Synthesising ideas on formation of ionic bond

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Malaysia SPM Form 4 Chemistry, Chapter 4: Chemical Bond.

Contents 1 Formation of Ion 1.1 Formation of Negative Ion 1.1.1 Difference Between Fluoride Ion and Neon Atom 1.2 Formation of Positive Ion 2 Example 2.1 A Group 1 metal + a Group 7 non-metal 2.2 A Group 2 metal + a Group 7 non-metal 2.3 A Group 2 metal + a Group 6 non-metal 3 Predicting The Formula for Ionic Compound

[edit] Formation of Ion

• An ion is an atom or group of atoms carrying positive or negative charge. Example Ca²⁺, O^2-, SO₄^2- 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 transfered from atom to another atom.
• If electron(s) is removed from an atom, the number of protons will be more than number of electrons. In this case, the atom will has excess positive charge and hence form a positive ion (cation).
• If aton gains negative electrons, there is an excess negative charge in the atom, so a negative ion is formed.
• In other words,

1. The atom losing electrons forms a positive ion (cation) and is usually a metal.
2. The atom gaining electrons forms a negative ion (anion) and is usually a non-metallic element.

[edit] Formation of Negative Ion

Example: Formation of Fluoride Ion

• A fluorine atom has 9 protons and 9 electrons.
• Since the number of protons is equals to the number of electrons, the fluorine atom is neutral.
• The electron arrangement of fluorine atom is 2.7. This is not a stable arrangement of electrons.
• To achieve the stable electron arrangement of noble gases (octet electron arrangement), fluorine need to receive 1 electron from the other atom.

• As shown in the diagram above, after receiving 1 electron, the electron arrangement of fluorine become 2.8, which is an octet arrangement of electrons.
• At the same time, the number of electrons has increased by 1 and become 10 electrons while the number of proton remain unchanged.
• Hence the charge of the fluoride ion is -1.
• The table below shows the difference between a fluorine atom and a fluoride ion in term of its number of proton and electron and its electron arrangement.

Fluorine Atom	Fluoride Ion
Number of proton = 9 Number of electron = 9 Eelctron Arrangement = 2.7 (Not Octet) Charge = 0	Number of proton = 9 Number of electron = 10 Eelctron Arrangement = 2.8 (Not Octet) Charge = -1

[edit] 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.
• By referring to the electron arrangement, sometime, students may mistaken an ion as noble gas.
• We should take notes that even though the fluoride ion and the neon atom have similar electron arrangement, fluoride ion carry charge whereas neon atom is neutral.

[edit] Formation of Positive Ion

Example: Formation of Magnesium Ion

• A magnesium atom has 12 protons and 12 electrons and it is neutral.
• The electron arrangement of magnesium atom is 2.8.2 .
• To achieve the stable electron arrangement of noble gases (octet electron arrangement), magnesium atom need to loss2 electrons.

• As shown in the diagram above, after losing 2 electrons, the electron arrangement of magnesium become 2.8, which is an octet arrangement of electrons.
• At the same time, the number of electrons has reduced by 2 and become 10 electrons while the number of proton remain unchanged.
• Hence the charge of the magnesium ion is +2.
• The table below shows the difference between a magnesium atom and a magnesium ion in term of its number of proton and electron and its electron arrangement.

Fluorine Atom	Fluoride Ion
Number of proton = 9 Number of electron = 9 Eelctron Arrangement = 2.7 (Not Octet) Charge = 0	Number of proton = 9 Number of electron = 10 Eelctron Arrangement = 2.8 (Not Octet) Charge = -1

Ionic Bonding 1. Ionic bonds are formed by one atom transferring electrons to another atom to form ions. Ions are atoms, or groups of atoms, which have lost or gained electrons. 2. The atom losing electrons forms a positive ion (a cation) and is usually a metal. The overall charge on the ion is positive due to excess positive nuclear charge (protons do NOT change in chemical reactions). 3. The atom gaining electrons forms a negative ion (an anion) and is usually a non-metallic element. The overall charge on the ion is negative because of the gain, and therefore excess, of negative electrons. 4. Ions of opposite charge will attract one another, thus creating an ionic bond. 5. The examples below combining a metal from Groups 1 (Alkali Metals), or 2, with a non-metal from Group 6 or Group 7 (The Halogens)

[edit] Example

[edit] A Group 1 metal + a Group 7 non-metal

In terms of electron arrangement, the sodium donates its outer electron to a chlorine atom forming a single positive sodium ion and a single negative chloride ion. The valencies of Na and Cl are both 1, that is, the numerical charge on the ions. NaF, KBr, LiI etc. will all be electronically similar. The atoms have become stable ions, because electronically, sodium becomes like neon and chlorine like argon.

[edit] A Group 2 metal + a Group 7 non-metal

In terms of electron arrangement, the magnesium donates its two outer electrons to two chlorine atoms forming a double positive magnesium ion and two single negative chloride ions. The atoms have become stable ions, because electronically, magnesium becomes like neon and chlorine like argon. NOTE you can draw two separate chloride ions, but in these examples a number subscript has been used, as in ordinary chemical formula. The valency of Mg is 2 and chlorine 1, ie the numerical charges of the ions. BeF2, MgBr2, CaCl2 or CaI2 etc. will all be electronically similar.

[edit] A Group 2 metal + a Group 6 non-metal

In terms of electron arrangement, one Magnesium atom donates its two outer electrons to one oxygen atom. This results in a double positive magnesium ion to one double negative oxide ion. All the ions have the stable electronic structures 2.8.8 (argon like) or 2.8 (neon like). the valency of both calcium and oxygen is 2. MgO, MgS, or CaS will be similar electronically (S and O both in Group 6)

[edit] Predicting The Formula for Ionic Compound

Element that combine				Formula of the ionic compound
Element X from	Charge of the ion	Element Y from	Charge of the ion
Group I	+1	Group V	-3
Group I	+1	Group VI	-2
Group I	+1	Group VII	-1
Group II	+2	Group V	-3
Group II	+2	Group VI	-2
Group II	+2	Group VII	-1
Group III	+3	Group V	-3
Group III	+3	Group VI	-2
Group III	+3	Group VII	-1