To create Lewis Dot structures for compounds involving polyatomic ions, such as calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2), follow these steps:
1. Identify the central atom: In both compounds, calcium (Ca) and magnesium (Mg) are the central atoms since they are less electronegative than the other atoms.
2. Determine the valence electrons: Find the total number of valence electrons for each atom in the compound. Calcium and magnesium are in Group 2 of the periodic table, so they each have 2 valence electrons. Oxygen (O) in carbonate (CO3) has 6 valence electrons, while hydrogen (H) and oxygen in hydroxide (OH) have 1 and 6 valence electrons, respectively.
3. Draw the skeletal structure: Connect the central atom(s) with the other atoms using single bonds. In calcium carbonate, the skeletal structure would be Ca-O-Cā”O, where the oxygen atoms are double-bonded to the carbon atom. In magnesium hydroxide, the skeletal structure would be Mg-O-H.
4. Distribute the remaining valence electrons: Subtract the number of valence electrons already used in the skeletal structure from the total number of valence electrons for each atom. Distribute the remaining electrons around the atoms, prioritizing octet formation for all atoms except hydrogen.
For calcium carbonate (CaCO3):
- Calcium has 2 valence electrons, but it's bonded to three oxygen atoms, so it has used all of its valence electrons.
- Carbon has 4 valence electrons and needs 4 more to complete its octet, so it forms two double bonds with the oxygen atoms.
- Each oxygen atom has 6 valence electrons and needs 2 more to complete an octet, so they each form single bonds with the carbon atom.
For magnesium hydroxide (Mg(OH)2):
- Magnesium has 2 valence electrons, but it's bonded to two oxygen atoms and two hydrogen atoms, so it has used all of its valence electrons.
- Each oxygen atom has 6 valence electrons and needs 2 more to complete an octet, so they each form single bonds with the magnesium atom.
- Each hydrogen atom has 1 valence electron and needs 1 more to complete an octet, so they each form single bonds with the oxygen atoms.
5. Check for octet rule fulfillment: Ensure that all atoms (except hydrogen) have either 8 electrons or a full valence shell.
6. Add formal charges: If necessary, assign formal charges to make sure all atoms have a charge of zero or as close to zero as possible. This step may not be required for these particular compounds.
That's it! You have successfully created the Lewis Dot structures for calcium carbonate and magnesium hydroxide.