Sodium Chloride, NaCl

The classic case of ionic bonding, the sodium chloride molecule forms by the ionization of sodium and chlorine atoms and the attraction of the resulting ions.

离子键的经典例子是氯化钠分子，它由钠和氯原子的离子化以及所产生的离子之间的吸引力形成。

An atom of sodium has one 3s electron outside a closed shell, and it takes only 5.14 electron volts of energy to remove that electron. The chlorine lacks one electron to fill a shell, and releases 3.62 eV when it acquires that electron (it's electron affinity is 3.62 eV). This means that it takes only 1.52 eV of energy to donate one of the sodium electrons to chlorine when they are far apart. When the resultant ions are brought closer together, their electric potential energy becomes more and more negative, reaching -1.52 eV at about 0.94 nm separation. This means that if neutral sodium and chlorine atoms found themselves closer than 0.94 nm, it would be energetically favorable to transfer an electron from Na to Cl and form the ionic bond.

钠原子有一个3s电子处于闭壳层之外，仅需5.14电子伏特的能量即可移除该电子。氯原子缺少一个电子以填满壳层，当它获得该电子时释放3.62 eV（其电子亲和能为3.62 eV）。这意味着当它们相距很远时，将钠的一个电子转移给氯原子仅需1.52 eV的能量。当形成的离子被带得更近时，它们的电势能变得越来越负，达到约0.94纳米分离距离时为-1.52 eV。这表明如果中性钠和氯原子之间的距离小于0.94纳米，将电子从Na转移到Cl并形成离子键会是能量上有利的。

The potential energy curve shows that there is a minimum at 0.236 nm separation and then a steep rise in potential which represents a repulsive force. This repulsive force is more than just an electrostatic repulsion between the electron clouds of the two atoms. It has a quantum mechanical character rooted in the Pauli exclusion principle, and is often called just the "exclusion principle repulsion". When the ions are widely separated, the wavefunctions of their core electrons do not significantly overlap and they can have identical quantum numbers. As they get closer, the increasing overlap of the wavefunctions causes some to be forced into higher energy states. No two electrons can occupy the same state, so as a new set of energy states is formed for the composite, two-nucleus system, the lower energy states are filled and some of the electrons are pushed into higher states. This requires energy and is experienced as a repulsion, preventing the ions from coming any closer to each other.

势能曲线显示，在0.236纳米的分离距离处有一个最小值，随后势能迅速上升，这代表一种排斥力。这种排斥力不仅仅是两个原子电子云之间的静电排斥。它具有量子力学特征，根植于泡利不相容原理，并通常被称为“排斥原理”。当离子相互远离时，它们的核心电子的波函数不显著重叠，因此可以具有相同的量子数。随着它们靠近，波函数的重叠增加，导致一些电子被迫进入更高的能量状态。由于没有两个电子可以处于相同的状态，当复合系统形成新的能量状态时，较低的能量状态被填满，一些电子被推入更高的状态。这需要能量，并表现为排斥力，阻止离子进一步靠近。

The potential diagram above is for gaseous NaCl, and the environment is different in the normal solid state where sodium chloride (common table salt) forms cubical crystals. The ion separation is 0.28 nm, somewhat larger than that in the gaseous state.

上面的势能图是针对气态NaCl的，而在常态下的固体状态中，氯化钠（普通食盐）形成立方晶体，环境不同。离子间距为0.28纳米，比气态中的略大。

A major part of the study of molecular structure is the description of the chemical bonds which are formed between atoms. The classic studies are the extremes of ionic bonding in sodium chloride and covalent bonding in the hydrogen molecule.

分子结构研究的重要部分是对原子之间形成的化学键的描述。经典研究包括氯化钠中的离子键和氢分子中的共价键。