Evolution of the Periodic Table Flashcards
Why is classification of elements needed
- To organize their knowledge in a systematic way
2. To be able to predict new elements for undertaking further study
Explain Dobereiner’s method of classification
Johann Dobereiner introduced triads - sets of three elements where the second element had an atomic mass = average of the other two and also properties between the other two
Give examples of the Triads
- Ca, Sr, Ba
- Li, Na, K
- Cl, Br, I
Explain Chancourtois method of classification
E.A.B Chancourtois arranged the elements in order of increasing atomic weight and put them in a cylindrical table to show recurrence of properties
What was the limitation of Dobereiner’s method
It only worked for a few elements.
What was the limitation of Chancourtois method
It did not gather much attention
Explain Newland method of classification
John Alexander Newland arranged the elements in order of their increasing atomic weight and noticed that every eighth element showed properties similar to the first - like the octaves (Hence, this was called the Law of Octaves)
What was the limitation of Newland method
It only held for elements upto calcium
Explain Mendeleev method of classification
Mendeleev, at the same time as Lothar Meyer, proposed that elements on arrangement in increasing order of atomic mass, show similarity in physical and chemical properties
Explain Lothar Meyer method of classification
Lothar Meyer, at the same time as Mendeleev, proposed regularity when arranged in order of increasing mass. He also plotted graphs like atomic volume-mass, MP-mass, BP-mass and so on.
He also observed a change in repeating length in the periodicity of these properties.
Periodic Law
The properties of elements are a periodic function of their atomic weights
Discuss Mendeleev’s table
- Elements were arranged in rows and columns such that elements with similar properties were in the same column.
- It had eight columns (I, II, III, IV,…, VIII)
- It had seven rows (some split into two or more sub rows)
- He mainly used the empirical formulas and properties of the compounds formed by the elements to group them
What are some issues with Mendeleev’s periodic table
- The placement of some elements ignores the ‘increasing mass’ rule so as to be a part of a group with similar properties - for example, Iodine which has lesser mass than Te (group 6) is still placed in group 7 due to similarity in properties with F, Cl and Br
What are some advantages of Mendeleev’s table
He left some spaces empty stating that those elements were undiscovered yet. For example, in his table, Eka-aluminum and Eka-silicon correspond to modern day Ga and Ge. Mendeleev was very close in predicting their mass values and some of their physical and chemical properties
Modern Periodic Law
The properties of elements are periodic functions of their atomic numbers
Explain Moseley method of classification
Henry Moseley observed regularities in the characteristic X Ray spectra of these elements. He plotted a graph root(v) - Z, where v is frequency of X Rays emitted and observed that it gave a straight line, not the graph with atomic mass
Discuss Modern table
- The long form of the periodic table is the accepted and most convenient form of the table.
- There are 7 rows (periods) and 18 columns (groups/families)
- Elements with the same outer electronic configuration are placed in the same column
- The period number corresponds to the highest principal quantum number of the elements
- 14 elements of the 6th and 7th period - lanthanoids, actinoids, are placed at separate planels
Number of elements in each period
First period has 2
Then 8, 8, 18, 18, 32
The seventh period also has 32 according to the quantum theory calculations but not all elements have been discovered yet
Nomenclature of elements with Z > 100
- Name = _ + _ +_ + -ium
- 0 - nil,(n) 1 - un(u), 2 - bi(b), 3 - tri(t), 4 - quad(q), 5 - pent(p), 6 - hex(h), 7 - sept(s), 8 - oct(o), 9 - enn(e)
- Symbol - first letters
- For example, 124 - Un + bi + quad + ium = Unbiquadium (Ubq)
Electronic configuration
Distribution of electrons into orbitals of an atom
Electronic configurations and period relationship
- Period number = highest principal quantum number
2. Number of elements in a period = 2 * number of orbitals available in that period
Discuss the periods of Modern PT
- First has only 2 elements (1s orbital)
- Second and third has 8 (1 s orbital and 3 p orbitals)
- Fourth and fifth has 18 (1 s orbital and 3 p orbitals + 5 d orbitals from the previous shell)
- Sixth and seventh has 32 (1 s orbital and 3 p orbitals + 5 d orbitals from previous shell + 7 f orbitals from the shell before that)
Electronic configurations and group relationship
All elements in a group have same outer electronic configuration
For example, group 15 all have ns2np3
Block chemistry
- The periodic table is classified into 4 blocks (s, p, d, f) depending on the type of atomic orbital that is filled
What are the two exception wrt block chemistry
- Hydrogen is placed separately because:
*it has 1 s electron and can be placed in group 1
*it can lose one electron to give noble gas
configuration and hence can be placed in group
17 - Helium is placed in group 18 even though it has two s electrons and should be placed in group 2, because of its noble gas configuration and similarity in properties to the other noble gases
Discuss s block elements
- They constitute of group 1 and 2 elements - they have ns1 and ns2 configurations
Group 1 - alkali metals
Group 2 - alkaline earth metals - They readily lose electron to form +1/+2 ions
- They are all reactive metals with low IE
- They are so reactive that they are rarely found pure in nature
- Metallic character and alkaline nature increase down the group
- Except with Li, Be, all compounds formed are mainly ionic
Discuss p block elements
- They constitute of group 13-18 elements - they have configuration ranging from ns2np1 tp ns2np6
- Group 18 consists of the noble gases which are incredibly stable (fully filled shell) and hence show very low reactivity
- Group 17 - halogens, Group 16 - chalcogens
- The above two groups have highly negative EGE and readily lose electrons to form noble gases
- Group 15 - nicogens
- Non metallic character increases from left to right and decreases down the group
Discuss d block elements
- They constitute of group 3-12 elements and have configurations ranging from (n-1)d0-10 ns2
- They are all metals
- They form colored ions, exhibit variable OS, Para magnetism and are often used as catalysts.
- Zn, Cd, Hg which have d10 configuration do not show any of these properties
- The d block elements form a bridge between the chemically active s block to the less active p block and hence are called transition elements
Discuss f block elements
- They constitute of the two rows of elements at the bottom of the periodic table
- Lanthanoids - Ce(58) to Lu(71)
- Actinoids - Th(90) to Lr(103)
- Electronic configuration - (n-2)f0-14 (n-1)d0-1 ns2
- These two series of elements - inner transition elements
- They are all metals
- Elements after uranium - transuranium elements
Why do we not know much about the actinoids
- They are more complicated because of the number of oxidation states they show
- They are radioactive
- They have been made only in nanogram quantities by nuclear reactions and their chemistry is not fully studied yet
Discuss metallic character in the periodic table
Metals comprise 78% of elements and appear on the left side of the periodic table.
Physical properties of metals
- They have high MP, BP (Cs, Ga have low MP)
- They are malleable and ductile and also good conductors of electricity
- They are solid at room temperature (except Hg)
Discuss non metals in the periodic table
They appear on the right side of the periodic table
Non metallic character increases as we move from left to right
Physical properties of non metals
- They have low MP, BP (except B, Diamond)
- They are brittle and neither malleable nor ductile
- They are generally solids or gases at room temperature
- They are poor conductors of heat and electricity (except graphite)
What are semi metals
Semi metals or metalloids are elements that show properties of both metals and non metals
eg: Si, Ge, As, Sb, Te
