CBSE NCERT Solutions Chemistry notes for Class 11 Dalton’s Atomic Theory
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Here, we will discuss Dalton’s Atomic Theory, its merits, and demerits. The following topic is for a better understanding of CBSE Class 11 students (Chemistry, unit 1).
DALTON’S ATOMIC THEORY
John Dalton (1766-1844), a British school teacher, introduced the concept of atoms. In 1808, he presented his atomic theory based on experiments and laws of chemical combination. The postulates of Dalton’s atomic theory are:
- All matter consists of tiny indivisible particles called The atoms cannot be created or destroyed.
- Atoms of the same element are similar in shape and mass but differ from the atoms of other elements.
- The atom is the smallest unit of matter that can take part in a chemical reaction.
- Atoms of one element cannot be converted into atoms of another element, rather during a chemical reaction;the atoms of the original matter recombine to form different substances.
- Compounds are formed from the chemical combination of atoms of different elements in specific ratios.
- Atoms of thesame element can combine in more than one ratio to form two or more compounds.
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Merits of Dalton’s Atomic Theory
The theory is in accordance with the laws of chemical combination. His postulate 1 and 4 explain the law of mass conservation. Postulate 5 supports the law of definite proportion.The law of multiple proportions is supported by postulate 6.
Dalton was the first person to find out the distinction between the fundamental particle of an element (atom) and that of the compound (molecule).
Demerits of Dalton’s Atomic Theory
- Dalton’s atomic theory did not predict the existence of subatomic charged particles, protons, neutrons and electrons. Thus, an atom can be subdivided into the subatomic particles.
- According to Dalton, the atoms of thesame element are similar in all respects. But, atoms of some elements vary in their masses and densities. These atoms are called as For example, chlorine has two isotopes with mass numbers 35 and 37.The isotopes are the atoms of an element that have the same atomic number but different atomic mass. For example, the element carbon with atomic number 6 has 6 protons and 6 electrons, but only 98.89% of naturally occurring carbon atoms have 6 neutrons (making the atomic mass A=12). A small percentage (1.11%) have 7 neutrons making the A= 13. While fewer (less than 0.01%) have 8 neutrons so that A= 14. They all are three naturally occurring isotopes; 12C, 13C and 14C.
- Dalton’s theory says that atoms of different elements are different in all respects. But elements like argon and calcium have asame atomic mass of 40 amu (atomic mass unit). Such atoms are known as isobars. The Isobars are the atoms that have the same number of nucleons/ same atomic mass (sum of protons and neutrons in the nucleus of an atom) but differ in atomic number (number of protons in the nucleus). Some examples are sulphur, chlorine, argon, potassium, and calcium all having 40 amu. The nuclei of these nuclides (atoms) contain 40 nucleons(protons+neutrons)
- Dalton’s theory failed to explain the existence of An allotrope is defined as two or more different physical forms (solid, liquid or gas) in which an element can exist.The allotropes differ each other in their physical and chemical properties.For example, graphite, charcoal, and diamond are all allotropes of carbon.Gaseous oxygen exists in three allotropic forms, monoatomic oxygen (O), a diatomic molecule (O2), and triatomic oxygen (O3) known as ozone. The Dalton’s theory does not account for the differences in the properties of charcoal, graphite, diamond.