Basic Chemistry for High-Schoolers - Valency and Ions
Valency is essentially the number of electrons an atom can give away or receive to become stable and to react. For metals it is the number of electrons in the outer shell it gives away. For non metals it's different. Naturally non metals do not give away electrons but rather they opt to receive electrons from metals. For non- metals their valency is the number of electrons they can receive before being stable. Think of it as ( 8 - the number of electrons in the outer shell). I recommend that you do not use this to solve related questions but rather mull on it to get an idea of whats happening.The electrons in the last shell are referred to as valance electrons as they are the electrons commonly used in reactions.
"The words valence and valency have two related meanings in chemistry.
- Valence describes how easily an atom or radical can combine with other chemical species. This is determined based on the number of electrons that would be added, lost or share if it reacts with other atoms.
- Valence is denoted using a positive or negative integer used to represent this binding capacity. For example, common valences of copper are 1 and 2" from http://chemistry.about.com/od/electronicstructure/a/Valences.htm
All elements or radicals that give away electrons have a positive(+) sign on the left of the number. All metals give away electrons and hydrogen also gives away an electron.
All elements or radicals that receive electrons have a negative(-) sign on the left of the number. All non metals receive electrons except for hydrogen (usually) as it has an isotope of it; hydride to receive electrons.
The number next to the symbol (+/-) indicates how many electrons it can give or take.
If an element does not have a capacity to receive or give a certain amount of electrons you can have multiple of those atoms until you have enough to receive the amount or give away the amount.
e.g 3O + 6e- —> 3O2-
One oxygen atom would only have been able to receive 2 electrons whereas 3 can receive 6 ( 3 x 2=6).
4Al - 12e- —> 4Al3+
One aluminium atom would only have been able to give away 3 electrons whereas 4 can give away 12 ( 3 x 4=12) .
Quick difference between valency and valence electrons
Giving away and receiving electrons
Does this mean Cl1- will receive an electron?
No as Cl1- is chlorine after it has received an electron to become an ion meaning that it has one more electron than normal
e.g O2- means it has two extra electrons
Will Na1+ be able to give away an electron?
No as Na1+ has already given away an electron thus having one less electron than normal.
However there is no such thing as Na2- as sodium can only give away an electron. chlorine will only receive one electron in normal high school questions unless told otherwise.
This is also true for elements like oxygen who receives 2 electrons and aluminium who gives away 3 electrons.
However there are instances where the receiving/giving away of electron changes depending on the question.
There are however, transition metals which are a group of metals that vary on the number of electrons they give. When this happens you are usually told in the question e.g Fe(III)
(III)- means valency of 3
This can be done differently in different schools but for a high school question unless told otherwise assume transition metals in a reaction have a valency of 2 unless there's a bracket e.g Cu(I)- copper with a valency of +1
Cl + e- –> Cl1-
This shows chlorine receiving an electron to become an ion with one more electrons than protons.
Na - e- —> Na1+
This shows sodium giving away an electron to become an ion with one less electron than protons.
There is nothing like this Na1+ + Cl —> NaCl
But there can be something like this Na1+ + Cl1- —> NaCl This is mainly used as an explanation for Na+Cl —>
For something like aluminium to react with oxygen you need to make sure they have enough atoms to create a neutral compound. Aluminium has a valency of +3 while oxygen has a valency of -2. Now find the LCM which is 6. 3 x 2=6 and 2 x 3 =6. This means that you need 2 atoms of aluminium and 3 atoms of oxygen resulting in the chemical formula Al2O3.
What is an ion?
"An ion is a charged atom or molecule. It is charged because the number of electrons do not equal the number of protons in the atom or molecule. An atom can acquire a positive charge or a negative charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.
When an atom is attracted to another atom because it has an unequal number of electrons and protons, the atom is called an ION. If the atom has more electrons than protons, it is a negative ion, or ANION. If it has more protons than electrons,it is a positive ion." from www.qrg.northwestern.edu
An ion is basically an atom or molecule with a charge. This charge can be either negative or positive. When an atom or molecule gives away or receive electrons to participate in a reaction then it is known as an ion. Ions also have more or less electrons than protons. The number of electrons more or less than the number of protons is the number next to a charge in an ion. These numbers can also indicate how many electrons they have received or given away.
e.g Cl-1 has one more electron than protons and has received one electron
An atom usually gives away or receives enough electrons to have the electron configuration of it's period's or row's noble gas.
The charge (+/-) indicates whether or not it has received or given away an electron. If an ion has a positive sign then it is a cation. A cation is an ion with a positive charge indicating that it has given away a certain amount of electrons and now has a certain amount of electrons less than the number of protons. To find the number of electrons all you have to do is minus the number of protons by the number next to the charge. A cation is formed through oxidation which is the process of giving away electrons. Metals and hydrogen's can give away electrons to be a cation.
e.g Na - e- —> Na1+
If an ion has a negative sign then it is an anion. An anion is an ion with a negative charge indicating that it has received a certain amount of electrons and now has a certain amount of electrons more than the number of protons. To find the number of electrons all you have to do is add the number of protons by the number next to the charge. An ion is formed through reduction which is the process of receiving electrons or a reduction of the oxidation state. Non-metals can receive electrons to become anions
Br - e- —> Br 1-
Radicals are also ions hence why they can be called poly-atomic ions. Radicals can have either positive charges or negative charges. The charge and number next to the charge depend on the insufficient/excess amount of electrons or not bonded electrons which is determined by the valency of its elements.
e.g CO3 2-
Carbon has a valency of +4 in this reaction and oxygen has a valency of -2. Because there are 3 oxygen's the total becomes -6. You can think of carbon having 4 less electrons and oxygen having 2 less electrons each for a total of 6. The radical then gets its charge through the electrons.
This is how I do it: +4 ( Carbon) -6 ( 3 oxygen) = -2
Now the radical has a negative charge with 2 extra electrons or a valency of -2. You can think of it as carbon having 4 less electrons and the 3 oxygen having a total of 6 extra electrons. Then when they bond to make a radical they have 2 extra electrons (6 -4 = 2).
If there was one less oxygen then the carbon and 2 oxygen's could react to form carbon dioxide; a neutral compound.
For a radical with a positive charge like NH4 adjust the method. Nitrogen has 3 extra electrons and 4 hydrogen have 4 less electrons. So -3 + 4 = +1 meaning ammonium has a valency of +1. Think of it as nitrogen having 3 extra electrons and 4 hydrogen's requiring / having 4 less electrons meaning that that when they bond they need one more electron to become neutral or that ammonium has one less electron ( 4 - 3 =1 ).
If there was one less hydrogen the the nitrogen and hydrogen's could react to form ammonia; a neutral compound.
Anions and Cations
Whats an ion?