3g. The covalent bonding in the water and hydrogen sulfide molecules

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes


 Covalent bonding diagram for WATER covalent molecule, molecular formula H2O

Two atoms of hydrogen (1) combine with one atom of oxygen (2.6) to form the molecule of the compound water H2O

* metals \ non-metals (zig-zag line)

Pd metals Part of the modern Periodic Table

Pd = period, Gp = group

metals => non–metals
Gp1 Gp2 Gp3 Gp4 Gp5 Gp6 Gp7 Gp0
1 1H  Note that hydrogen does not readily fit into any group but is a non-metal 2He
2 3Li 4Be atomic number Chemical Symbol eg 4Be 5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba Transition Metals 81Tl 82Pb 83Bi 84Po 85At 86Rn
The covalent molecules of water (hydrogen + oxygen) and hydrogen sulfide (hydrogen + sulfur)

 

Hydrogen is one electron short of a full shell, oxygen is two electrons short of a full outer shell of 8, so two hydrogen atoms share their electrons with the six outer electrons of oxygen, so all three atoms now have a full outer shell.

(c) doc b and (c) doc band (c) doc b combine to form (c) doc b so that the hydrogen atoms are electronically like helium and the oxygen atom becomes like neon (2.8, but only the outer shell of oxygen's electrons are shown).

Electronically, hydrogen (1) becomes like helium (2) and oxygen (2.6) becomes like neon (2.8), so the hydrogen and oxygen atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.

(Lewis diagram of water) simplified 'dot and cross' electronic diagram for the covalently bonded water molecule.

The water molecule is held together by the strong H–O hydrogen–oxygen single covalent bonds by sharing electrons.

Note that the inner shell of oxygen's electrons are shown in this diagram, but NOT in the bonding diagram above. Only the outer shell of oxygen's electrons are involved in the covalent bonding here.

Although these electron arrangement diagrams show how the covalent bond is formed and the electronic structure of the molecule, they do not give any idea on the relative size of the atoms in the molecule or the 3D spatial arrangement of the atoms in the molecules, nor indeed how the molecules stack together in a crystal lattice.

The molecule can be shown as (c) doc b (displayed formula of water) with two hydrogen – oxygen single covalent bonds.

The two pairs of double dots represent pairs of electrons not involved in the covalent bonding in water. Its actually referred to as a bent or V shape (A level comment).

Advanced note: water has a V or bent shape, the H–O–H bond angle is 105o.

The 'pros and cons' of molecular diagrams and understand the limitations of diagrams:

(c) doc b

(c) doc b The dot and cross diagram (left) is good for showing the electronic detail of the bonding but does not show the relative size of the atoms or the 3D shape. It can also show whether a bond is single (ox) or double (oxox).

This displayed formula (right) may give you, but not usually for larger molecules, some idea of the shape, but not the full electron detail, nor does it show the relative size of the atoms. Importantly, what it does show is precisely how all the atoms are connected e.g. which atoms connect to each other (H-O-H NOT H-H-O) and whether the bonds are single (as in water H-O-H) or double (as in carbon dioxide O=C=O).

 

The ball and stick model (left) gives you an idea of the shape of the molecule, but nothing about the relative size of the atoms and nothing about the electronic structure of the molecule (which you get from the dot and cross diagrams).ADD symbols

space filling model of water H2OA space filling model (right) gives you the best 2D representation of the real 3D shape of the molecule and the relative size of the atoms and the space they fill. However, it does not say anything about the electronic structure and it doesn't indicate whether the covalent bonds are double, single or treble.

These comments apply to the rest of the molecular structures described below.

You should appreciate different styles of representing a molecule's structure all having something useful to say!

 

dot and cross diagram of the water moleculeOn the above right is the ball and stick diagram for the water molecule.

The diagrams on the left and right are the full 'dot and cross' electronic diagram for the covalent bonding in the water molecule. All the electrons are shown in this electronic dot & cross Lewis diagrams for covalent bonding in the water molecule.

The dot and cross diagram does not tell you anything about the size of the atoms in the molecule or the accurate 3D spatial arrangement of the atoms, that is, the shape of the molecule

Comments

Melting point of water 0 oC

Boiling point of water 100 oC

Water is a colourless liquid at room temperature


HYDROGEN SULFIDE

Hydrogen sulphide will be similar, since sulphur (electrons 2.8.6) is in the same Group 6 as oxygen.

Valency of oxygen is 2 and sulphur is also 2 here.

The dot and cross Lewis diagram for hydrogen sulfide is similar to water.

Sulfur is in group 6 with six outer shell electrons, two of which pair up with the electrons from the two hydrogen atoms.

It has a bent shape like water.

Comments

Melting point of hydrogen sulfide -86 oC

Boiling point of hydrogen sulfide -61 oC

Hydrogen sulfide is a poisonous egg smelling gas at room temperature.


What next?

Recommend next: The covalent bonding in the ammonia molecule

Explaining the properties of small covalently bonded molecules

 

Sub-index for Part 3. Covalent Bonding: small molecules & properties

 

Index for ALL chemical bonding and structure notes

 

Perhaps of interest?

Overview of Periodic Table

 

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