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TOPICS: PARTICULATE NATURE OF MATTER
CONTENTS:
- Â Â Â Electronic configuration
- Atomic number and mass number (nucleon number)
- Relative atomic mass based on C-12 isotope
- Isotopy of elements
PERIOD 1:Â Â Â ELECTRONIC CONFIGURATIONÂ
Electrons are found revolving around the nucleus of an atom in circular paths known as rings, orbits, energy levels or shells. Each shell contains electrons with similar energy. Those with the lowest energies being nearest to the nucleus.
Thus, the arrangement of electrons in the atom according to energy is called ELECTRONIC CONFIGURATION. Letters and figures are associated with these orbits or shells as shown above. The maximum possible number of electrons that can be accommodated in a shell is given by the formula:
Nmax= 2n2. Where Nmax = Maximum no of electron. n= no, of shell.
Thus K- shell can contain 2 × 12 = 2 electrons.
L- Shell can contain 2 × 22 = 8 electrons.
M- Shell can contain 2 × 32 = 18 electrons etc.
The electron structures of the atoms of the first twenty elements are given in the table below.
| Element | Symbol | Number of protons (or atomic number) | Number of electrons and their distribution in the shells.                                 K          L         M        N |
| Hydrogen | Â H | 1 | 1 |
| Helium | He | 2 | 2 |
| Lithium | Li | 3 | 2Â Â Â Â Â Â Â Â Â Â Â 1 |
| Beryllium | Be | 4 | 2Â Â Â Â Â Â Â Â Â Â Â 2 |
| Boron | B | 5 | 2Â Â Â Â Â Â Â Â Â Â Â 3 |
| Carbon | C | 6 | 2Â Â Â Â Â Â Â Â Â Â Â 4 |
| Nitrogen | N | 7 | 2Â Â Â Â Â Â Â Â Â Â Â 5 |
| Oxygen | O | 8 | 2Â Â Â Â Â Â Â Â Â Â Â 6 |
| Fluorine | F | 9 | 2Â Â Â Â Â Â Â Â Â Â Â 7 |
| Neon | Ne | 10 | 2Â Â Â Â Â Â Â Â Â Â Â 8 |
| Sodium | Na | 11 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â 1 |
| Magnesium | Mg | 12 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â 2 |
| Aluminium | Al | 13 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 3 |
| Silicon | Si | 14 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 4 |
| Phosphorus | P | 15 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 5 |
| Sulphur | S | 16 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 6 |
| Chlorine | Cl | 17 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 7 |
| Argon | Ar | 18 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 8 |
| Potassium | K | 19 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â 1 |
| Calcium | Ca | 20 | 2Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â Â Â Â Â Â Â Â 8Â Â Â Â Â Â Â 2 |
The electronic configurations of some elements are shown below:
Hydrogen, H (atomic number 1)
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