Dalton's Atomic Theory

Dalton's Atomic Theory

Dalton's atomic theory provided an explanation for the law of conservation of mass and the law of definite proportions. According to Dalton’s atomic theory, all matter (whether an element, a compound or a mixture), is compounds of small particles, called atoms.

The Main Postulates of Dalton's Atomic Theory are

  • Every matter is made up of very small particles, called the atoms.
  • Atoms are indivisible particles which can neither be created nor be destroyed in a chemical reaction.
  • Atoms of a given element are identical in mass as well as in chemical properties.
  • Atoms of different elements have different masses and chemical properties.
  • Atoms combine in the ratio of small whole numbers to form compounds.
  • The relative numbers as well as kinds of atoms are constant in a given compound.

Merits of Dalton's Atomic Theory

Dalton's atomic theory has proved to be very useful in various ways

  • This theory has enabled us to explain the laws of chemical combination.
  • Dalton was the first to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).

Demerits of Dalton's Atomic Theory

  • Atom is no longer considered as the smallest indivisible particle. It has been established that it is made up of electrons, protons and neutrons.
  • Atoms of the same element may have slightly different masses (Isotopes).
  • Atoms of different elements may have the same masses (Isobars).
  • Substance made up of the same kind of atoms may have different properties.
  • The ratio in which the different atoms combine to form compound may be fixed and integral but may not be simple.

Illustration

Dalton atomic theory postulated that "Atoms combine in simple whole number ratio". How has this postulate been modified?

 Solution

According to the modified postulate, 'Atoms combine in whole number ratio but this ratio may not be simple." For example. in C12H22O11 (sucrose) ratio of C : H : O is 12 : 22 : 11, which is a whole number ratio but not the simple.

Illustration

Carbon and oxygen combine to form carbon dioxide gas. They do so in the same ratio 3 : 8. What mass of oxygen would be required to react completely with 6.0 g of carbon? Also calculate the mass of carbon dioxide gas so formed.

 Solution

Carbon and oxygen combine as per law of constant proportions, i.e. 3 g of carbon will always combine with 8 g of oxygen or

  3g carbon combines with oxygen = 8g

   6g carbon will combine with oxygen = 8/3 x 6 = 16g

Total amount of carbon dioxide = 6 ± 16= 22 g

Atoms

Atoms are the smallest particles of an element which may or may not have independent existence but take part in a chemical reaction. These are the building blocks of all natter.

For Example:- atoms of hydrogen, oxygen, nitrogen, etc, are not capable in independent existence whereas atoms of helium, noon, etc are capable of independent existence.

Size of Atoms

Atoms are very small and their radius is measured in nanometres.

1/109 M =1 nm or lm = 109 nm

Radius of hydrogen atom is 0.1 nm.

Modern Symbols of Atoms of Different Elements

Although Dalton was the scientist who introduced symbols for representing elements for the first time but modern symbols for the elements were introduced by JJ Berzilius. These are defined as a short hand representation of the name of an element.

Now a days, it is the IUPAC who approves the names of the letters of the element’s name in English. The first letter of a symbol is always written in capital letter and the second letter as small letter.

e.g., chlorine – Cl, zinc – Zn and aluminium – Al.

Symbols of some elements have been taken from their names in different languages such as Latin, German, Greek, etc.

For Example:-

  • Iron – Fe from Ferrum (Latin Name)
  • Gold – Au from Aurum (Latin Name)
  • Potassium – K from Kalium (Latin Name)
  • Chlorine – Cl from Chloros (Greek Name)
  • Cobalt – Co from Kobold (German Name)
  • Sodium– Na from Natrium (Latin Name)

Atomic Mass

According to Dalton, each element has a characteristic atomic mass. But determining the mass of an individual atom was a relatively difficult task due to its very small size.

Hence, their relative atomic masses were determined using the laws of chemical combination and the compounds formed. For this purpose, initially oxygen was taken as standard because of the following two reasons:

(i):- Oxygen reacted with a large number of elements and formed compounds.

(ii):- This unit gave masses of most of the elements as whole numbers.

Relative Atomic Mass

It is defined as the number of times a given atom is heavier than 1/12th of mass of 1 atom of carbon-12 (C-12) or it is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom.

Atomic Mass Unit

It is defined as the mass unit equal to exactly 1/12th of the mass of one atom of C-12 isotope. Earlier, it was abbreviated as amu but according to latest recommendations of IUPAC, it is now written as `u'— unified mass.