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What role do Chemical Elements play in the Evolution of Symbols?
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- Dalton and the Birth of Elemental Symbols: He was the first scientist to use the symbols for elements.
- Symbols and Quantities in Chemical Elements: When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element.
- Berzilius: He suggested that the symbols be made from one or two letters of the name of the element.
 Symbols proposed by Dalton
- In the beginning, the names of elements were derived from the name of the place where they were found for the first time.
- Example: The name copper was taken from Cyprus.
- Some names were taken from specific colours.
- Example: Gold was taken from the English word meaning yellow.
- Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) is an international scientific organisation which approves names of elements, symbols and units.
- Many of the symbols are the first one or two letters of the element’s name in English.
- Example: Hydrogen- H, Aluminium – Al, Cobalt – Co.
- Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name.
- Example: Chlorine – Cl, Zinc – Zn etc.
- Other symbols have been taken from the names of elements in Latin, German or Greek.
- Example: The symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.
 Symbols for some elements
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How are chemical Elements defined through their Atomic Numbers?
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- It is defined as the number of protons present in the nucleus of an atom.
- All atoms of an element have the same atomic number, denoted by ‘Z’.
- In fact, elements are defined by the number of protons they possess.
- For hydrogen, Z = 1, because in a hydrogen atom, only 1 proton is present in the nucleus.
- Similarly, for carbon, Z = 6.
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Mass Numbers and the Essence of Chemical Elements:
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- The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom and it is denoted by ‘A’.
- Since protons and neutrons are present in the nucleus they are also called nucleons.
- Mass of carbon is 12 u because it has 6 protons and 6 neutrons.
- Similarly, the mass of aluminium is 27 u (13 protons+14 neutrons).
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How do Isotopes differ in Atomic Composition, and what special properties do they offer in various Chemical Elements?
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- Isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.
- Example: Hydrogen atoms have three atomic species, namely protium ( 1H1), deuterium ( 1H2or D) and tritium ( 1H3 or T).
- The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.
- Similarly (i) carbon, 6C12 and 6C14 , (ii) chlorine, 17Cl35 and17Cl37 , etc.
- Many elements consist of a mixture of isotopes.
- Each isotope of an element is a pure substance.
- The chemical properties of isotopes are similar but their physical properties are different.
- Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1.
- The mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element.
- Example: The average mass of Cl will be 35.5u and if an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it.
- Practical Applications of Isotopes: Some isotopes have special properties such as;
- An isotope of uranium is used as a fuel in nuclear reactors.
- An isotope of cobalt is used in the treatment of cancer.
- An isotope of iodine is used in the treatment of goitre.
 [Atomic Masses of a few elements]
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Isobars: Chemical Elements Pairs with Identical Mass Numbers in Calcium and Argon
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- Atoms of different elements with different atomic numbers but same mass number, are known as isobars.
- Example: Calcium (atomic number 20) and Argons (atomic number 18) are isobars.
- The number of protons in these atoms is different, but the mass number of both these elements is 40 i.e. the total number of nucleons is the same in the atoms of this pair of elements.
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What is the significance of the Carbon-12 Isotope in determining Atomic Masses of Chemical Elements?
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- Dalton’s Atomic theory: Characteristic Atomic Mass of chemical elements: According to Dalton’s atomic theory, each element had a characteristic atomic mass.
- Mass Determination: Laws, Compounds, and Atomic Mass: Since determining the mass of an individual atom was a relatively difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.
- Initial Development: Oxygen’s Influence on Chemical Elements Mass: While searching for various atomic mass units, scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit due to two reasons:
- (i) Oxygen reacted with a large number of elements and formed compounds.
- (ii) This atomic mass unit gave masses of most of the elements as whole numbers.
- However, in 1961, carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
- One atomic mass unit (earlier abbreviated as ‘amu’, but according to the latest IUPAC, it is now written as ‘u’ – unified mass) is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.
- The relative atomic masses of all elements have been found with respect to an atom of carbon-12.
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