Atoms and Molecules Online Test 10th Science Lesson 7 Questions in English

Matter is made of atoms. Curiously the idea of atom was first proposed by the Greek philosophers in the fifth century BC (BCE). But their theory was more philosophical than scientific.

The first scientific theory of the atom was proposed by John Dalton.

Few of the postulates of Dalton’s theory about an atom were found incorrect by the later on studies made by J.J. Thomson, Rutherford, Neil’s Bohr and Schrodinger.

Atoms of the same element may have different atomic mass. (Discovery of isotopes 17Cl35, 17Cl37). Atoms of different elements may have same atomic masses (discovery of Isobars 18Ar40, 20Ca40).

Atoms of one element can be transmuted into atoms of other elements. In other words, atom is no longer indestructible (discovery of artificial transmutation).

Atoms may not always combine in a simple whole number ratio (E.g. Glucose C6H12O6 C:H:O = 6:12:6 or 1:2:1 and Sucrose C12H22O11 C:H:O = 12:22:11).

Atom is the smallest particle that takes part in a chemical reaction. The mass of an atom can be converted into energy (E = mc2).

Protons and neutrons have considerable mass, but electrons don't have such a considerable mass. Thus, the mass of an atom is mainly contributed by its protons and neutrons and hence the sum of the number of protons and neutrons of an atom is called its mass number.

Individual atoms are very small and it is difficult to measure their masses. You can measure the mass of macroscopic materials in gram or kilogram. The mass of an atom is measured in atomic mass unit (amu).

The mass of a proton or neutron is approximately 1 amu.

The symbol ‘amu’ is no longer used in the modern system and instead it uses the symbol ‘U’ to denote unified atomic mass.

As an atom is very small, its absolute mass cannot be determined directly. The early pioneers of chemistry used to measure the atomic mass of an atom relative to an atom of another element. They measured the masses of equal number of atoms of two or more elements at a time, to determine their relative masses.

The established one element as a standard, gave it an arbitrary value of atomic mass and using this value they measured the relative mass of other elements. The mass obtained by this way is called relative atomic mass.

Later hydrogen atom was replaced by oxygen atom as the standard. Now the stable isotope of carbon (C-12) with atomic mass 12 is used as the standard for measuring the relative atomic mass of an element.

‘The main postulates of modern atomic theory’ are as follows: An atom is no longer indivisible (after the discovery of the electron, proton, and neutron).

Atoms are the building blocks of matter. Since matter has mass, it must be due to its atoms. According to the modern atomic theory, an atom contains subatomic particles such as protons, neutrons and electrons.

Atomic mass unit is one-twelfth of the mass of a carbon-12 atom; an isotope of carbon, which contains 6 protons and 6 neutrons.

The mass of hydrogen atom was chosen as a standard and masses of other atoms were compared with it because of the existence of isotopic character of hydrogen (1H1, 1H2, 1H3).

Protons and neutrons have considerable mass, but electrons don't have such a considerable mass. Thus, the mass of an atom is mainly contributed by its protons and neutrons and hence the sum of the number of protons and neutrons of an atom is called its mass number.

Relative atomic mass of an element is the ratio between the average mass of its isotopes to 1/ 12th part of the mass of a carbon-12 atom. It is denoted as Ar. It is otherwise called “Standard Atomic Weight”.

Modern methods of determination of atomic mass are done by Mass Spectrometry which uses C-12 as standard.

Relative Atomic Mass is only a ratio so it has no unit. If the atomic mass of an element is expressed in grams it is called as Gram Atomic Mass.

Gram Atomic Mass of hydrogen = 1 g Gram Atomic Mass of carbon = 12 g Gram Atomic Mass of nitrogen = 14 g Gram Atomic Mass of oxygen = 16 g

It is somewhat more complicated because most of the naturally occurring elements exist as a mixture of isotopes each of which has its own mass. Thus it is essential to consider this isotopic mixture while calculating the atomic mass of an element.

The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

Average atomic mass = (Mass of 1st isotope × % abundance of 1st isotope) + (Mass of 2nd isotope ×% abundance of 2nd isotope) Thus for the given element the average atomic mass = (10 × 75 / 100) + (20 × 25 / 100) = 12.5

The atomic masses of elements given in the periodic table are average atomic masses. Sometimes the term atomic weight is used to mean average atomic mass. It is observed from the periodic table that atomic masses of most of the elements are not whole numbers.

To calculate the atomic mass of carbon both of its natural isotopes such as carbon-12 and carbon-13 are considered. The natural abundance of C-12 and C-13 are 98.90 % and 1.10 % respectively. The average of the atomic mass of carbon is calculated as follows:Average atomic mass of carbon = (12 × 9 8 . 9 ) 100 + (13 × ) 1.1 100

Isotopes of oxygen

Except noble gases atoms of most of the elements are found in the combined form with itself or atoms of other elements. It is called as a molecule.

A molecule is a combination of two or more atoms held together by strong chemical forces of attraction, i.e. chemical bonds.

A molecule may contain atoms of the same element or may contain atoms of two or more elements joined in a fixed ratio, in accordance with the law of definite proportions. Thus, a molecule may be an element or a compound.

If the molecule is made of similar kind of atoms, then it is called homoatomic molecule.

The molecule that consists of atoms of different elements is called heteroatomic molecule. A compound is a heteroatomic molecule.

The number of atoms present in the molecule is called its ‘atomicity’.

Ozone (O3) contains three oxygen atoms and hence it is called homotriatomic molecule. If a molecule contains more than three atoms, then it is called polyatomic molecule.

Consider hydrogen chloride. It consists of two atoms, but of different elements, i.e. hydrogen and chlorine. So, its atomicity is two. It is a heterodiatomic molecule.

The water molecule contains two hydrogen atoms and one oxygen atom. So its atomicity is three. It is a heterotriatomic molecule.

The Relative Molecular Mass of a molecule is the ratio between the mass of one molecule of the substance to 1 /12th mass of an atom of Carbon -12.

Relative molecular mass of water (H2O) is calculated as follows: A water molecule is made of 2 atoms of hydrogen and one atom of oxygen. So, the relative molecular mass of water = (2 × mass of hydrogen) + (1 × mass of oxygen) = (2 × 1) + (1 × 16) = 18 i.e., one molecule of H2O is 18 times as heavy as 112th of the mass of a carbon –12.

Relative Molecular Mass is only a ratio. So, it has no unit. If the molecular mass of a compound is expressed in grams, it is called Gram Molecular Mass.

Gram Molecular Mass of water = 18 g Gram Molecular Mass of carbon dioxide = 44 g Gram Molecular Mass of ammonia = 17 g Gram Molecular Mass of HCl = 36.5 g

Atomic mass units provide a relative scale for the masses of the elements. Since the atoms have such small masses, no usable scale can be devised to weigh them in the calibrated units of atomic mass units.

Chemists measure atoms and molecules in ‘moles’. So you can now understand that ‘mole’ denotes a number of particles.

In the SI system, the mole (mol) is the amount of a substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 g (or 0.012 kg) of the carbon-12 isotope.

The actual number of atoms in 12 g of carbon-12 is determined experimentally. This is called Avogadro’s number (NA)

NA is named after an Italian scientist Amedeo Avogadro who proposed its significance. Its value is 6.023 × 1023. So one mole of a substance contains 6.023 × 1023 entities.

Mole Concept: The study of the collection of particles by using mole as the counting unit in order to express the mass and volume of such unit particles in a bulk of matter is known as mole concept.

The number of moles of a substance can be calculated by various means depending on the data available, as follows: Number of moles of molecules. Number of moles of atoms. Number of moles of a gas (Standard molar volume at STP = 22.4 litre). Number of moles of ions.

STP-Standard Temperature and Pressure (273.15 K,1.00 atm)

One mole of an element contains 6.023 × 1023 atoms and it is equal to its gram atomic mass .i.e., one mole of oxygen atom contains 6.023 × 1023 atoms of oxygen and its gram atomic mass is 16 g.

One mole of matter contains 6.023 × 1023 molecules and it is equal to its gram molecular mass.

One mole of any gas occupies 22.4 liter or 22400 ml at S.T.P. This volume is called as molar volume.

Calculation of number of moles by Different modes Number of moles = Mass / Atomic Mass = Mass / Molecular mass = Number of Atoms / 6.023 × 1023 = Number of Molecules / 6.023 × 1023

The percentage composition of a compound represents the mass of each element present in 100 g of the compound.

Mass % of an element = mass of that element in the compound / molecular mass of the compound * 100 Now, molecular mass of H2O = 2(1) + 16 = 18 g Mass % of hydrogen = 2 × 100 18 = 11.11 % Mass % of oxygen = 16 × 100 18 = 88.89 % This percentage composition is useful to determine the empirical formula and molecular.

In 1811 Avogadro framed a hypothesis based on the relationship between the numbers of molecules present in equal volumes of gases in different conditions.

The Avogadro’s law states that “equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules”

It follows that the volume of any given gas must be proportional to the number of molecules in it. If ‘V’ is the volume and ‘n’ is the number of molecules of a gas, then Avogadro law is represented, mathematically, as follows: V α n V = constant × n

The chemical formula for potassium permanganate is KMnO4 which has one potassium, one manganese and four oxygen atoms.

APPLICATIONS OF AVOGADRO’S LAW: It explains Gay-Lussac’s law. It helps in the determination of atomicity of gases. Molecular formula of gases can be derived using Avogadro’s law. It determines the relation between molecular mass and vapor density. It helps to determine gram molar volume of all gases (i.e, 22.4 liter at S.T.P)

Relative molecular mass: (Hydrogen scale) The Relative Molecular Mass of a gas or vapor is the ratio between the mass of one molecule of the gas or vapor to mass of one atom of Hydrogen.

Vapor density is the ratio of the mass of a certain volume of a gas or vapor to the mass of an equal volume of hydrogen measured under the same conditions of temperature and pressure. Vapor Density (V.D.) = Mass of a given volume of gas or vapor at S.T.P. / Mass of the same volume of hydrogen

V.D. at S.T.P = Mass of ‘n’ molecules of a gas or vapor at S.T.P. / Mass of ‘n’ molecules of hydrogen

Relative molecular mass = 2 × Vapor density

Molecular mass of H2SO4 = (1 × 2) + (32 × 1) + (16 × 4) = 2 + 32 + 64 = 98 g % of H in H2SO4 = Mass of hydrogen / Molecular mass of H2SO4 * 100 % of H in H2SO4 = 2/ 98 * 100 = 2.04

Number of moles of CO2 = Volume at S.T.P / Molar volume = 10 / 22.4 = 0.446