 Welcome to Shapes of Simple Molecules Part 1. Determining the shape of molecules with one central atom starts with the Lewis dot structure. The example below represents ammonia. It has atomic symbols. These represent the atomic nucleus and the core electrons. Lines. The lines represent the electrons involved in covalent bonds. Each bond contains two electrons. And dots. These are used to represent lone pairs of electrons. The groups of valence electrons around the central atom determine the molecule's shape. For example, in ammonia there are four groups of electrons around the nitrogen atom. There is one lone pair of electrons. Each lone pair contains one group of electrons. There are three bonds. Each bond contains one group of electrons. One group of electrons can be either one non-bonding pair of electrons, one single bond, one double bond, or one triple bond. Each counts as one group, even though some have more than two electrons. For example, in sulfur dioxide there are three groups of electrons around the sulfur atom. Since we count the two double bonds as one group each, and the lone pair of electrons is one group. Or in hydrogen cyanide there are two groups of electrons around the carbon atom. Since we count the triple bond as one group and the single bond as one group. And in carbon dioxide there are two groups of electrons around the carbon atom. Since we count each double bond as one group. Now you try it. In the following examples, how many groups of electrons are around the central atom? In formaldehyde the two single bonds and the one double bond make three total groups of electrons. In germane the four single bonds make four total groups of electrons. Next we'll look at how the groups of electrons repel each other to create the molecule shape. Groups of electrons are negatively charged and repel each other. Groups can be two electrons in a single bond or a non-bonding pair. Four electrons in a double bond or six electrons in a triple bond. In all of these groups the electrons move as far away from each other as possible to minimize repulsions. The two groups of electrons in beryllium hydride move to opposite sides of the central beryllium atom. Ball and stick models show how the valence electrons move around the central atom to avoid each other. Space-filled models are more realistic but obscure the geometry. In molecular models typically specific elements are given specific colors. For example, in the carbon dioxide molecule shown here, carbon is typically shown as black and oxygen is red. Here are some elements and the colors used to represent them. Now let's look at the different shapes the simple molecules take on. Both beryllium hydride and carbon dioxide have two groups of electrons around the central atom. The angle between the groups is 180 degrees. This results in the molecules adopting a linear shape. The trigonal planar shape is flat and has angles of 120 degrees between the bonds. Any atom that contains three groups of electrons around the central atom will arrange the electrons in this pattern. In sulfur dioxide the three groups of electrons separate into the trigonal planar arrangement. However, the shape of the molecule is bent because the shape of any molecule is defined only by the bonded atoms. The angle between the bonded atoms is about 120 degrees. In methane there are four groups of electrons around the central carbon atom bonded to four hydrogen atoms. A tetrahedral shaped molecule is produced when each bonded pair moves as far away as possible from the other three electron pairs. The tetrahedral shape, so named because the bonds point toward the four corners of a tetrahedron, has angles of 109.5 degrees between the bonds. Four groups of electrons around a central atom are always arranged in this pattern. The tetrahedral shape can be drawn on paper by using a solid line to represent a bond that lies in the plane of the paper, a solid wedge for a bond that projects out in front of the dashed line for a bond that projects behind the paper. If one of the four groups is a lone pair, the groups of electrons still arrange themselves in the tetrahedral shape. However, the shape of the molecule is defined by the four atoms and is pure middle. If two of the four groups are lone pairs, the groups of electrons arrange themselves in a tetrahedral shape. The shape of the molecule is defined by the three atoms and is bent. The lone pairs of electrons are closer to the nucleus than are bonding electrons. They therefore repel the bonding electrons more than bonding electrons repel each other. This results in a slightly smaller angle between bonding electrons as the number of lone pairs increases, for example methane, ammonia and water. What would the shape of the following molecules be? The shape is trigonal planar. The shape of carbon disulfide is linear. The shape of germane is tetrahedral. The shape of formaldehyde is trigonal planar. You have completed this video, Shapes of Simple Molecules Part 1.