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.
Question: What is the electronic configuration of an oxygen atom?
- 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?
- 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?
- 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?
-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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