In this video, we will draw dot and cross diagram of carbon dioxide molecule. From there, we will determine the structural formula of carbon dioxide.
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 carbon dioxide (CO₂), 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:
- Carbon has 6 electrons → 2 in the first shell and 4 in the second shell.
- Oxygen has 8 electrons → 2 in the first shell and 6 in the second shell.
From this, we can see that:
- Carbon needs 4 more electrons to achieve a stable octet configuration.
- Each oxygen atom needs 2 more electrons to achieve a stable octet configuration.
Since there are two oxygen atoms in carbon dioxide, carbon will share 2 electrons with each oxygen atom.
Drawing the dot and cross diagram:
- Carbon shares all 4 of its valence electrons (2 with each oxygen).
- Each oxygen atom uses 2 of its electrons for bonding and keeps the remaining 4 electrons in its outer shell.
After drawing the diagram, it is good to check whether each atom has achieved a noble gas electronic configuration (octet rule):
- Left oxygen: 8 electrons ✓
- Carbon: 8 electrons ✓
- Right oxygen: 8 electrons ✓
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 shells. Only show the valence electrons involved in bonding.
Converting to structural formula: From the dot and cross diagram, we can see that 4 electrons are shared between carbon and each oxygen atom. Since 4 electrons are shared, this means 2 bonds are formed. Therefore, carbon dioxide contains double bonds between carbon and oxygen (O=C=O).
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