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Chemistry
Classification of Elements
| Term | Definition |
|---|---|
| No. of elements discovered. natural ? | 118, 92 are natural |
| Name the scientists and their attempts of classification of elements. | Dobereiner's Triad, Newland's Law of Octaves, Lother Mayer's atomic volume curve, Medeleev's atomic law and atomic table. |
| Dobereiner's Atomic Triad. Triad Property | He suggested that atoms having similar properties can be arranged in groups of 3 called a triad. Here the middle element has the mass which is mean of the 2 element around it. |
| Example of the triad. | Li, Na, K and Cl, Br, I |
| Newlands law of Octaves | He suggested that elements, arranged in increasing order of atomic weight, the eight element starting from one is repetition of firsts like the eight note of octaves in music. |
| Lother Mayor's Atomic volume curve | He suggested that if elements are plotted with respect to their atomic volume, the elements sharing similar properties share similar positions in the graph. Like Alkali metals occupied the highest points. |
| Atomic Volume | Ratio of Atomic mass to their density. |
| Mendeleev's Periodic Law | He states that, " The physical and Chemical properties of elements are the periodic function of their atomic weight". Ie. When elements are arranged in increasing order of atm. weight, their properties change gradually but repeat periodically. |
| Periods and Groups | Periods are horizontal rows and groups are vertical columns. 7 and 18. |
| Anomalies of Mendeleev's periodic table. | 1. Position of H : It resembles both like alkali metals and halogens. So its position in not explained. 2. Position of isotopes: isotopes should be given different position if atomic mass is basis of classification 3. Lanthanides and actinides: Separately |
| 4. Anomalous pairs; some higher mass elements are placed before, 5. separation of similar elements and grouping of dissimilar one, 6.no explanation for periodicity can be given by this law | |
| Discoverer of modern atomic law | Henry Moseley |
| Modern periodic law | States that, " The chemical and physical properties of elements are the periodic function of their atomic number" |
| Explanation of periodicity by MPL | Elements have periodic nature because theiy are arranged in basis of no. of protons. Hence, in certain intervals, the atomic structure can resemble previous structure hence showing similar properties. We can conclude that E.C governs the periodicity. |
| What is periodicity? | Periodicity is the repetation in regular intervals. When elements are arranged in increasing num. of A.Num, they show periodicity. |
| Anomalies of MPT | 1. Position of Hydrogen 2. Position of He 3. Position of Lanthanides and Actinides. |
| S-block | Lass orbit in s orbit. Highly electro+ve , soft metals, low B.P and low M.P, Highly reactive and for ionic compounds, etc. Can be detected by flame test. |
| p-block | Last orbit is p-orbit, Almost all non metals including nobel gasses, mostly form covalent compounds among themselves. |
| d-block | Last orbit is d-orbit, transitional elements, metals, high b.p and high m.p high density, good conductors of heat and electricity, exibit variable Oxidation states, more +ve than p block but less +ve than s block |
| f-block | last orbit is f-orbit, radiocative elements , innertransitional elements. heavy meatals, variable oxidation states, luminascent properties. |
| Bohr's classification on the basis of incomplete shell. | Nobel gasses, representative elements, transitional and inner transitional elements. |
| Screening effect or shielding effect | The phenomena where the nucleus attracts the electrons but the inner electrons repulse the valence electrons. In this way, the valence electrons experiences less amount of nuclear charge. |
| Stater's rule | valence contributes 0.35, penultimate contributes 0.85 and inner shell contributes 1. to shielding constant |
| Atomic radii | The distance between the centre and valence shell of nucleus. |
| It is difficult to determine atomic radius beacause? | i. Electron cloud has no boundary ii. Difficult to isolate atom and measure its radii iii. Size varies with neighbouring atoms and type of bonds |
| Types of atomic radii | i. Covalent radii: Half the distance between two homonuclear diatomic molecule bounded by single covalent bond. ii. Metallic radii: Half distance betw atoms bounded by metallic bond iii. Van der walls radii: 1/2 distance bet 2 nonbonded atm held VWB |
| Strengths of bonds | Covalent> metallic >> Van der Waal's force of attraction |
| IONIC RADIUS | Distance betw centre and a point to which the atom has influence over its electron in an ion. |
| Ionization energy/potential | The minimum amount of energy required to remove the most loosely held valence electron from an isolated gaseous atom in its ground state. For an element IE1 < IE2 < IE3 < .... |
| Factors affecting Ionization energy | 1. Sheilding effect, 2. Atomic radii 3. Nuclear charge and Zeff 4. Penetration effect of e- 5. electronic configuration |
| Electron Affinity (E.A) | The amount of energy released when an e- is added to an isolated gaseous atom in its ground state. |
| Diagonal relationship | IN the periodic table. periods 2 and 3 show similar characteristics with their diagonal element. This is caused due to similar atomic radii and ionic radii. |
| Bridge elements | In Mendeleev's PT those elements that show similar properties with both the subgroups of corresponding groups are called bridge elements. Ex. Na and Mg |
| Differentiating electron | Is the electron that differentiates an element's atom with their corresponding element's atoms in the periodic table. |
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Saksm
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