The Lewis dot structures, or Lewis structures, are a system of expressing the valency of the atoms in a compound or an ion. It is similar to electron dot diagrams, with the addition of the depiction of bonds between atoms. To understand Lewis structures, you need to understand the concept of valency. Here's a primer on the topic. If you know the role of valency in chemistry, you can safely skip to the next section, which describes how to depict a substance in terms of Lewis structures. Valency, or valence, is the capacity of an element to participate in a chemical bond. It is represented by the number of electrons the atom needs to gain or lose to achieve a stable state. In other words, it is the number of electrons found in an atom's outermost unfulfilled orbital. For instance, an oxygen atom has an electronic configuration of 1s² 2s² 2p⁴, or in simpler terms, [2,6]. The closest inert gas to oxygen is neon, which has a configuration of 1s² 2s² 2p⁶. In neon's configuration, all the orbitals of the atom are fulfilled ('s' orbitals can only contain 2 electrons, and 'p' orbitals can only contain 6), and the atom is thus stable. In the case of oxygen, though the number of protons and electrons remains the same, i.e., even though the atom doesn't become an ion, the '2p' orbital is unfulfilled, and needs 2 more electrons to be stable. Thus, an oxygen atom has 6 valence electrons, i.e., it has 6 electrons in its outermost unfulfilled orbital. Now that we have refreshed up on the concept of valence, let's move on to Lewis dot structures. In Lewis dot structures, the valency of the element in question is shown as illustrated in the following diagrams. The chemical symbol of the element is placed at the center, and only valence electrons are shown around it. As can be seen in the images, oxygen and sulfur have exactly the same Lewis dot structure. This is due to the fact that although the two compounds have different electronic configurations, the two have the same arrangement of electrons in their outermost orbits. Elements in the same group of the periodic table, as oxygen and sulfur are, have the same valence electron configuration. A Lewis structure of an atom doesn't show all the electrons in it―only the valence ones. Thus, oxygen and sulfur have the same Lewis structure but a different electronic configuration. In case of single bonds, a single line is drawn between the two atoms of a compound. This is seen in the adjoining image, depicting the Lewis structure of water. The single bond can also be illustrated through two atoms next to each other. Similarly, double bonds are shown with a double line connecting the two atoms, or else with two pairs of electrons close to each other. The pairs of electrons that are not participating in the bonding between the atoms are called the lone pairs.