## Tuesday, 31 December 2013

### Determine electronic configuration

Part 1: Understanding the periodic table
Remember these definitions:

• Relative atomic mass: The average mass of one atom of the element relative to 1/12 the mass of one atom of carbon-12.
• Proton number (atomic number): number of protons in one atom of the element.
• Relative molecular mass: the mass of one molecule of the compound relative to 1/12 the mass of one atom of carbon -12.
• Isotopes are atoms of with the same number of protons but different number of neutrons.

Now, what you need to do know as well:

• The larger number is the relative atomic number (equals to number of proton plus electron). It is also the called mass number or nucleon number
• The smaller number is the proton (atomic) number. And:
• Mass number - proton number  =number of neutrons
• Protons and neutrons are found in the nucleus; electrons are found in the electron shells that orbit round the nucleus.
• Proton has a charge of +1, electron has a charge of -1, neutron has no charge
• Relative mass of proton is 1, relative mass of neutron is 1, relative mass of electron is almost 0.
• For atoms, number of proton = number of electrons
• For ions, number of electrons= number of protons - charge of ions
• Electronic configuration depends on number of electrons, so you look at proton number (the smaller number)

Part 2: What is the charge?

Atoms have no charge.

Ions have charge. In general, the following are the charges:

Group I  --> +1
Group II  --> +2
Group III --> +3
Group V --> -3
Group VI --> -2
Group VII --> -1

Part 3: Electronic configuration
For O level chemistry, remember this:
1. First shell, maximum 2 electrons
2. The rest, maximum 8 electrons.

Try these!
 Question:  What is the electronic configuration of an oxygen atom? Thought process: - atomic number (proton number) = 8 -  first shell 2, left 6 , so 2nd shell 6. Answer: electronic configuration of oxygen : (2,6) Question:  What is the electronic configuration of an oxygen ion? Thought process: - oxygen is in Group VI. It has a charge of -2. -for ions, number of electrons = proton number -charge =8-(-2)=10 -  first shell 2, left 8 , so 2nd shell 8. Answer: electronic configuration of oxygen ion, or O2- : (2,8) Question:  What is the electronic configuration of a calcium atom? Thought process: - atomic number (proton number) = 20 -  1st shell 2, left 18.    2nd shell 8,left 10    3rd shell 8, left 2    4th shell 2. Answer: electronic configuration of calcium : (2,8,8,2) Question:  What is the electronic configuration of a calcium ion? Thought process: -Calcium is in group II and has a charge of +2 - Number of electrons = proton number - charge =20-(+2)=18 -  1st shell 2, left 16.    2nd shell 8,left 8.    3rd shell 8. Answer: electronic configuration of calcium ion or Ca2+ : (2,8,8)

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1. Positive site, where did u come up with the information on this posting? I'm pleased I discovered it though, ill be checking back soon to find out what additional posts you include. Best Electric Water Kettles

2. Hi Nancy,
I am a tutor and I write these notes based on the Singapore GCE O level syllabus.