Drawbacks of Dalton’s Atomic Theory

The idea of atoms dates back two millennia, long before John Dalton introduced the concept of atoms in 1803. Kanada, an ancient Indian philosopher, discussed the existence of indivisible particles, which he referred to as “anu.” The rule of conservation of matter was introduced by French scientist Lavoisier in 1773. In which he suggested that in a chemical process, the matter is always preserved. In a chemical process, it is neither formed nor destroyed. This indicates that the quantity of each element in the reactants is equal to the amount of each element in the products created in a chemical reaction. Every time we balance chemical equations, we employ the rule of conservation of mass! 

Later, a French scientist named Joseph Proust proposed his law of definite proportions. This law states that a chemical compound’s component components are always present in a set ratio (by mass) regardless of its source or method of manufacture. Proust’s law was another name for this law.

Postulates of Dalton’s Atomic Theory

• All matters are made up of tiny, indivisible particles called atoms.
• A specific element of all kinds of atoms is the same in mass, size, and other properties. However, different atoms element exhibit different properties and different masses and sizes.
• Atoms can neither be created nor be destroyed. It cannot be further divisible into smaller particles.
• Elements of different atoms can combine with each other in fixed ratios (in whole-number ratio) in order to form compounds.
• In chemical reactions, atoms can be combined, rearranged, or separated.

Limitations of Dalton’s Atomic Theory

1. This theory did not define subatomic particles: According to Dalton’s atomic theory, it stated that atoms were indivisible. However, the discovery of the existence of subatomic particles (such as electrons, protons and neutrons) disproved this postulate.
2. It does not define the isotopes:  Dalton’s atomic theory states that all atoms of an element have identical masses and densities. However, there are atomic masses for different isotopes of elements (Example: Isotopes of hydrogen are hydrogen, deuterium, and tritium).
3. It does not define the isobars: Theory states that the masses of the atoms of two different elements must differ. However, two different elements can share the same mass number. these atoms are called isobars (Example: ⁴⁰Ar and ⁴⁰Ca).
4. Elements need not combine in whole-number ratios or simply to form compounds: Some certain complex organic compounds do not form simple ratios of constituent atoms. Example: sugar(C₁₁H₂₂O₁₁).
5. Allotropes were not defined: Allotropes of Carbon i.e. graphite and diamond they both have different properties to each other. that cannot be explained by Dalton’s atomic theory.

Why Dalton’s atomic symbol are not considerable?

• During Dalton Proposed his atomic theory there were few atoms were in exist but after that there were lots of elements were in exist so there are lots of symbols to remember it become difficult to remember that symbols.
• The symbols of elements proposed by Dalton were difficult to draw.

Therefore, Berzelius, proposed an alternative method of representing elements, He suggested alphabets could be used as symbols to represent elements.

Modern Atomic Symbols

The modern atomic theory states that atoms of one/identical element are the same, while atoms of different elements are different. The basic characteristic that all the atoms of the same element share are the number of protons. All atoms of carbon have six protons in the nucleus; all atoms of sodium have 11 protons in the nucleus. This number of protons is very important to identify which kind of atom is present, it is called the atomic number of the element. Thus, carbon has an atomic number of 6, while sodium has an atomic number of 11. Each and every element has its own unique identity i.e. atomic number.

There are different numbers of neutrons in the atoms of similar elements however, atoms of the same atomic number (atoms with the same number of protons) with different numbers of neutrons or mass numbers are called isotopes. Mostly natural occurring elements exist as isotopes. For example, hydrogen has three Isotopes, In case hydrogen atoms have a proton and a neutron in their nuclei. this is deuterium. If a hydrogen atom has one proton and two neutrons in the nucleus; this isotope is called tritium. 

The mass number of the isotope is defined as the sum of the number of protons and neutrons in the nucleus.

Mass Number = Number of protons + Number of neutron

In 1869, Dimitri Mendeleev proposed the original periodic table, in this the elements were arranged according to increasing atomic mass at the time of establishing periodic table nucleus was not discovered, and the interior structure of the atom was not understandable at all at that time, that is why the first atomic mass was the basis to create. But, after the structure of the nucleus was clear or understood, it became fair that it was the atomic number that governed the properties of the elements.

Sample Questions

Question 1: Define atom?

Answer:

Atom is define as, the smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element. simply, the atom is the basic building block of chemistry.

Question 2: What are polyatomic ions? Give examples.

Answer:

Polyatomic ions are the ions that contain more than one atom but they are as a single unit.

Examples of polyatomic ions are : CO₃^2-, H₂PO^4–

Question 3: The law of conservation of mass was followed by which postulate of Dalton atomic theory?

Answer:

The law of conservation of mass is define by the postulate of Dalton’s Atomic theory:

“Atoms can neither be created nor destroyed”.

Question 4: It is not possible to detect an atom with naked eyes. Why?

Answer:

• Atoms are microbial in nature, measured in very small units i.e. nanometers.
• They do not exist independently except for atoms of noble gases, Hence, an atom cannot be visible to the naked eyes.

Question 5: Find the number of atoms in each molecule:

(i) H₂S molecule and

(ii) PO₄^3- ion?

Answer:

The number of atoms present are as follows:

(i) In H₂S molecule 

Number hydrogen atom = 2 and 

Number of Sulphur atom = 1

Hence 3 atoms are present in total.

(ii) In PO₄^3- ion

Number Phosphorus atom = 1 and

Number of Oxygen atom = 4

Hence 5 atoms are present in total.