All Students Notes

Electron Affinity  Electron affinity may be defined as the amount of energy released when an electrons add up in the valence shell of an isolated gaseous atom.                   X + e ⎯⎯→ X- + electron affinity  Units   Units of electron affinity are kJ/mole-1 or kJ/mole.  Trend in Periodic Table   In period : Generally, electron affinity values of elements increase from left to right within the period. In fact, that atomic size decreases and nuclear charge (atomic number) gradually increases on moving left to right within the period. As a result, nucleus bind with incoming extra electron more tightly and hence has more electron affinity.   For example, halogens have higher electron affinity than alkali metals in each period because of smaller atomic size and greater nuclear charge than alkali metals.  In group : Electron affinity decreases from top to bottom within the group because s...

Question: What is ionization energy? Give its units. Also explain its trend with reason in periodic table . Answer Ionization Energy  Ionization energy may be defined as the minimum amount of energy required to remove the valence electron from an isolated gaseous atom.   M(g) + ionization energy ⎯⎯→ M(g) + + e-  Ionization energy indicates the attraction between nucleus and outermost electron. A higher ionization energy means stronger attraction between the nucleus and the outermost electron. While a lower ionization energy indicates a weaker force of attraction between the nucleus and the outermost electron.  Units Units of ionization energy are kJ/mole-1 or kJ/mole.  Trends in Periodic Table  In period: Generally, ionization energy of elements increases from left to right within the period. Reason is that atomic size decreases and nuclear charge (atomic number) gradually increases by one unit in each element on moving lef...

Question: What is meant by term atomic size? Give its trend in periodic table. Answer . Atomic Size The size or radius of an atom is the average distance between the nucleus of an atom and its valence shell.  Trend in Periodic table   In period:  Generally, atomic size decreases from left to right across the period. This is because valence shell remains same in the period and nuclear charge is increased by one unit in each atom as we move from left to right across the period. At the same time, valence electron is added to the same shell which gradually increases the attractive forces between nucleus and valence electrons. As a result, atomic size decreases.  For example, atomic size of beryllium is smaller than lithium. This is because Be (at right side of lithium) has one more proton and electron as compared to lithium and hence more force of attractions. So, it has smaller atomic size as compared to Li.  In group : Generally, at...

Question: What is shielding  effect? Give its trends in the periodic table. Answer . Shielding Effect  The decrease in force of attraction between nucleus  and valence electrons due to presence of inner shell electrons is called shielding effect.  It is also known as screening effect. It increases with increase in inner shell electrons. For example, Mg has greater shielding effect than Be. This is because Mg has eight more inner-shell electrons than Be. Trend in Periodic Table In period As we move from left to right in a period, shielding effect remains constant. Reason is that the number of inner shell electrons remains constant along the period. In group As we move from top to bottom in a group, shielding effect increases. This is due to fact that the number of inner shell increases and hence, inner shell electrons also increase. For example, Na has greater shielding effect than Li because of eight more inner shell electrons than Li...

Example 3.1: Identifying the group and period of following elements on the basis of electronic configuration.  Solution   n = 1 for K  n = 2 for L  n = 3 for M (valence shell) No. of electrons in valence shell= 3  Hence,  Period number = ‘n’ value for valence shell  Period number = 3  Group number = No. of electrons in valence shell  Group number = 3  So, Al is present in 3rd period and 3rd group.   - : -:-:- i.   n = 1 for K   n = 2 for L (valence shell) No. of electrons in valence shell= 3   Hence,  Period number = ‘n’ value for valence shell  Period number = 2  Group number = No. of electrons in valence shell  Group number = 3  So, B is present in 2nd period and 3rd group.                                                ...

Periodic table and periodicity of properties Question 1: Define following terms: Periodic table; periodic  law; periods of elements; groups of elements; short periods and long periods. Answer Periodic table: A table that shows a systematic arrangement of elements is called periodic table. Periodic law: According to this law, the elements are arranged with increasing order of the atomic numbers their properties are repeated in periodic  manners. Periods:   The horizontal rows of periodic  table are known as periods.  Short periods:  First three periods are called short periods.  Long periods:   Periods number 4, 5, 6 and 7 are called Long periods.  Groups:   The vertical columns of periodic tables are known as groups. All elements are divided into two groups.  Question 2: Differentiate between normal a Representative elements and transition elements Answer.   Normal or Representative elements...