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# 化学代写|无机化学代考Inorganic Chemistry代写|CHEM220 Bond length

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## 化学代写|无机化学代考Inorganic Chemistry代写|Bond length

Key points: The equilibrium bond length in a molecule is the separation of the centres of the two bonded atoms; covalent radii vary through the periodic table in much the same way as metallic and ionic radii.

The equilibrium bond length in a molecule is the distance between the centres of the two bonded atoms. A wealth of useful and accurate information about bond lengths is available in the literature, most of it obtained by X-ray diffraction on solids (Section 8.1). Equilibrium bond lengths of molecules in the gas phase are usually determined by infrared or microwave spectroscopy, or more directly by electron diffraction. Some typical values are given in Table $2.6$.

To a reasonable first approximation, equilibrium bond lengths can be partitioned into contributions from each atom of the bonded pair. The contribution of an atom to a covalent bond is called the covalent radius of the element (19). We can use the covalent radii in Table $2.7$ to predict, for example, that the length of a $\mathrm{P}-\mathrm{N}$ bond is $110 \mathrm{pm}+74 \mathrm{pm}=184 \mathrm{pm}$; experimentally, this bond length is close to $180 \mathrm{pm}$ in a number of compounds. Experimental bond lengths should be used whenever possible, but covalent radii are useful for making cautious estimates when experimental data are not available.

Covalent radii vary through the periodic table in much the same way as metallic and ionic radii (Section 1.7a), for the same reasons, and are smallest close to F. Covalent radii are approximately equal to the separation of nuclei when the cores of the two atoms are in contact: the valence electrons draw the two atoms together until the repulsion between the cores starts to dominate. A covalent radius expresses the closeness of approach of bonded atoms; the closeness of approach of nonbonded atoms in neighbouring molecules that are in contact is expressed in terms of the van der Waals radius of the element, which is the internuclear separation when the valence shells of the two atoms are in nonbonding contact (20). van der Waals radii are of paramount importance for understanding the packing of molecular compounds in crystals, the conformations adopted by small but flexible molecules, and the shapes of biological macromolecules (Chapter 27).

## 化学代写|无机化学代考Inorganic Chemistry代写|Bond strength

Key points: The strength of a bond is measured by its dissociation enthalpy; mean bond enthalpies are used to make estimates of reaction enthalpies.

A convenient thermodynamic measure of the strength of an AB bond is the bond dissociation enthalpy, $\Delta H^{\ominus}(\mathrm{A}-\mathrm{B})$, the standard reaction enthalpy for the process
$$\mathrm{AB}(\mathrm{g}) \rightarrow \mathrm{A}(\mathrm{g})+\mathrm{B}(\mathrm{g})$$
Bond dissociation enthalpy is always positive as energy is required to break bonds. The mean bond enthalpy, $B$, is the average bond dissociation enthalpy taken over a series of A-B bonds in different molecules (Table 2.8).

Mean bond enthalpies can be used to estimate reaction enthalpies. However, thermodynamic data on actual species should be used whenever possible in preference to mean values because the latter can be misleading. For instance, the $\mathrm{Si}-\mathrm{Si}$ bond enthalpy ranges from $226 \mathrm{~kJ} \mathrm{~mol}^{-1}$ in $\mathrm{Si}{2} \mathrm{H}{6}$ to $322 \mathrm{~kJ} \mathrm{~mol}^{-1}$ in $\mathrm{Si}{2}\left(\mathrm{CH}{3}\right)_{6}$. The values in Table $2.8$ are best considered as data of last resort: they may be used to make rough estimates of reaction enthalpies when enthalpies of formation or actual bond enthalpies are unavailable.

## 化学代写|无机化学代考Inorganic Chemistry代写|Bond strength

$\mathrm{AB}$ 键强度的一个方便的热力学测量是键解禽焓, $\Delta H^{\ominus}(\mathrm{A}-\mathrm{B})$, 过程的标准反应焓
$$\mathrm{AB}(\mathrm{g}) \rightarrow \mathrm{A}(\mathrm{g})+\mathrm{B}(\mathrm{g})$$

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## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。