Atoms: Atomic Number, Mass Number and Percentage composition
ATOMIC NUMBER AND MASS NUMBER
Atoms:THE ATOMIC NUMBER
Atoms:The atomic number of an element is the number of protons present in the atomic nucleus of the element. It is denoted by letter Z and increased by one when the elements were arranged in order of ascending relative atomic mass. In a neutral atom, the number of protons is equal to the number of electrons. Thus, the atomic number is also the number of electrons. The atomic number of an element is a whole number, where the relative atomic mass of an element is not a whole number.
Atoms with the same atomic number may have different masses and still have their properties identical. However, no change in atomic number is possible without a major change in properties.
Atomic number of an element determines the nature of the atom and also distinguishes it from atoms of all other elements.
Atoms:The mass Number
The mass number of an element is the sum of the protons and the neutrons in the atomic nucleus of the element, e.g., Oxygen has a mass number of 16. The number of protons is 8 while neutron is 8. We can describe an atom of an element by writing its symbol together with its atomic number and mass number, e.g.
Isotopes are atoms with the same atomic number but different mass number. This is due to differences in the number of neutrons. Isotopes of an element have different physical properties, but exhibit the same chemical properties, because neutrons appear to have no influence on the chemical properties of the atom but their major effect is in contributing to the mass of the atom
An atom showing these characteristic properties is said to show Isotopy and the varieties of the atoms are called Isotopes of the element.
A well-known example of isotopy occurs in chlorine.
The relative atomic mass of an element which exhibits ‘isotopy is the average mass of its various Isotopes as they occur ‘naturally in any quantity of the element. For example, the relative atomic mass of chlorine is 35.5 because a given quantity of chlorine contains:
75% of and 25% of
(75/100 x 35) + (25/100 x 37)
= 26.25 + 9.25
The relative atomic mass can also be calculated using the ratio instead of percentage abundance. For example, two Isotopes of chlorine with mass number 35 and 37 exist in the ratio 3:1 respectively.
What is the relative atomic mass of chlorine?
i.e. (3 x 35) +0 x 37)
= 105 +37
Total ratio = 3 + 1 = 4
Relative atomic mass = 142/4 = 35.5
Other examples of isotopes include:
Carbon with the atomic number of 6
Most carbon atoms have a mass number of 12 while a few carbon atoms have mass number of 14.
Oxygen with mass number 16 and 17
Some isotopes of different elements have the same mass number, Such Isotopes are called Isobars. e.g., , and are Isobars.
Isobars are not chemically alike since chemical characteristics depend on the number of electrons which is determined by the atomic number.
Atoms: RELATIVE ATOMIC MASS
The relative atomic mass of an element is the number of times the average mass of one atom of the element is heavier than one-twelfth of the mass of one atom of carbon 12. i.e.? it is the average mass of the atoms of the element on a scale on which one atom of the carbon – 12 isotope is 12 units,
The relative atomic mass carries no units; the formula is
The relative atomic mass of each element has been verified accurately with the aid of the mass spectrometer introduced by Aston.
RELATIVE MOLECULAR MASS
The relative molecular mass of a compound is the sum of the relative atomic masses of all the atoms in one molecule of the substance. It is the mass of the correctly written formula or molecule of a compound obtained by adding together the appropriate relative atomic masses of all the atoms of the elements present. Relative molecular mass is also referred to as the formula mass.
Calculate the relative molecular mass of sodium tetraoxosulphate (VI), Na2SO4 (Na = 23, s = 32, O=16)
1 mole of contains 2 sodium
1sulphur and 4 oxygen atoms
Hence relative molecular mass of
=(2 x 23) + (1 x 32) + (4 x 16)
= 46 + 32 + 64
The mole is the amount of substance that contains as many elementary entities as there are carbon atoms in 12 grammes of carbon 12 isotope, i.e., it is the amount of a substance that contains the same number of units as the number of atoms in exactly 12g of carbon -12. The elementary entities can be atoms, molecules, ions, electrons or other particles which must be specified.
The number of atoms in a mole, known as
Avogadro Number (NA) is 6.02 x 1023
Thus, a mole of any substance is that amount of the substance which contains 6.02 x 1023 particles of that, substance, for example, a mole of Helium atom contains 6.02 x1023 helium atom and a mole, of Beryllium contains 6.02 x 1023 atoms of Beryllium.
The relative atomic mass of any element in grams contains one mole of atom (i.e. 6.02 x 1023 atoms) of the element and the relative molecular mass of an element or compound in grams contains One mole of molecules (i.e., 6,02 X 1023 molecules) of that element or compound, e.g. a mole of oxygen atoms, (O), contains 6.02 x 1O23 atoms and weighs 16.0g but a mole of oxygen molecules O contains 6.02 X 1023 molecules and weighs
A mole of an ionic substance contains 6.02 x 1023 formular units; e.g a mole of chloride, CaCl2 consists of 6.02 x 1O23 (1 mole of Ca2+ ions) and 2 (6.02 x 1023) Cl– (2 moles of Cl– ions).
Moles = mass in grams / relative molecular mass
Number of moles = number of particles / NA (Avogadro number)
Determine the number of moles in 95g of MnO2.
Mass of MnO2 = 95g
Mass of one mole MnO2 = 86.94g
Number of moles formula is
Number of moles = given mass / relative molecular mass
Number of moles = 95 / 86.94
Number of moles = 1.092 mol
try this: Calculate the number of moles of carbon dioxide molecules in 22 g of CO2.
The molar mass of any substance is the mass of one mole of the substance expressed in grams. Its unit is in grams per mole (gmol-l). The molar mass of a substance is its formula mass expressed in grams. This is because a mole of any substance can be represented by its formula. For example, the molar mass of an element will be its relative atomic mass expressed in grams and the molar mass of a compound will be its relative molecular mass expressed in grams.
The formula of a compound indicates the kinds of atoms present in the substance and their number: thus, the mas of 1 mole of a compound is the sum of the masses of the moles of its component element.
Calculate the formula mass of
(H = 1, S = 32, O = 16)
=H + S + O
=1 X 2 + 32 + 16 X 4
=2 + 32 + 64
Atoms: Percentage composition
The percentage composition of a given compound is defined as the ratio of the amount of each element to the total amount of individual elements present in the compound multiplied by 100.
For the compound (Ca(OH)2) or calcium hydroxide, find the per cent composition of the element Calcium(Ca).
Given that the chemical formula for calcium hydroxide = (Ca(OH)2)= 74
Using the above formula, we calculate the formula mass of this compound.
For calcium hydroxide, the formula mass can be calculated as,
Ca/ Ca(OH)2 X 100
= 40/74 X 100 = 54.1%