Electron arrangment

Figure 15.8 Pyridine and pyrimidine are nitrogen-containing aromatic heterocycles with tt electron arrangements much like that of benzene. Both have a lone pair of electrons on nitrogen in an sp2 orbital in the plane of the ring.

In this chapter, we focus on electron arrangements in atoms, paying particular attention to the relative energies of different electrons (energy levels) and their spatial locations (orbitals). Specifically, we consider the nature of the energy levels and orbitals available to—... 

With this background, we show how electron arrangements in multielectron atoms and the monatomic ions derived from them can be described in terms of—... 

The alkali metals are extremely reactive. Thus, there is a dramatic change in chemistry as we pass from the inert gases to the next column in the periodic table. The chemistry of the alkali metals is interesting and often spectacular. Thus, these metals react with chlorine, water, and oxygen, always forming a +1 ion that is stable in contact with most substances. The chemistry of these +1 ions, on the other hand, is drab, reflecting the stabilities of the inert gas electron arrangements that they have acquired. 

Apparently the diatomic molecules of the halogens already have achieved some of the stability characteristic of the inert gas electron arrangement. How is this possible How could one chlorine atom satisfy its need for one more electron (so it can reach the argon stability) by... 

Explain how these energies are consistent with the proposal that the electron arrangements of the inert gases are specially stable. 

With these two assumptions, we can propose the electronic arrangement of lowest energy for each atom. We do so by mentally placing electrons successively in the empty orbitals of lowest energy. The electron orbital of lowest energy is the Is orbital. The single electron of the hydrogen atom can occupy this orbital. In the helium... 

Once we have identified the arrangement of the most distant locations of these regions, which is called the electron arrangement of the molecule, we note where the atoms lie and identify the shape of the molecule by giving it the name of the corresponding shape in Fig. 3.1. In naming the molecular shape, we consider only the positions of atoms, not any lone pairs that may be present on the central atom, even though they affect the shape. 

A molecule with only two atoms attached to the central atom is BeCl2. The Lewis structure is CI — Be — CE, and there are no lone pairs on the central atom. To be as far apart as possible, the two bonding pairs lie on opposite sides of the Be atom, and so the electron arrangement is linear. Because a Cl atom is attached by each bonding pair, the VSEPR model predicts a linear shape for the BeCL molecule, with a bond angle of 180° (4). That shape is confirmed by experiment. 

Methane, CH4, has four bonding pairs on the central atom. To be as far apart as possible, the four pairs must take up a tetrahedral arrangement around the C atom. Because the electron arrangement is tetrahedral and an H atom is attached to each bonding pair, we expect the molecule to be tetrahedral (see 1), with bond angles of 109.5°. Thar is the shape found experimentally. 

A sulfur hexafluoride molecule, SF6, has six atoms attached to the central S atom and no lone pairs on that atom (8). According to the VSEPR model, the electron arrangement is octahedral, with four pairs at the corners of a square on the equator and the remaining two pairs above and below the plane of the square (see Fig. 3.2). An F atom is attached to each electron pair, and so the molecule is predicted to be octahedral. All its bond angles are either 90° or 180°, and all the F atoms are equivalent. 

STRATEGY Write down the Lewis structure and identify the electron arrangement around each central atom (each C atom, in this case). Treat each multiple bond as a single unit. Then identify the overall shape of the molecule (refer to Fig. 3.2 if necessary). 

Identify the electron arrangement around each central atom. 

Predict the electron arrangement and the shape of a nitrogen trifluoride molecule, NF,. 

STRATEGY For the electron arrangement, draw the Fewis structure and then use the VSEPR model to decide how the bonding pairs and lone pairs are arranged around the central (nitrogen) atom (consult Fig. 3.2 if necessary). Identify the molecular shape from the layout of atoms, as in Fig. 3.1. 

Seef-Test 3.2A Predict (a) the electron arrangement and (b) the shape of an lf 5 molecule. 

Sei e-Tfst 3.3A (a) Give the VSEPR formula of an NH3 molecule. Predict (b) its electron arrangement and (c) its shape. 

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