى عزم ثنائي القطب الرابطة ، والذي يشار إليه بالحرف اليوناني μ. قيمته هي نتاج حجم الشحنات الجزئية والمسافة بينهما.
عادة ما يتم التعبير عن لحظات ثنائية القطب في debyes. ديباي واحد يساوي 3.336 × 10-30 كولوم متر.
يشير المتجه من الذرة الأقل إلى الذرة الأكثر كهربية ويشير إلى عزم ثنائي القطب للرابطة. يتناسب طوله مع حجم فرق الكهربية بين الذرتين. معظم الجزيئات ثنائية الذرة التي تحتوي على ذرات من عناصر مختلفة لها لحظات ثنائية القطب وبالتالي فهي جزيئات قطبية.
في المركبات متعددة الذرات ، يتم تحديد صافي عزم ثنائي القطب من خلال عزم ثنائي القطب للرابطة الفردية وهندسة المركب.
ضع في اعتبارك جزيء ماء له رابطتان قطبيتان. لها شكل منحني وهي جزيء قطبي.
في المقابل ، جزيء ثاني أكسيد الكربون خطي. الرابطتان بين الكربون والأكسجين قطبية ولكنهما موجهتان في اتجاهين متعاكسين ، مما يلغي عزم ثنائي القطب لبعضهما البعض ويجعل الجزيء الكلي غير قطبي.

الهندسة الجزيئية وعزوم ثنائي القطب

<div style="direction:rtl">يمكن استخدام نظرية VSEPR لتحديد الأشكال الهندسية لأزواج الإلكترونات والهياكل الجزيئية على النحو التالي: <ol> <li style="text-align: right;">اكتب بنية لويس للجزيء أو الأيون متعدد الذرات.</li> <li style="text-align: right;">احسب عدد مجموعات الإلكترون (الأزواج الوحيدة والروابط) حول الذرة المركزية. تعتبر الرابطة الفردية أو المزدوجة أو الثلاثية منطقة واحدة من كثافة الإلكترون.</li> <li style="text-align: right;">حدّد هندسة زوج الإلكترون بناءً على عدد مجموعات الإلكترون.</li> <li style="text-align: right;">استخدم عدد الأزواج المنفردة لتحديد التركيب الجزيئي. إذا كان من الممكن وجود أكثر من ترتيب واحد للأزواج المنفردة والروابط الكيميائية، فاختر الترتيب الذي يقلل من عمليات التنافر.</li> </ol> <h4>عزم ثنائي القطب للجزيء</h4> عندما تشكل ذرات ذات كهروسالبية مختلفة رابطة، يتم سحب الإلكترونات نحو الذرة الأكثر كهروسالبية، مما يترك ذرة واحدة بشحنة موجبة جزئية (δ+) والذرة الأخرى بشحنة سالبة جزئية (δ–). تسمى هذه الروابط بالروابط التساهمية القطبية، ويؤدي فصل الشحنات إلى ظهور عزم ثنائي القطب للرابطة. يتم تمثيل مقدار عزم ثنائي القطب الرابطة بالحرف اليوناني µ ويعطى بواسطة:  μ = Qr حيث Q هو حجم الشحنات الجزئية (التي يحددها فرق الكهروسالبية)، وr هي المسافة بينهما. يتم التعبير عن لحظات ثنائي القطب عادة بوحدات ديباي، حيث يساوي ديباي الواحد 3.336 × 10−30 C·m. عزم ثنائي القطب للرابطة هو متجه يمثله سهم يشير على طول الرابطة من الذرة الأقل كهروسالبية نحو الذرة الأكثر كهروسالبية، مع وجود علامة زائد صغيرة على الطرف الأقل كهروسالبية. قد يكون للجزيء بأكمله أيضًا شحنة منفصلة، اعتمادًا على تركيبه الجزيئي وقطبية كل رابط من روابطه. ويقال إن مثل هذه الجزيئات قطبية. تقيس لحظة ثنائي القطب مدى فصل الشحنة الصافية في الجزيء ككل. في الجزيئات ثنائية الذرة، يحدد عزم ثنائي القطب الرابطة القطبية الجزيئية. عندما يحتوي الجزيء على أكثر من رابطة واحدة، يجب أن يُؤخذ الشكل الهندسي في الاعتبار. إذا كانت الروابط في الجزيء مرتبة بحيث يكون المجموع المتجه لعزوم روابطها يساوي الصفر، فإن الجزيء يكون غير قطبي (e.g., CO2). يحتوي جزيء الماء على بنية جزيئية منحنية، ولا يتم إلغاء لحظات الرابطة. ولذلك، الماء هو جزيء قطبي مع عزم ثنائي القطب الصافي. تم اقتباس هذا النص من <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>.</div>

يمكن استخدام نظرية VSEPR لتحديد هندسة زوج الإلكترون والهياكل الجزيئية على النحو التالي:

اكتب بنية لويس للجزيء أو ...

Education

JoVE Core

Organic Chemistry

الكيمياء العضوية

Unit 1

الوحدة الأولى

Covalent Bonding and Structure

الترابط التساهمي والبنية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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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Chemical Bonds: Types and Strength

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intermolecular forces and physical properties

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Predicting Shape and Molecular Polarity

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