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It is interesting to note that atoms of a given atomic number can have different number of neutrons. Atoms of elements having the same atomic number with different mass numbers are called isotopes


Some examples are listed below:

Isotopes of Hydrogen

Hydrogen atom (Z=1) has no neutrons.

Number of protons = 1

Number of electrons = 1

Number of neutrons = 0

It has been reported that the hydrogen element has atoms with mass number 2 and 3 also i.e.


Isotopes of Chlorine

Nuclear composition of isotopes of chlorine:

Isotopes of Carbon

Nuclear composition of isotopes of carbon:


Table of some elements that exist as mixtures of isotopes

Relative Atomic Mass (Ram) of Isotopes

The relative atomic mass (Ar) is the average mass of an element, taking account of its natural isotopes and their percentage abundance.

The strict definition of relative atomic mass is that Ar = average mass of all the isotopic atoms present in the element compared to 1/12th the mass of a carbon-12 atom.

Example: chlorine consists of 75% chlorine-35 and 25% chlorine-37.

So the relative atomic mass of chlorine is 35.5 or Ar (Cl) = 35.5


Ion Formation

By the loss or gain of electrons a neutral atom is changed to an ion. Ions are charged atoms or a group of atoms. In other words, ions are particles formed by atoms by the donation or acceptance of electrons. Listed below are some elements that attain the octet configuration of Noble gases. Let us see how this happens. Study the given table:

The elements can be grouped thus:

Na, Mg, K, Ca g lose electrons

S, O, F, Cl g gain electrons


Most of these atoms try to attain the configurations of either neon (2,8) or argon (2,8,8). The charge on the cation indicates the number of electrons donated. In an anion it shows the number of electrons gained.


Differences between atoms and ions

The physical and chemical properties of an atom and ion of an element are quite different, as studies have proved.  Let us consider the example of a sodium atom and the sodium ion.


Differences between sodium atom and sodium ion

Ionization energy and electron affinity

  1. Ionization energy

Ionization potential (or ionization energy) is the amount of energy required to remove one or more electrons from the outermost shell of an isolated atom in the gaseous state.

Atom(g) + IE Positive ion(g) + electron(g)

Thus, the ionization energy gives the ease with which the electron can be removed from an atom. The smaller the value of the ionization energy, the easier it is to remove the electron from the atom.


An electron is held in an atom by the electrostatic force of the positively charged protons in the nucleus and the negative charge of the electrons. By supplying enough energy, it is possible to remove an electron from an atom. The element is first brought into the vapour state. Then the electron is removed by supplying energy equivalent to the ionization potential.


Factors affecting ionization energy

  1. a) The inert gases have very high ionization energy, due to the stability of the outer shell. Helium has the highest ionization energy.
  2. b) Within a group, the ionization energy generally decreases with increasing atomic number. Increasing atomic number results in increasing atomic radii.


Thus, the electrons of the outer shell are further away than those of the previous element and can be removed easily.

  1. c) Ionization energy decreases down the group because of increase in the number of shells.

The effective nuclear charge decreases as atomic size increases.

Thus it is easier to pull one electron from the outermost shell of the atom.


Electron Affinity

This is the enthalpy change when 1 mole gaseous atoms gains 1 mole of electrons under standard conditions.

Cl (g) + e- Cl-(g)

The elements in group 7 have the highest electron affinities, they form negative ions easily, as go down the group the electron affinity decreases so reactivity decreases. The second electron affinity is the energy needed to to add an electron to 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions under standard conditions (where standard conditions are 100kpa and 298K).

Cl- (g) + e- Cl2-(g)


This process involves adding a negatively charged electron to a negative ion – naturally this process is endothermic since energy needs to be supplied to overcome the repulsive forces between the negative ion and the negative incoming electron.


Characteristics of isotopes


See also:







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