In this video, we will draw dot and cross diagram of water molecule. From there, we will determine the structural formula of water.
Knowing how to write accurate structural formula is particularly important when students deal with Organic Chemistry.
Covalent bond is formed when electrons are shared between non-metals. After bonding, each atom attains the noble gas electronic configuration.
Elements and compounds with covalent bonds are called molecules. A molecule is a group of two or more atoms held together by covalent bonds.
Before drawing the dot and cross diagram of water (H₂O), we first need to determine the number of electrons in each atom.
We can find the number of electrons from the periodic table. The number at the top of each element is the atomic number, which tells us the number of protons. In an atom, the number of protons and electrons is the same. Therefore, this number also represents the number of electrons.
Electron arrangement:
- Hydrogen has 1 electron in its first shell. It needs 1 more electron to achieve a stable noble gas (duplet) configuration.
- Oxygen has 8 electrons → 2 electrons in the first shell and 6 electrons in the second shell. It needs 2 more electrons to achieve a stable octet configuration.
Since there are two hydrogen atoms, each hydrogen atom will share 1 electron with oxygen. Oxygen will share 2 of its electrons (1 with each hydrogen atom) to form bonds.
Drawing the dot and cross diagram:
- Each hydrogen atom shares its single electron with oxygen.
- Oxygen shares 2 of its valence electrons (one with each hydrogen).
- The inner shell of oxygen is not involved in bonding.
After drawing the diagram, we should check whether each atom has achieved a noble gas electronic configuration:
- Oxygen: Now has 8 electrons in its outermost shell ✓
- Each Hydrogen: Now has 2 electrons in its outer shell ✓
All atoms have achieved a stable electronic configuration.
Note for exams: If the question asks you to draw only the outermost electrons, you can omit the inner shell of oxygen. Only show the valence electrons involved in bonding.
Converting to structural formula: From the dot and cross diagram, we can see that 2 electrons are shared between oxygen and each hydrogen atom. Since 2 electrons are shared, this represents 1 single bond. Therefore, water has the structural formula with two single bonds between oxygen and hydrogen (H₂O).
Note on molecular shape: In water, the molecule has a V-shape (bent shape). However, at O-Level, the focus is mainly on the number of bonds rather than the shape of the molecule. You will learn about molecular shapes in greater detail at A-Level.
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