Part A. You are to research the task below and submit as a written piece of work i) John Dalton proposed his atomic theory in 1808. Outline his theory. (4 marks) ii) Explain which part of Dalton’s atomic theory was later found to be incorrect. (3 marks) iii) Dalton developed a way to measure the relative atomic mass of the different elements. Using examples research and describe the meaning of the term ‘relative atomic mass’. (3 marks)
i) 1) Elements are made up of tiny particles called atoms. 2) Atoms cannot be destroyed or created but can be rearranged during chemical changes. 3) Atoms of a particular element are identical in mass, size, shape and other properties. 4) Atoms of different elements are different in their mass, size, shape and other properties. 5) Atoms cannot be divided into smaller particles.
6) Atoms of different elements combine in small whole-number ratios to form compounds.
ii) 1) One part of Dalton’s atomic theory that was later discovered to be incorrect was the part where atoms could not be divided into smaller pieces. New research has proved that atoms can be sub-divided into sub-atomic particles like protons, neutrons and electrons. 2) Due to the discovery of isotopes it has been discovered that atoms of the same element are not exactly identical. They have different masses due to the number of neutrons. 3) Also it has been discovered that different elements have the same mass. These elements are called isobars. 4) It has been recently discovered that atoms can actually be destroyed by fission processes in atomic reactors or by atomic bombs. 5) Another defect in Dalton’s atomic theory is that not all compounds have small whole-number ratios. They can actually have large whole-number ratios as well.
iii) The relative atomic mass is defined as: “the average mass of the isotopes in a naturally occurring sample of the element, taking into account the proportion of each isotope present.” The symbol for relative atomic mass is Ar. For example naturally occurring chlorine has two isotopes Cl-35 and Cl-37. Cl-35 has a relative abundance (which is the percentage of that isotope on Earth in relation to the total abundance of the atom on Earth) of 75% while Cl-37 has a relative abundance of 25%. To calculate the Ar you must put the above into a formula.
Ar= (75/100×35) + (25/100×37) =35.5
Another example of calculating relative atomic mass is:
For an element E with the naturally occurring isotopes aE, bE, cE, and with the respective abundances of A%, B%, C% etc, the relative atomic mass (Ar) = (A/100 x a) + (B/100 x b) + (C/100 x c)
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