Atoms and Molecules

An ancient Greek philosopher Democritus (460 - 370 BC) and Leucippus suggested that if we go on dividing matter, a stage will come when further division of particles will not be possible. Democritus called these individual particles 'atoms' (which means indivisible).

Law of Chemical Combinations

Law of Conservation of Mass

In every chemical reaction, total masses of all the reactants is equal to the masses of all the products.

Law of Definite or Constant Proportions

In a given chemical compound, the proportions by mass of the elements that compose it are fixed, independent of the origin of the compound or its mode of preparation.

For example, in pure water the ratio of mass of hydrogen to the mass of oxygen is always 1:8 irrespective of the source of water. In other words, pure water contains 11.11% of hydrogen and 88.89% of oxygen by mass whether water is obtained from well, river or from a pond.

Dalton's Atomic Theory

In 1803, Dalton published a new system of chemical philosophy in which the following statements comprise the atomic theory of matter:

  1. Matter consists of indivisible atoms.
  2. All the atoms of a given chemical element are identical in mass and in all other properties.
  3. Different chemical elements have different kinds of atoms and in particular such atoms have different masses.
  4. Atoms are indestructible and retain their identity in chemical reactions.
  5. The formation of a compound from its elements occurs through the combination of atoms of unlike elements in small whole number ratio.

Dalton’s fourth postulate is related to the law of conservation of mass. Every atom of an element has a definite mass. Also in a chemical reaction there is rearrangement of atoms. Therefore after the reaction, mass of the product should remain the same.

The fifth postulate is an attempt to explain the law of definite proportions. A compound is a type of matter containing the atoms of two or more elements in small whole number ratio. Because the atoms have definite mass, the compound must have the elements in definite proportions by mass.

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element in these compound for a fixed mass of the other element are in the ratio of small whole numbers. 

For example, carbon and oxygen form two compounds: Carbon monoxide and carbon dioxide. Carbon monoxide contains 1.3321 g of oxygen for each 1.000g of carbon, whereas carbon dioxide contains 2.6642 g of oxygen for 1.0000 g of carbon. In other words, carbon dioxide contains twice the mass of oxygen as is contained in carbon monoxide (2.6642 g = 2 × 1.3321 g) for a given mass of carbon.

Atomic Mass

Dalton gave the concept of atomic mass. According to him, atoms of the same element have same atomic masses but atoms of different elements have different atomic masses.

Since Dalton could not weigh individual atoms, he measured relative masses of the elements required to form a compound. From this, he deduced relative atomic masses. For example, you can determine by experiment that 1.0000 g of hydrogen gas reacts with 7.9367 g of oxygen gas to form water. If you know formula of water, you can easily determine the mass of an oxygen atom relative to that of hydrogen atom.

In 1961, C-12 (or 126C) atomic mass scale was adopted. This scale depends on measurement of atomic mass by an instrument called mass spectrometer.

Atomic Mass Unit

The masses of atoms are obtained by comparison with C-12 atomic mass scale. C-12 isotope is chosen as standard and arbitrarily assigned a mass of exactly 12 atomic mass units. Therefore, one atomic mass unit (amu) equals exactly one twelfth of mass of a carbon-12 atom.

Isotopes and Atomic Mass

Dalton considered an atom as an indivisible particle. Later researches proved that an atom consists of several fundamental particles such as electrons, protons and neutrons. An electron is negatively charged and a proton is positively charged particle.

Number of electrons and protons in an atom is equal. Since charge on an electron is equal and opposite to charge of a proton, therefore an atom is electrically neutral. Protons remain in the nucleus in the centre of the atom, and nucleus is surrounded by negatively charged electrons.

The number of protons in the nucleus is called atomic number denoted by Z. For example, there are 8 protons in the oxygen nucleus, 6 protons in carbon nucleus and only one proton in hydrogen nucleus. Therefore atomic numbers of oxygen, carbon and hydrogen are 8, 6 and 1 respectively. There are also neutral particles in the nucleus and they are called "neutrons". Mass of a proton and of a neutron is nearly the same.

Total mass of the nucleus = mass of protons + mass of neutrons

Total number of protons and neutrons is called mass number (A). By convention atomic number is written at the bottom of left corner of the symbol of the atom of a particular element and mass number is written at the top left corner. For example, symbol 126C indicates that there is a total of 12 particles (nucleons) in the nucleus of a carbon atom, 6 of which are protons. Thus, there must be 12-6 = 6 neutrons.

Atomic number (Z) differentiates the atom of one element from the atoms of the other elements. An element may be defined as a substance where all the atoms have the same atomic number. But the nuclei of all the atoms of a given element do not necessarily contain the same number of neutrons.

Isotopes

Atoms of an element that have the same atomic number (Z) but different mass number (A) are called isotopes. For example, atoms of oxygen, found in nature, have the same number of protons which makes it different from other elements, but their neutrons (in nucleus) are different. This is the reason that the masses of atoms of the same element are different. For example, one type of oxygen atom contains 8 protons and 8 neutrons in one atom, second type 8 protons and 9 neutrons and the third type contains 8 protons and 10 neutrons.

Molecule

A molecule is an aggregate of two or more than two atoms of the same or different elements in a definite arrangement. These atoms are held together by chemical forces or chemical bonds.

An atom is the smallest particle of a substance but can not exist freely. Contrary to this, a molecules can be considered as the smallest particle of an element or of a compound which can exist alone or freely under ordinary conditions.

Molecular Mass

Molecular formula of a compound is normally used for determining the molecular mass of that substance. If a substance is composed of molecule (for example: CO2, H2O or NH3), it is easy to calculate the molecular mass. Molecular mass is the sum of atomic masses of all the atoms present in that molecule. For example, the molecular mass of CO2 is obtained as

Mass of C: 1 x 12.0 u = 12.0 u

mass of 2 O: 2 x 16.0 u = 32.0 u

Mass od CO2 = 44.0 u

Mole Concept

When we mix two substances, we get one or more new substances. For example, when we mix hydrogen and oxygen and ignite the mixture, we get a new substance - water. This can be represented in the form of a chemical equation,

2H2 (g) + O2 (g) → 2H2O (l)

In the above equation, 2 molecules (four atoms) of hydrogen react with 1 molecule (2 atoms) of oxygen and give two molecules of water.

Mole

The word mole was, apparently introduced in about 1896 by Wilhelm Ostwald who derived the term from the Latin word ‘moles’ meaning a ‘heap’ or ‘pile’. The mole whose symbol is ‘mol’ is the SI (international system) base unit for measuring amount of substance.

A mole is the amount of substance that contains as many elementary entities (atoms, molecules, formula unit or other fundamental particles) as there are atoms in exactly 0.012 kg of carbon-12 isotope. In simple words, mole is the number of atoms in exactly 0.012 kg (12 grams) of C-12.

It is experimentally found that the number of atoms contained in exactly 12 g of C-12 is 6.022×1023. This number is called Avogadro’s number.

NA = 6.02×1023 mol–1

Molar Mass

Mass of one mole of a substance is called its molar mass. Mass of one mole atoms of oxygen means mass of 6.02×1023 atoms of oxygen. It is found that one mole atoms of oxygen weighs 16.0 g.

Converting Moles to Grams

Example: How many grams are there in 3.5 mol of oxygen?

Molar mass of oxygen (O2) = 32 g mol–1

Therefore, number of grams of oxygen in 3.5 mol

= 3.5 mol of oxygen × 32.0 g mol–1

= 112.0 g of oxygen

Converting Grams to Molecules

Example: How many number of molecules are there in 27 g of water?

Mole concept provides a relationship between number of particles and their mass. Thus it is possible to calculate the number of particles in a given mass.

Number of mole of H2O = Mass of water (H2O) / Molar mass of H2O

= 27/18 = 1.5 mol

Since 1 mol of water contains 6.02×1023 molecules.

1.5 mol of water contains = 6.02 × 1023 molecules mol–1 × 1.5 mol

= 9.03 × 1023 molecules of water