INTRODUCTION

• NAME : JEET SHAH• CLASS : 9TH B• ROLL NO : 12• SUB : CHEMISTRY• TOPIC : FORMATION OF IONS

FORMATION OF IONS

WHAT IS ION ?• An ion is a charged atom or

molecule. It is charged because the number of electrons do not equal the number of protons in the atom or molecule. An atom can acquire a positive charge or a negative charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.

How do atoms form ions?• Atoms have an equal number of

protons and electrons and so do not have an overall charge.

• Atoms with incomplete outer electron shells are unstable. By either gaining or losing electrons, atoms can obtain full outer electron shells and become stable, just like the noble gases.

• When this happens, atoms have an unequal number of protons and electrons and so have an overall charge. This is how atoms become ions.

What Are Negative And Positive Ions?

• Ions are all around you right now and, whether you are aware of them or not, there is a good chance they are affecting the way you feel and quite possibly your health as well.

• So what is an ion? An ion is a molecule that has lost or gained an electron through various atmospheric forces or environmental influences.

• There are both positive and negative ions.

Cations and Anions

• A cation is an atom that has lost a valence electron and therefore has more positive protons than negative electrons, so it is positively charged.

• An anion is an atom that has gained a valence electron and is negatively charged.

Formation of monatomic ions

• Atoms consist of electrons moving around a nucleus. When some phenomena occur (chemical transformations, mechanical friction, exposure to radiations) the atom may lose one or more electrons.

• An electron loss also corresponds to a loss of negative charges that leads to the formation of an electrically charged compound: it is an ion. The electrons lost by one atom can not subsist alone for very long time and are quickly captured by another atom that gains negative charges and also loses its electrical neutrality: this atom is therefore also transformed in ion.

Formation of a cation• A cation is a positive ion that is formed from an atom that

loses some of its electrons.Indeed, after a loss of electrons, the negative charges become less numerous than the positive charges.

• Example: formation of the aluminum ionThe aluminum atom is composed of:- 13 electrons which carry a total of 13 negative charges.- 13 positive charges in its nucleus.This atom can lose 3 electrons to become an aluminum ion which is then made of :- 13 - 3 = 10 electrons which carry a total of 10 negative charges.- 13 positive charges in its nucleus (the nucleus is not affected by the loss of electrons)If we compare the positive and negative charges it can be seen that the aluminum ion has three positive charges in excess (13 positive charges and 10 negative charges).This excess charge is denoted by a superscript (top right) in the formula of the aluminum ion : Al3+ .

Formation of an anion• An anion is a negative ion that is formed from an atom

that gains electrons.Indeed, after a gain of electrons, the negative charges become more numerous than positive charges.

• Example: formation of the chloride ionThe chlorine atom is composed of:- 17 electrons which carry a total of 17 negative charges.- 17 positive charges in its nucleus.This atom can gain an electron to become a chloride ion which is then made of :- 17 + 1 = 18 electrons which carry a total of 18 negative charges.- 17 positive charges in its nucleus.If we compare the positive and negative charges we see that the chloride ion has a negative excess charge (17 positive charges and 18 negative charges ).The formula of chloride ion is: Cl- .

Polyatomic ions• Ions can also be formed from

molecules whose atoms lose or gain one or more electrons. These ions are then called polyatomic ions.Examples of polyatomic ions:- The sulfate ion whose formula is SO4

2-

- The carbonate whose formula is CO3

2-