The shape of a molecule or ion is governed by the arrangement of the electron pairs around the central atom. All you need to do is to work out how many electron pairs there are at the bonding level, and then arrange them to produce the minimum amount of repulsion between them. You have to include both bonding pairs and lone pairs. Work out how many of these are bonding pairs, and how many are lone pairs.
You know how many bonding pairs there are because you know how many other atoms are joined to the central atom assuming that only single bonds are formed. For example, if you have 4 pairs of electrons but only 3 bonds, there must be 1 lone pair as well as the 3 bonding pairs.
Arrange these electron pairs in space to minimize repulsions. How this is done will become clear in the examples which follow. The only simple case of this is beryllium chloride, BeCl 2. The electronegativity difference between beryllium and chlorine is not enough to allow the formation of ions. Beryllium has 2 outer electrons because it is in group 2.
It forms bonds to two chlorines, each of which adds another electron to the outer level of the beryllium. There is no ionic charge to worry about, so there are 4 electrons altogether - 2 pairs. It is forming 2 bonds so there are no lone pairs. The molecule is described as being linear. Boron is in group 3, so starts off with 3 electrons. It is forming 3 bonds, adding another 3 electrons. There is no charge, so the total is 6 electrons - in 3 pairs. Because it is forming 3 bonds there can be no lone pairs.
The 3 pairs arrange themselves as far apart as possible. The arrangement is called trigonal planar. In the diagram, the other electrons on the fluorines have been left out because they are irrelevant. There are lots of examples of this. The simplest is methane, CH 4. Carbon is in group 4, and so has 4 outer electrons. It is forming 4 bonds to hydrogens, adding another 4 electrons - 8 altogether, in 4 pairs.
Because it is forming 4 bonds, these must all be bonding pairs. Four electron pairs arrange themselves in space in what is called a tetrahedral arrangement. A tetrahedron is a regular triangularly-based pyramid. The carbon atom would be at the centre and the hydrogens at the four corners. All the bond angles are It is important that you understand the use of various sorts of line to show the 3-dimensional arrangement of the bonds. In diagrams of this sort, an ordinary line represents a bond in the plane of the screen or paper.
A dotted line shows a bond going away from you into the screen or paper. A wedge shows a bond coming out towards you. Nitrogen is in group 5 and so has 5 outer electrons. Each of the 3 hydrogens is adding another electron to the nitrogen's outer level, making a total of 8 electrons in 4 pairs.
Because the nitrogen is only forming 3 bonds, one of the pairs must be a lone pair. The electron pairs arrange themselves in a tetrahedral fashion as in methane. Dots are used to show the valence electrons, whereas the lines to represent bonds in the structure. Here is the step-by-step procedure to understand the Lewis structure of NH3.
Now that we know the valence electrons for the molecule, we can predict its Lewis structure. Hydrogen atoms never take the central position, so we will place the Nitrogen atom in the centre. Place all the Hydrogen atoms around the Nitrogen atom and the valence electrons of both the atoms like this. Each Hydrogen atom only needs one electron to become stable, as it is an exception to the octet rule. Nitrogen will share three of its valence electrons for forming a stable structure. Thus there are three single bonds formed between Nitrogen and Hydrogen atoms, and there is one pair of nonbonding electrons on the nitrogen atom.
Ammonia has a tetrahedral molecular geometry. All the Hydrogen atoms are arranged symmetrically around the Nitrogen atom which forms the base, and the two nonbonding electrons form the tip which makes the molecular geometry of NH3 trigonal pyramidal. The Nitrogen atom has the electronic configuration of 1s2 2s2 2px1 2py1 2pz1.
When it shares the electrons with Hydrogen atoms, one s-orbital and three p-orbitals hybridize and overlaps with s orbitals of a Hydrogen atom to form sp3 hybridization.
Thus, Ammonia or NH3 has sp3 hybridization. There are three single bonds and one lone pair of electrons in NH3 molecule. Ammonia has 4 regions of electron density around the central nitrogen atom 3 bonds and one lone pair. These are arranged in a tetrahedral shape. The resulting molecular shape is trigonal pyramidal with H-N-H angles of Average rating 4. Vote count: No votes so far! Be the first to rate this page. Tell us how we can improve this page in your own language if you prefer?
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