Ion
An atom or molecule that is positively (cation) or negatively (anion) charged due to the loss or gain of one or more electrons.
In this lesson, we will learn about what ions are, and how they are formed.
When an atom gains or loses electrons, they develop a charge.
Recall that electrons are negatively charged, so the gain or loss of electrons will create a charge.
These charged particles are called ions.
When an atom loses electrons, they become positively charged.
Positively charged ions are called cations.
An easy way to remember this is to recall that cats have paws.
When an atom gains electrons, they become negatively charged.
Negatively charged ions are called anions.
Let's look at sodium as an example.
Its electronic structure is 2, 8, 1.
It has one electron in its valence, or outermost shell.
When sodium loses this valence electron, it becomes a positively charged sodium ion.
This charge is written as a superscript when writing the formula.
When drawing the electronic structure, enclose it in square brackets, and write the charge on the top right hand corner.
Now, let's look at oxygen as another example.
Its electronic structure is 2, 6.
Therefore, there are six electrons in its valence shell.
Oxygen gains two electrons to form a negatively charged oxide ion.
Remember to include the charge when writing the formula and when drawing the electronic structure.
If we look at the electronic structures of these two ions, we will see that they have a complete outer shell.
We will also see that they have the same electronic structure as that of a noble gas.
The sodium ion and the oxide ion have the same electronic structure as a neon atom.
How are you feeling about ions? Are you ready to form ions on your own? Let's try.
Please pause the lesson and draw the electronic structure of a calcium ion and a chloride ion.
The electronic structure of Ca is 2, 8, 8, 2.
It loses its two valence electrons to form a Ca2+ ion.
The electronic structure of chloride is 2, 8, 7.
It gains an electron to form a Cl- ion.
Both ions also have the same electronic structure as an argon atom.
Did you get the structures right? Did you remember to specify the charges?
What do sodium and Magnesium have in common?
They are both metals, and metals lose their valence electrons to form cations.
Group 1 metals, such as sodium, have one valence electron, and they lose this electron to form ions with a +1 charge.
Group 2 metals, such as Magnesium, have two valence electrons, and they lose these electrons to form ions with a 2+ charge.
Oxygen and Iron are both non-metals – non-metals tend to gain electrons to form anions.
O is in Group 6, and has six valence electrons.
It gains two electrons to form an ion with a -2 charge.
F is in Group 7, and has seven valence electrons.
It gains one electron to form an ion with a -1 charge.
Group 3 elements, such as aluminium, lose three electrons to form ions with a +3 charge.
Group 5 elements, such as nitrogen, gain three electrons to form ions with -3 charge.
Group 4 elements, such as carbon, tend form covalent compounds – stay tuned!
In conclusion, an ion is a charged particle formed from either the gain or loss of electrons.
A positively charged ion is known as a cation, and a negatively charged ion is known as an anion.
The ions formed have the same electronic structure as their nearest noble gas.
In this lesson, you will learn how to test for the presence of carbonate (CO32-), sulphate (SO42-), and nitrate (NO3-) ions.
To test for carbonates, CO32-, add dilute acid, and bubble the gas produced through limewater.
This reaction releases carbon dioxide, which turns limewater milky due to the formation of calcium carbonate.
Since many carbonates are insoluble in water, you will likely be working with solids instead of solutions.
To test for sulphates, SO42-, add a few drops of barium chloride solution.
The white precipitate formed is barium sulphate, noted for its insolubility in water.
Write down the equations for the above reactions and resume the lesson once you are done.
Remember to balance the equations and use the correct state symbols.
Carbonates:
CO32-(s)/(aq) + 2H+(aq) CO2(g) + H2O(l)
Carbonate ions react with acid to produce carbon dioxide gas and water.
Sulphates:
Ba2+(aq) + SO42-(aq) BaSO4(s)
Barium ions and sulphate ions combine to produce a barium sulphate precipitate.
To test for nitrates, NO3-, add sodium hydroxide and aluminium powder and heat gently in a warm water bath.
Aluminium powder reduces the nitrate ion to an ammonium ion.
The ammonium ion then reacts with hydroxide to release ammonia.
NH4+(aq) + OH-(aq) NH3(aq) + H2O(l)
Ammonia can be tested using damp red litmus paper, which turns blue.
You can dip a glass rod in concentrated hydrochloric acid and placed it near the mouth of the reaction vessel.
This should produce ammonium chloride in the form of white smoke if ammonia is present.
The production of carbon dioxide from the addition of dilute acid confirms the presence of carbonates.
If a white precipitate is formed from the addition of an aqueous barium solution, this means that sulphates are present.
Nitrates can be reduced to ammonium ions, which can then react with hydroxide ions to produce ammonia.
Ammonia turns damp red litmus paper blue and forms a white smoke of ammonium chloride in the presence of concentrated hydrochloric acid.
Water impurity is a very big environmental concern.
Pure water contains no dissolved ions, and many tests have been devised to determine the presence of ions in a sample of water.
In this lesson, we will learn how to test for chloride, bromide, and iodide ions.
If we are working with unknown solids, they must first be dissolved in water.
The resulting solution must then be acidified with dilute nitric acid.
This removes any impurities that may be present to give a clear result.
Then, add a few drops of silver nitrate.
Silver halide precipitates are formed and each has a characteristic colour.
If chloride ions are present, the resulting silver chloride precipitate is white.
If bromide ions are present, the resulting silver bromide precipitate is cream.
If iodide ions are present, the resulting silver iodide precipitate is yellow.
It may be difficult to distinguish between the white silver chloride precipitate and the cream silver bromide precipitate.
Addition of dilute aqueous ammonia dissolves silver chloride but not silver bromide.
Therefore, this test is used to distinguish between silver chloride and silver bromide.
Concentrated ammonia dissolves both silver chloride and silver bromide.
Silver iodide dissolves in neither dilute nor concentrated ammonia.
Write down the ionic equations for the precipitation reactions that we have learned about. Resume the video once you are done.
The equations are as follow:
Ag+(aq) + Cl-(aq) AgCl(s)
Ag+(aq) + Br-(aq) AgBr(s)
Ag+(aq) + I-(aq) AgI(s)
Remember that since the silver halides are precipitates, the subscript is (s).
Halide ions can be detected through addition of silver nitrate, which forms a silver halide precipitate.
To conclude:
Silver chloride is white and dissolves in dilute aqueous ammonia.
Silver bromide is cream and only dissolves in concentrated ammonia.
Silver iodide is yellow and does not dissolve in either dilute or concentrated ammonia.