How to Write the Formulae for Ionic Compounds

Learning Objective

It has always been interesting to see how a teacher writes down the formula of various compounds with different subscripts of each symbol in the formula, example Na₂S, HCl, K₂O, MgCl₂ etc. You might have wondered how much time they might have spent on memorizing the formulae for all these compounds.

How do you know that the formula of Magnesium chloride is MgCl₂, and not Mg₂Cl or MgCl, or Mg₂Cl₄! Here we will learn all the trick to write the formulae for ionic compounds. Ionic compounds are generally formed by mixing metal and nonmetal ions or rather positive and negative ions.

Learning Objective

Predicting the charge of metal and nonmetal ions with fixed charges
Formulating a neutral compound by mixing and matching cations and anions
Illustrating the ratio of cations to anions to make a neutral ionic compound
Predict the ions formed in the aqueous dissociation (hydration) of ionic compounds in water

Chemical Safety, handling and storage

Step 1- Read the lab safety protocol given under materials<content<week 1. Complete and sign the lab safety contract on the last page. Save and upload it to assessments<assignment<week 1 materials.If you have completed it already, no need to upload it again. If you have not, complete it ASAP. Your lab experiments will be graded only if you have submitted your signed lab safety contract.
Step 2-Wear gloves and safety goggles when you handle chemicals.
Step 3-Google the SDS (Safety Data Sheet)for each chemical and material that you use for each experiment. Read and understand the safety measures to be taken during handling, storage, and disposal.
Step 4-Read the two documents given under Week1 materials under content on D2L about the storage and handling of chemical lab kits.
Step 5-Dispose the chemicals as mentioned in the above-mentioned documents. Wash/flush them down the drain with plenty of water as they are relatively safer chemicals in minute amounts.
Step 6-Wash all the labware including the pipet and well-plates. Turn off the multimeter and store it safely. Tighten all the solutions caps and store them in their original Ziplocs, do not mix them.

Prior Knowledge

Interpret the data from the Periodic Table of Elements like the chemical symbols, classification of groups, rows, main group, and transition elements.

Model

1A	2A		3A	4A	5A	6A	7A	8A
1+
1+	2+				3-	2-	1-
1+	2+	Transition metals and the rest of the unlabeled metals show variable ionic charges (exception Zn, Ag, and Hg). They are known as TYPE II Metals Iron-Fe⁺², Fe⁺³ Lead-Pb⁺², Pb⁺⁴ Metals with fixed charge like 1A, 2A metals, Al, Zn, Ag, Hg are known as TYPE I Metals Noble (inert) gases given in column 8A generally do not react or form ions	3+		3-	2-	1-
1+	2+					2-	1-
1+	2+						1-
1+	2+

1+	2+

Task 1

Ionic Charges and subatomic particles (Protons and electrons)-Review

Protons are the positively charged particles found in an atom. Electrons are the negatively charged particles. They are equal and opposite in the magnitude of charges. An atom is neutral with zero charge. This is because the number of positive protons is equal to the number of electrons.

However, when an atom loses or gains electrons, the number of electrons and protons are not the same. Therefore, the atom is no more neutral. It will acquire the charge of the particles found in excess. If there are two excess protons, it will be a cation with +2 charge. If there are three excess electrons compared to the protons, it will be an anion with -3 charge.

Charge = # of Protons (Atomic Number) - # of Electrons
Neutral atom	# of Protons =	# of Electrons	Examples 10p=10eà 10-10=0 Ne atom
Positive ions (Cations)	# of Protons >	# of Electrons	11p > 10eà 11-10=+1 Na+ ion
Negative ions (Anions)	# of Protons <	# of Electrons	9p < 10e à9-10=(-)1 F- ion

Materials required

Beads, dice, and the bigger well-plate (or any tray or lid to hold the beads and dice), periodic table to identify the element from the atomic number (=# of protons), camera to capture pictures

Method
Pick the right number of beads and dice in the well plates (tray) to show the models for corresponding ions or atoms and vice versa. Remember the atomic number is the number of protons.

So, you should be able to find the atomic symbol from the Periodic Table. See the example given in the first box. Take pictures and paste in the corresponding boxes. Complete all the cells with the correct information

	Bead =Proton	Dice=Electron	Picture	Charge / Symbol
1	6 p=6+	6 e=6(-)		Charge =p-e =6-6 =0 Neutral atom P=6=Atomic #6 Carbon atom
2	8p	6e		Charge =p-e =___-___ = Atom or ion? P=__=Atomic #_ Atom/ion symbol
3	16p	18e		Charge =p-e =___-___ = Atom or ion? P=__=Atomic #_ Atom/ion symbol
4				Charge =p-e =20-18 =+2 Neutral atom P=20=Atomic #20 Atom/ion symbol
5				Charge =p-e =16-18 = Atom or ion? P=16=Atomic #16 Atom/ion symbol
6	1p	Zero electrons		Charge =p-e =___-___ = Atom or ion? P=__=Atomic #_ Atom/ion symbol
7	Take all the beads	Take all the dice		Charge =p-e =___-___ = Atom or ion? P=__=Atomic #_ Atom/ion symbol

Introduction

Compounds are mainly classified as Ionic and molecular (covalent) compounds.

Ionic compounds are formed by the combination of opposites, positive and negative ions or simply metal (NH⁴⁺ -ammonium is an exception) and nonmetal atoms to make a neutral combination. Molecular (covalent) compounds are formed by the likes, only the nonmetal atoms and so no opposite ions or charges are involved.

Chemical Safety, handling and storage

Metal ions (positive cations) in nature electrostatically attracts enough number of counter ions (oppositely charged negative anions) to balance or neutralize the charge on it. This makes an ionic bond. In an ionic compound the entire material is made of repetitive units of negative and positive ions.

Positive metal ions are known by the name of the metal say Calcium ion, Sodium ion etc. However negative anions of nonmetal atoms are tweaked as “ide” in ions like chlorine as chloride ion, oxygen as oxide ion etc.

Questions

Identify and write down the names and formulae of some elements from column 1A

Identify and write down the names and formulae of some elements from column 7A

Write the chemical formula for some noble gases.

Based on the information from Table 1 given under the model, assign the ionic charges for the following main group elements

Element	H	Al	Br	N	S	Ba	Rb	Ne
Ion with Charge	H⁺
Name the ions	Hydrogen ion

Based on the charges assigned to the above ions, predict the charges on the counter ion to neutralize the charge.

Element	H	Al	Br	N	S	Ba	Rb	Ne
Ion with Charge	H⁺
Charge on the ion	1+
Total charge on the counter ion to neutralize it	1-

If there is one Na⁺ion, how many chloride ions are required to neutralize the charge? Why?
If there is one O^2-ion how many potassium ions are required to neutralize the charge? Why?

TASK 2

Purpose

To elucidate the mechanism behind the formation of ionic compound (bond) in fixed ratios

Materials required

Right-click copy, and paste the following pictures (sticky rectangle with charges written) with correct ionic charges in the compound table (Table 2) to make the model of the neutral compounds.

Example

Table 1

Make a model for NaCl using the opposite positive and negative sticky rectangles. First find the charge on each ion like Na is +1, Cl is -1. Refer the model given on page# 2. Make a combination of positive and negative ions to get a total zero charge. Copy paste that many stickies from the above table and make the model and write the formula of the neutral ionic compound. Paste and label model in the space given below. Model of MgF₂is also shown.

Table 1

Elements

Model for the neutral compound

Formula for the neutral compound

Name

Na and

Na is +1 and Cl is -1

Na+ Cl- Charge=0 Paste the model here

*Label with chemical formula and ionic charges

NaCl

Neutral compound

Sodium chloride

Mg and F

Mg is +2

And F is -1

Mg 2+ F- F- Charge =0

Mg with two Fluoride ions

MgF₂

Neutral compound

Magnesium fluoride

No charge is present

Table 2

Complete the following table by pasting the model for the following ionic compounds Label them with chemical formula and ionic charge.

#	Elements	Model for the neutral compound	Formula for the neutral compound
1	Ca and Br
2	Ca and O
3	K and S
4	Al and Br
5	K and N
6	Al and O
7	Mg and N

TASK 3

Dissociation of Ions and Conductivity in water (aqueous solutions)

Purpose

To illustrate the reverse process of ionic bond formation in Task 2. Most of the ionic compound dissolve in water and form solvated (hydrated) free ions back. When free ions are available in a solution it can act as an electrolyte to conduct electricity. The more the ions present higher the conductivity of the electrolyte solution.

Materials Required

Multimeter to measure conductivity (we will measure resistivity instead of conductivity and would calculate conductivity as the inverse), well-plates to hold solutions, pipet, paper towel, gloves, plain white paper strips to smear the solutions on, a pencil for labelling, and the following solutions/materials; Copper metal strip, Iron metal strip, corn starch, Bromothymol blue, Bromophenol blue, Red food coloring, Hydrochloric acid, Copper sulfate, Zinc nitrate, oil( optional-if you have at home), pencil lead.

Method

Wear gloves. Take 10 drops of each solution mentioned in the following table in separate clean wells of the bigger well-plate. Connect the multimeter to the black and red leads. Turn the knob to 2000kΩ.

Turn the second well plate upside down to use as a platform to streak the solution as shown in the first picture. Otherwise take a white aper strip and lay it on top of the plate, label with a pencil and smear the solutions on it.

Turn on the multimeter and pin the probes into the solution smear or metal or powder strip. The leads should be at least one centimeter apart as shown above. Record the resistivity value displayed. It may be unsteady, but take a rough estimate and record as the resistivity. Say the value is jumping between 450 and 550 kΩ, write it as 500 kΩ.

Before putting the probe in the next material solution wash the probe and dry with a paper towel. If there is very little or no resistivity (highly conductive)it will show 0.00, If it shows “1” that means the resistivity is very high (no conductivity) and is an insulator.

Lower the resistivity value higher the conductivity, as both are inversely related. Remember free flowing ions/charges in a material makes it conductive. Take pictures of two of the following measurements (not the ones shown in the following pictures) and paste it below the following pictures.

Data table

No	Solutions	Resistivity kΩm	Conductivity	Conductivity 1/resistivity value 1/(kΩm)
1	Touch the end of the red and black leads together	It should give a zero	Yes	NA
2	Copper strip			NA
3	Iron strip			NA
4	Corn starch			NA
5	Red food coloring	Example- 61 kΩm		1/61 /kΩm =0.016/ kΩm
5a	Red food coloring (repeat)
6	Tap water
7	Bromophenol blue
8	Bromothymol blue
9	Copper nitrate
10	Zinc nitrate
11	Hydrochloric acid
12	Oil

Questions

Which of the solution showed the highest conductivity? Why?
Which of the solutions showed the least conductivity? Why?
Show the dissociation of the ionic compound NaCl into hydrated ions in water using an equation.
Complete the following dissociation process for the following ionic compounds in water

#	Compound	Dissociation equation with appropriate ionic charges
1	NaBr	NaBr(s) à Na⁺ (aq) + Br^-(aq) *aq-dissolved in water (aqueous)
2	CaCl₂	CaCl₂(s) à Ca²⁺ (aq) + 2Cl^-(aq) *aq-dissolved in water (aqueous)
2	K₂S
3	AlCl₃
4	CO
5	C₁₂H₂₂O₁₁