In this video, we will draw dot and cross diagram of hydrogen molecule. From there, we will determine the structural formula of hydrogen.
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 hydrogen (H₂), 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 of hydrogen:
- Hydrogen has 1 electron in its first (and only) shell.
- It needs 1 more electron to achieve a stable noble gas (duplet) configuration, just like helium.
Since there are two hydrogen atoms in a hydrogen molecule (H₂), each hydrogen atom will share its single electron with the other.
Drawing the dot and cross diagram:
- Each hydrogen atom shares its one electron with the other hydrogen atom.
- This results in 2 electrons being shared between the two hydrogen atoms.
- Both atoms now have 2 electrons in their outer shell.
After drawing the diagram, we should check whether each atom has achieved a noble gas electronic configuration:
- Both hydrogen atoms now have 2 electrons in their outer shell ✓ (same as helium)
Both atoms have achieved a stable electronic configuration.
Converting to structural formula: From the dot and cross diagram, we can see that 2 electrons are shared between the two hydrogen atoms. Since 2 electrons are shared, this represents 1 single bond. Therefore, the structural formula of hydrogen gas is H–H.
Important Note: A single bond represents the sharing of two electrons between two atoms.
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