Periodic Classification of Elements

The organization of all elements known to man.

Made by: Karma Dolkar, Anmol Pant, Kavya

Mishra and Harshul Malik {XC}

Why Do We Need Classification?

• With the passage of time, new elements are being discovered. To study them in an orderly manner and to predict their properties easily, we require classification. The following scientists have made some significant contributions in the achieving of the same:

• Johann Wolfgang Dobereiner• John Newlands • Mendeleev• Henry Moseley

Early AttemptsDobereiner’s Triads• In 1817, Dobereiner, a German

scientist, tried to arrange the elements with similar properties into groups. He called these ‘triads’.

• If placed in the order of their increasing atomic masses, Atomic mass of middle element = Atomic mass of 1+ Atomic mass of 2/2

• A major drawback was that only 3 triads were formed. Nevertheless, his attempt was lauded.

Newlands’ Law Of Octaves• In 1866, John Newlands, an

English scientist, placed elements in the increasing order of their atomic masses.

• He found that every 1st element had properties similar to every 8th one, thus resembling octaves in music.

• Drawbacks: Applicable up to Ca only, assumption that only 56 elements existed, placing of unlike elements together.

Major Contribution Mendeleev is regarded as the main

contributor. When he started his work, only 63 elements were known. He took the compounds that elements made with oxygen and hydrogen as one of the basic criteria. He then pinned the elements according to their similarity on a wall. He noticed a periodic recurrence and gave the Periodic Law: ‘The properties of an element are the periodic functions of their atomic masses’.

Mendeleev’s Periodic Table Mendeleev realized that the

physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.

Gaps and predictions Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered. He was also able to work out the atomic mass of the missing elements, and so predict their properties.

Advantages of Mendeleev’s Table

Systematic arrangements of 63 elements in groups and periods.

corrected doubtful masses of some elements like Be, Pd, Pt etc.

Predicted properties of elements which were not discovered -like scandium , gallium , germanium , which were correct and left gaps for them.

Limitations of Mendeleev’s

Table The position of hydrogen was not correctly defined. It was placed in group I although it resembles both the group I elements - the alkali metals and the group VII elements-the halogens, in their properties. 

In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6). 

Isotopes were not given separate places in the periodic table although Mendeleev's classification is based on the atomic masses. 

Some similar elements were grouped separately while some dissimilar elements were grouped together. For example copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group. 

Mendeleev could not explain the cause of periodicity in the elements.  The position for lanthanides and actinides were not included in this table.

Modern Periodic Table

Periodic TableThe periodic table organizes the elements in a

particular way. A great deal of information about an element can be gathered from its position in the

period table.

For example, you can predict with reasonably good accuracy the physical and chemical properties of the element. You can also predict what other elements a

particular element will react with chemically.

Understanding the organization and plan of the periodic table will help you obtain basic information

about each of the 118 known elements.

Key to the Periodic TableElements are organized

on the table according to their atomic number, usually found near the top of the square.The atomic number

refers to how many protons an atom of that element has.

For instance, hydrogen has 1 proton, so it’s atomic number is 1.

The atomic number is unique to that element. No two elements have the same atomic number.

Valency- The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.

LEFT TO RIGHT : When we move from left to right first the valency increases from 1 to 4, then it decreases from 4 to 0. TOP TO BOTTOM : Valency remains the same.

TRENDS IN THE MODERN PERIODIC TABLE

Atomic size – The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of the isolated atom. (picometer, 1pm = 1×10−12 m). Example -atomic radius of hydrogen atom is 37 pm.

FROM LEFT TO RIGHT : Moving from left to right across a period, electrons are added one at a time to the outer energy shell. Electrons within a shell cannot shield each other from the attraction to protons. Since the number of protons are also increasing, the effective nuclear charge increases across a period. This causes the atomic radius to decrease. FROM TOP TO BOTTOM : Moving down a group in the periodic table, the number of electrons and filled electron shells increases, but the number of valence electrons remains the same. The outermost electrons in a group are exposed to the same effective nuclear charge, but electrons are found farther from the nucleus as the number of filled energy shells increases.

Metallic properties The metals like Na, Mg are towards the left-hand side of the periodic table. As metals tend to lose electrons while forming bonds, that is, they are electropositive in nature. As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease . Down the group , the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus .therefore they can be lost easily. Hence metallic character decreases across a period and increases down a group.

ElectronegativityElectronegativity measures an atom's tendency to attract and form bonds with electrons. This property exists due to the electronic configuration of atoms. Most atoms follow the octet rule (having the valence, or outer, shell comprise of 8 electrons). Because elements on the left side of the periodic table have less than a half-full valence shell, the energy required to gain electrons is significantly higher compared with the energy required to lose electrons. As a result, the elements on the left side of the periodic table generally lose electrons when forming bonds. Conversely, elements on the right side of the periodic table are more energy-efficient in gaining electrons to create a complete valence shell of 8 electrons. The nature of electronegativity is effectively described thus: the more inclined an atom is to gain electrons, the more likely that atom will pull electrons toward itself.

From top to bottom down a group, electronegativity decreases. This is because atomic number increases down a group, and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.

From left to right across a period of elements, electronegativity increases. If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.