A-Level课程证书被几乎所有英语授课的大学作为招收新生的入学标准。在中国开设A-Level课程旨在为中国学生提供进入国外大学的有效途径，具体目标为：培养在国内初高中成绩优秀的学生进入世界顶尖大学;培养在国内初高中成绩中等的学生进入世界一流大学;培养在国内初高中成绩一般的学生考取适合自己的大学。大部分英国学生都是用两年的时间修完这种课程，但能力很强的学生有时也可在更短的时间内修完。学生甚至可以直接在国内自学三到四门A-Level课程然后去北京、上海、广州等地英国文化委员会参加考试。这种课程要求学生学习三门或四门主科课程并参加毕业考试，考试合格者即可进入大学就读。学生的考试成绩及其所选修的A-Level课程在很大程度上决定着能否进入理想的大学和学习所选择的学位课程。为了方便同学们的A-Level备考，课窝考试网小编为大家收集整理了A-Level考试的相关信息，供大家学习参考，希望能为同学们的考试提供帮助。
Revision: Structure and Bonding
Ionic bonding
Ionic bonds are a type of chemical bond based on electrostatic forces between two oppositely-charged ions. In ionic bond formation, a metal donates an electron, due to a low electronegativity to form a positive ion or cation. Often ionic bonds form between metals and non-metals. The non-metal atom has an electron configuration just short of a noble gas structure. They have high electronegativity, and so readily gain electrons to form negative ions or anions. The two or more ions are then attracted to each other by electrostatic forces. Such bonds are stronger than hydrogen bonds, but similar in strength to covalent bonds.
Ionic bonding occurs only if the overall energy change for the reaction is favorable when the bonded atoms have a lower energy than the free ones. The larger the resulting energy change the stronger the bond.
Pure ionic bonding is not known to exist. All ionic bonds have a degree of covalent bonding or metallic bonding. The larger the difference in electronegativity between two atoms, the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water.
Covalent Bonding
Covalent bonding is a description form of chemical bonding that is characterized by the sharing of one or more electrons between two atoms. In general bonds are defined by a mutual attraction that holds the resultant molecule together. Often bonding occurs in such a way that the the outer electron shells of the participating atoms becomes filled. Such bonds are always stronger than the intermolecular hydrogen bond and similar in strength to or stronger than the ionic bond.
In contrast to the ionic and metallic bond, the covalent bond is directional, i.e. the bond angles have a great impact on the strength of the interaction. This impact arises because covalent bonds are formed by the overlap of atomic orbital, with greater overlap producing a greater strength of interaction. Atomic orbital all have highly directional character, resulting in a highly directionally-dependent interactions in bonding.
Covalent bonding most frequently occurs between atoms with similar electronegativities. For this reason, non-metals tend to engage in covalent bonding more readily since metals have access to metallic bonding.
Metallic bonding
Metallic bonding is the bonding within metals. It involves the delocalized sharing of free electrons among a lattice of metal atoms. Thus, metallic bonds may be compared to molten salts.
Metal atoms typically contain a high number of electrons in their valence shell compared to their period or energy level. These become delocalized and form a sea of electrons surrounding a giant lattice of positive ions. The surrounding electrons and the positive ions in the metal have a strong attractive force between them. This means that more energy is required to negate these forces. Therefore metals often have high melting or boiling points. The principle is similar to that of ionic bonds.
译文：
修订：结构和粘接
离子键是基于两个相反电荷离子之间的静电力的一种化学键。在离子键形成过程中，金属由于电负性低而形成正离子或阳离子，从而提供电子。通常在金属和非金属之间形成离子键。非金属原子的电子结构刚好不及惰性气体结构。它们具有高电负性，因此容易获得电子以形成负离子或阴离子。然后两个或更多个离子通过静电力相互吸引。这种键比氢键强，但与共价键相似。
只有当键合原子比自由基的能量低时，反应的整体能量变化才有利，离子键才会发生。产生的能量越大，债券就越强。
未知纯离子键存在。所有离子键都具有一定程度的共价键或金属键。两个原子之间的电负性差异越大，键的离子越多。离子化合物在熔化或溶解时导电。它们通常具有高熔点并倾向于可溶于水。
共价键是化学键的描述形式，其特征在于两个原子之间共享一个或多个电子。一般而言，债券是由相互吸引的因素共同决定的。键合通常以这样的方式发生，即参与原子的外部电子壳被填充。这样的键总是比分子间氢键更强并且与离子键的强度相似或比离子键强。
与离子和金属键相反，共价键是定向的，即键角对相互作用的强度有很大的影响。这种影响的产生是因为共价键是由原子轨道的重叠而形成的，更大的重叠产生了更大的相互作用强度。原子轨道都具有高度方向性，导致键合中高度定向依赖的相互作用。
共价键最常发生在具有类似电负性的原子之间。由于这个原因，非金属趋向于更容易地进行共价键合，因为金属可以获得金属键合。
金属结合是金属内的结合。它涉及金属原子晶格间自由电子的离域共享。因此，可以将金属键与熔盐进行比较。
与其周期或能量水平相比，金属原子在其价层中通常包含大量电子。它们变得非本地化，并形成围绕正离子巨大晶格的电子海。周围的电子和金属中的正离子在它们之间有很强的吸引力。这意味着需要更多的能量来抵消这些力量。因此金属通常具有高熔点或沸点。原理与离子键相似。
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为了方便同学们的A-Level备考，课窝考试网小编为大家收集整理了A-Level考试的相关信息，供大家学习参考，希望能为同学们的考试提供帮助。
A-LEVEL在线试听课程
