Q:- Define atomic number. Give examples.
Ans. The number of protons in the nucleus of an atom or number of electrons around the nucleus of an atom of an element is called atomic number of that element. It is represented by “Z”.
For example, atomic number of sodium (2311Na ) is 11 and (3517Cl) is 17. Atomic number is shown at subscript position of the symbol for an element.
Remember that the number of proton is equal to the number of electrons in a neutral atom.
Q:- Define mass number. Give examples.
Ans. The sum of number of protons and number of neutrons is called mass number. For example, the mass number of carbon (12 6C) is 12 and oxygen (168O) is 16.
Mass number is shown at superscript position of the symbol for an element.
Q:- How to calculate number of electrons, number of protons and number of neutrons.
Ans. Remember that number of electrons in an atom of an element is equal to number of protons. The number of neutrons can be calculated by subtracting atomic number from mass number as given.
For example, the number of electrons, protons and neutrons in chlorine (3517Cl) can be calculated as,
Number of electrons = 17
Number of protons = 17
Number of neutrons = 35-17 = 18
Q:- Define relative atomic mass. Give examples.
Ans. The mass of an atom of an element related to the mass of one atom of carbon-12 (C-12) atom is called relative atomic mass or simply atomic mass of that element. The unit of atomic mass is atomic mass unit (amu). For example, the relative atomic mass of hydrogen is 1.008 amu, oxygen is 15.9994 amu and sodium is 22.9898 amu.
Q:- Who were the first scientists to work on relative atomic masses of elements?
Ans. John Dalton, Gay Lussac, Lavoisier, Avogadro and Berzelius worked on relative atomic masses of elements and presented quantitative information about atomic masses. The chemists in nineteenth century observed the proportions in which elements combine to form different compounds and in this way they calculated relative atomic masses.
Q:- How is relative atomic mass determined?
Ans. An atom is extremely small so we cannot determine the mass of a single atom of an element. However, we can determine the atomic mass of an atom relative to another atom experimentally by mass spectrometer. This can be done by giving a value to the mass of one atom of a given element as a standard value.
In 1961, the international agreement selected light isotope of carbon-12 as a standard and assigned a mass of exactly 12 atomic mass unit. The atomic masses of all elements are determined by comparing to the mass of carbon-12 atom in mass spectrometer.
Q:- What is atomic mass unit?
Ans. One atomic mass unit (amu) is the mass exactly equal to one-twelfth (1/12th) the mass of one atom of light isotope of carbon-12. Mathematically, 1 amu = 1/12th of carbon-12 atom
Q:- Calculate the atomic mass of hydrogen atom which is 8.40% as massive as one atom of C-12.
Ans. If the percentage of an atom related to carbon-12 is given, then we can calculate the atomic mass of the given atom is the following formula.
Percentage/100 x 12 amu
Relative percentage by mass of hydrogen = 8.40%
So put the value in the above formula,
8.40/100 x 12 amu = 1.008 amu
Q:- Define chemical formula. Give examples.
Ans. A symbolic representation which tells us about the type of elements and whole number ratio between atoms of elements in covalent compounds and ionic compounds. For example, the chemical formulas of sodium chloride is NaCl, calcium oxide is CaO, ammonia is NH3 and hydrochloride acid is HCl. The numerical subscripts in chemical formulas tell us the number of each type of atoms in the compound. Chemical formula has two types, i.e molecular formula and empirical formula.
Q:- Define molecular formula. Give examples.
Ans. A symbolic representation which tells us about the actual whole number ratio between atoms of elements in a covalent compound. For example, the molecular formulas of glucose is C6H12O6, sulphuric acid is H2SO4, ammonia is NH3, benzene is C6H6 and sucrose (table sugar) is C12H22O11.