5.2.1 Formation of Ion
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Formation of Ion
- An ion is an atom or group of atoms carrying a positive or negative charge. Example Ca2+, O2-, SO42- etc.
- If a particle has equal numbers of protons (+) and electrons (-), the particle charge is zero, and the particle is said to be neutral.
- In a chemical reaction, electron(s) can be transferred from atom to another atom.
- If electron(s) is removed from an atom, the number of protons will be more than the number of electrons. In this case, the atom will have excess positive charge and hence form a positive ion (cation).
(An atom losses electron to form positive ion) - If an atom gains electrons, there is an excess negative charge in the atom, so a negative ion is formed.
- In other words,
- The atom losing electrons forms a positive ion (cation) and is usually a metal.
- The atom gaining electrons forms a negative ion (anion) and is usually a non-metallic element.
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| (An atom gain electron to form negative ion) |
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Formation of Positive Ion
- Usually, the atom of metals consists of 1, 2 or 3 valence electrons.
- In order to achieve octet electrons arrangement, the atoms will release the valence electrons.
- After releasing the valence electrons, the protons in the nucleus will outnumber the electrons. As a result, positive ions formed.
Example: Formation of Ion with +1 Charge
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| (A sodium atom release one electron to form a sodium ion with +1 charge) |
Example: Formation of Ion with +2 Charge
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| (A magnesium atom release one electron to form a sodium ion with +2 charge) |
Example: Formation of Ion with +3 Charge
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| (An aluminium atom release one electron to form a sodium ion with +3 charge) |
Formation of Negative Ion
- Negative ions are formed among atoms of non-metal elements except group-18 elements.
- Non-metal elements usually located in group 15, 16 and 17 in the periodic table and have 5, 6 or 7 valence electrons.
- To achieve octet arrangement of electrons, their atoms will receive 3, 2 or 1 electron.
- As a result, the electrons will outnumber the protons and hence negative ions formed.
Formation of Negative Ions of -1 Charge
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| (A fluorine atom receives one electron to form a fluoride ion with -1 charge) |
Formation of Negative Ions of -2 Charge
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| (An oxygen atom receives two electrons to form an oxide ion with -2 charge) |
Difference Between an Atom and an Ion That Have Same Electrons Arrangement
Difference Between Fluoride Ion and Neon Atom
- After forming an ion, the electron arrangement of the ion is similar to the noble gases.
- For example, the electron arrangement of fluoride ion is 2.8 which is similar to a Neon atom, 2.8.
- Fluoride ion and the neon atom have similar electron arrangement.
- Fluoride ion carries charge whereas neon atom is neutral.
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| (The electrons arrangement of a fluoride ion is the same as a neon atom. However, they are so much different chemically) |