Formulae Of Ionic Compounds & Their Names: Part 2 | Properties of Matter | Chemistry | FuseSchool

Summary

This lesson teaches the nomenclature and formula derivation of ionic compounds made from polyatomic ions, which consist of more than one atom. Examples of common polyatomic ions like hydroxide, sulfate, nitrate, and others are discussed, along with the 'criss-cross' method to balance charges in formulas. The importance of correctly using brackets to avoid misinterpretation is highlighted through examples. The lesson also touches on the naming of compounds with transition metals, emphasizing the use of Roman numerals to denote their valency.

Takeaways

• 🔬 Ionic compounds can be named and their formulae deduced from the charges of their constituent ions.
• 🌐 Polyatomic ions consist of more than one atom and are different from monatomic ions.
• 📝 Common polyatomic ions include Hydroxide (OH-), Sulphate (SO42-), Nitrate (NO3-), Phosphate (PO43-), Carbonate (CO32-), and Ammonium (NH4+).
• ⚠️ Except for the Ammonium ion, all listed polyatomic ions are anions.
• 📚 Memorizing the formulae and charges of polyatomic ions is essential for understanding ionic compounds.
• 📝 The process of writing ionic compounds involves 'criss-crossing' the charges of the ions to balance them.
• 🧩 In the formula NaNO3, 'Na' stands for sodium and 'NO3' for nitrate, making it sodium nitrate.
• 🔄 The structure of polyatomic ions remains unchanged in ionic compounds, as seen with Mg(OH)2 being magnesium hydroxide.
• 📐 Brackets and subscripts are used to indicate the number of polyatomic ions needed to balance the charge, as in Mg(OH)2.
• 🔄 The formula K2SO4 represents potassium sulphate, with two potassium ions balancing the charge of one sulphate ion.
• 🏷 Transition metals like copper can exhibit multiple valencies, requiring specification in the compound's name, e.g., copper (II) chloride for CuCl2.

Q & A

What is the difference between monatomic and polyatomic ions?

Monatomic ions are ions made of a single atom, while polyatomic ions consist of more than one atom.

What are some common polyatomic ions mentioned in the script?

The common polyatomic ions mentioned are Hydroxide (OH-), Sulphate (SO4^2-), Nitrate (NO3^-), Phosphate (PO4^3-), Carbonate (CO3^2-), and Ammonium (NH4^+).

Why is it important to criss-cross the charges when writing the formula of an ionic compound?

Criss-crossing the charges helps to balance the overall charge of the compound, ensuring that the ionic compound is electrically neutral.

What is the formula for the ionic compound formed by Na+ and NO3- ions?

The formula for the ionic compound formed by Na+ and NO3- ions is NaNO3.

How is the name of the ionic compound NaNO3 derived?

The name 'sodium nitrate' is derived by taking the name of the metal (sodium) and the polyatomic ion (nitrate) as it is.

Why do we use brackets and subscripts in the formula Mg(OH)2 for magnesium hydroxide?

Brackets and subscripts are used to indicate that there are two OH- ions needed to balance the +2 charge of the Mg2+ ion, maintaining the compound's electrical neutrality.

What is the formula and name of the ionic compound formed from K+ and SO4^2- ions?

The formula is K2SO4, and the name is potassium sulphate.

Why is it necessary to specify the valency of transition metals in the name of an ionic compound?

Specifying the valency in the name (e.g., copper (II) chloride) indicates the charge of the transition metal ion and helps distinguish between different compounds that the metal can form.

What does the term 'multivalent' mean in the context of transition metals?

'Multivalent' means that transition metals can form ions with different charges, such as Cu+ or Cu2+ for copper.

How is the valency of a transition metal represented in the name of an ionic compound?

The valency is represented by Roman numerals in round brackets directly after the transition metal, as in 'copper (II) chloride' for CuCl2.

What would be the incorrect formula if OH was not placed in brackets for the compound formed with Mg2+ and OH- ions?

The incorrect formula would be MgOH2, which would imply one oxygen and two hydrogen atoms for each Mg2+ ion, instead of the correct two hydroxide ions.