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Electrone Configuration
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VALENCE SHELL AND VALENCE ELECTRONS:
The outermost shell of an atom is called its valence shell and the electrons that it contains are known as the valence electrons. These are the only electons that are involved in any chemical bonding; they’re either lost, gained or shared but always, only the valence shell and the valence electrons are involved. Also, elements with the same no. of valence electrons show similar chemical properties. This is the reason behind the similar properties of all alkali metals (all have one valence electron).
VALENCY:It is the capacity of the atom of an element to take part in chemical bonding. It tells us how many electrons of an atom are capable of freely interacting with the electrons of otheratoms. *VALENCY = 8 – NUMBER OF VALENCE ELECTRONS (if the number of valence electrons is more than 4) *Valency = the number of valence electrons (if the number of valence electrons is four or less than four)
(*it is applicable only for the first twenty elements. Also, for the first 20 elements, the electron configuration is 2,8,8,8.) |
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| ELEMENT |
SYMBOL |
ATOMIC NUMBER |
ELECTRON CONFIGURATION |
VALENCY |
| HYDROGEN |
H |
1 |
1 |
+1 |
| HELIUM |
He |
2 |
2 |
0 |
| LITHIUM |
Li |
3 |
2,1 |
+1 |
| BERYLLIUM |
Be |
4 |
2,2 |
+2 |
| BORON |
B |
5 |
2,3 |
+3 |
| CARBON |
C |
6 |
2,4 |
4 |
| NITROGEN |
N |
7 |
2,5 |
-3 |
| OXYGEN |
O |
8 |
2,6 |
-2 |
| FLUORINE |
F |
9 |
2,7 |
-1 |
| NEON |
Ne |
10 |
2,8 |
0 |
| SODIUM |
Na |
11 |
2,8,1 |
+1 |
| MAGNESIUM |
Mg |
12 |
2,8,2 |
+2 |
| ALUMINIUM |
Al |
13 |
2,8,3 |
+3 |
| SILICON |
Si |
14 |
2,8,4 |
4 |
| PHOSPHORUS |
P |
15 |
2,8,5 |
-3 |
| SULPHUR |
S |
16 |
2,8,6 |
-2 |
| CHLORINE |
Cl |
17 |
2,8,7 |
-1 |
| ARGON |
Ar |
18 |
2,8,8 |
0 |
| POTASSIUM |
K |
19 |
2,8,8,1 |
+1 |
| CALCIUM |
Ca |
20 |
2,8,8,2 |
+2 |
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ISOTOPES:
When atoms of an element have different nos. of neutrons in their nuclei then they are known to be the isotopes of that element. The same no. of electrons in the atoms of a particular element but different nos. of neutrons results in the atomic number of the element being same but the mass number differing.
Most of the atoms of Hydrogen contain only one proton but there are some such atoms which have either one or two neutrons alongwith the single proton. These are the isotopes of hydrogen namely Deuterium and tritium. Deuterium has one neutron in addition to the proton in the nuclei resulting in its mass no. being two and Tritium has two neutrons in the nuclei resulting in its mass no. being three. The symbols for the isotopes of any element remains the same as the element symbol followed by the mass number. So the symbol for deuterium becomes H2 and for Tritium becomes H3.
NOTE: remember that only hydrogen has the atomic number one and therefore it has just one proton in all its isotopes. In the case of any other element, the no. of protons will differ because of the different atomic no. (all isotopes of helium will have two protons and of lithium will have three). |
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CHARACTERISTICS OF ISOTOPES:
• The number of valence electrons for all the isotopes of an element remain the same resulting in similar chemical properties
• The physical properties show a difference like a slight difference in the densities, melting and boiling points of different isotopes of an element.
It is due to the existence of the isotopes with varying masses that the atomic masses of elements are in fractions. |
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RADIOACTIVITY:
Isobars- atoms of different elements with the same mass number but different atomic numbers are called isobars. E.g. Argon, potassium, and calcium have the same mass no. 40 but different atomic nos. 18, 19 and 20 respectively. Isotones- atoms of different elements having the same number of neutrons but different no. of protons are called isotones. E.g. atoms of carbon, nitrogen and oxygen having the same no. of neutrons 8 but different nos. of protons 6, 7, 8 respectively. |
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