- Matter around us is present in the form of elements, compounds and mixtures.
- Elements are substances containing atoms of only one type. E.g., Na, Mg, Au, etc.
- There are 118 elements known to us. All these have different properties.
- To make the study of these elements easy, these elements have been divided into few groups in such a way that elements in the same group have similar properties.
- When elements were arranged in the order of increasing atomic masses, groups of three elements (known as traids), having similar chemical properties are obtained.
- The atomic mass of the middle element of the triad was roughly the average of the atomic masses of the other two elements.
- Limitations of Dobereiner’s Traids
Only three traids were recognized from the elements known at that time.
(i) Li, Na, K
(ii) Ca, Sr, Ba
(iii) Cl, Br, I
Newland’s Law of Octaves
- Newland arranged the then known elements in the order of increasing atomic masses and found that the properties of every 8th element is similar to that of the 1st element.
- He compared this to the octaves found in music and called it the ‘Law of Octaves’. For example, the properties of lithium (Li) and sodium (Na) were found to be the same.
Limitations of Newland’s Law of Octaves
- It was applicable upto calcium (for lighter elements only).
- Properties of new discovered elements did not fit into the law of octave.
- To fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties.
Mendeleev’s Periodic Table
- When elements are arranged in the order of increasing atomic masses, the element with similar properties occur at regular intervals.
- The properties of elements are a periodic function of their atomic masses.
- Mendleev’s periodic table is based on the chemical properties of elements. It contains 7 periods (horizontal rows) and 8 groups (vertical columns).
Merits of Mendeleev’s Periodic Table
- Some gaps were left for the undiscovered elements like gallium (Ga), Scandium (Sc) and Germanium (Ge).
- Predict properties of elements on the basis of their positions in the periodic table.
- Accommodate noble gases when they were discovered without disturbing the original arrangement.
Limitations of Mendeleev’s Classification
- Position of isotopes could not be explained.
- No fixed position for hydrogen.
- Wrong order of atomic masses of some elements could not be explained.
Modern Periodic Table
- Atomic number of an element is a more fundamental property than its atomic mass.
- According to the Modern Periodic law : The properties of elements are a periodic function of their atomic number.
- All the anomalies of Mendeleev’s classification disappear.
Explanation of the Anomalies by Modern Periodic Table
- Explanation for the position of isotopes (Same atomic number put at one place in the same group).
- Cobalt with atomic number 27 came first and nickel (28) should come later.
- Unlike atomic masses, atomic number is always a whole number, so there is no element between hydrogen and helium.
- Atomic Number: It is denoted by Z and equal to the number of protons in the nucleus of an atom.
- Modern Periodic table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.
- Elements with same number of valence electrons are placed in the same group.
Li : 2, 1
Na : 2, 8, 1
K : 2, 8, 8, 1
- Outermost or valence shell in all the three contains 1 electron. These elements have been placed in the same group.
- Number of shells increases as we go down the group.
- Elements with same number of occupied shells are placed in same period.
- For example, Li (2, 1); Be (2, 2); B (2, 3), C (2, 4), N(2, 5). These elements have same number of shells (two).
- Each period marks a new electronic shell getting filled.
- Number of elements placed in a particular period depends upon the fact that how electrons are filled into various shell.
- Maximum number of electrons that can be filled in a shell is given by 2n2 where n is shell number.
K shell n = 1 or 2n2 = 2(1)2 = 2 (First period has 2 elements.)
L shell n = 2 or 2n2 = 2(2)2 = 8 (Second period has 8 elements.)
- Position of an element in the periodic table tells us its chemical reactivity.
- Valence electron determine the kind and number of bonds formed by the element.
Trends in the Modern Periodic Table
- Valency: No. of valence electrons present in the outermost shell of its atom. On moving from left to right in each period, the valency of elements increases from 1 to 4 and then decreases to 0.
Valency remains the same down in a group.
- Atomic size: Atomic size refers to the radius of an atom. It may be visualized as the distance between the centre of the nucleus and the outermost shell.
- Atomic size or radius of an atom decreases as we move from left to right in a period because due to large +ve charge on the nucleus, the electrons are pulled in more close to the nucleus and size decreases.
- Atomic size increases as we move down the group because new shells are being added and this increases the distance between nucleus and outermost electron.
- Metallic character means the tendency of an atom to lose electron.
- Metals occupy the left hand side of the periodic table.
- On moving left to right in a period, the metallic character of an element decreases because the effective nuclear charge increases. It means tendency to lose electron decreases.
- Metals are electropositive as they tend to lose electrons while forming bonds.
- Metallic character increases as we go down a group as the effective nuclear charge is decreasing.
- Non-metals are electronegative as they tend to form bonds by gaining electrons.
- Non-metals occupies the right side of the periodic table.
- Non-metallic character increases across a period because due to increase in effective nuclear charge that means tendency to gain electron increase.
- Non-metallic character decreases as we move down a group due to decrease in effective nuclear charge experienced by the valence electron thus the tendency to gain electron decreases.In
- the middle of periodic table we have semi-metals or metalloid because they exhibit some properties of metals and non-metals.
- Oxides of metals are basic in nature while oxides of non-metals are acidic in nature.
Atomic size, Metallic character and non-metallic according to Periodic table