Concepts of Atomic Theory

Key Concepts

• Atomic theory
• Introduction to atomic theory
• Dalton’s atomic theory
• Postulate Dalton’s atomic theory

Introduction: 

Atomic Theory:  

Atoms are composed of particles, including protons, neutrons, and electrons. Atoms have tiny, positively charged and negatively charged species. Positively charged species are made up of protons and neutrons. Negatively charged species surround the nucleus with one or more electrons. Electrons occupy specific orbital or energy level in the circular path around the nucleus. The number of protons in an atom is called the atomic number. 

Scientists who put forward the explanation of atomic theory: 

Today, we know that everything is made up of huge number of tiny particles. But for thousands of years, people described the nature of matter in very different ways. The various descriptions of matter and how it behaves is called the atomic theory. 

Empedocles, a Greek scientist and philosopher, made one of the first attempts to explain matter 2500 years ago, claiming that matter was composed of four ‘elements’—earth, air, wind, and fire—in varying quantities.  

Another Greek scholar, Democritus, countered this theory with one of his own, reasoning that any substance, when cut into smaller and smaller pieces, would eventually be cut into a piece that could not be divided further. He called this piece ‘atomos’, from which we get the word ‘atom.’ 

 However, Democritus believed that atoms were uniformly placed, solid, hard, incompressible, and indestructible element that moved in infinite numbers through empty space until stopped. He argued that matter was subdivided into indivisible particles.

Aristotle, the most respected philosopher of the day, agreed with the theory proposed by Empedocles. He thought that all materials on earth were not made of atoms. He believed all substances like air, water, earth, and fire were made of small amounts of these four elements of matter. 

The ancient atomic theory was first proposed in the 5th century BC by the Greek philosophers Leucippus and Democritus and was further revived in the 1st century BC by the Roman philosopher Lucretius. 

Today, we know that everything is made of huge numbers of tiny particles. But for thousands of years, people described the nature of the matter in very different ways. The various descriptions of matter and how it behaves is called the atomic theory. 

What is the difference between a theory and a law?  

In science, laws are descriptions of events, patterns, or relationships that have been observed over and over again. Laws do not provide explanations of concepts; they simply state what happens. Theories are explanations of observations that are supported by reliable evidence. Laws are constant, but theories may change as new facts become available. 

Development of Atomic theory: 

The Greek philosopher Democritus introduced the idea of the atom in 1800. John Dalton re-introduced the atom. He provided evidence for atoms and developed atomic theory. 

Different atomic theories: 

• Dalton’s atomic theory 
• J. J. Thomson’s theory 
• Ernest Rutherford’s hypothesis – Rutherford’s Atom model 
• Bohr’s theory – Bohr Atom model 

• Einstein, Heisenberg and Quantum Mechanics 

Dalton’s atomic theory: 

John Dalton (1766–1844) was a British teacher and a scholar, credited for the development of atomic theory to explain about the new way of matter:  

Postulates of Dalton’s atomic theory: 

• Dalton’s atomic theory was the first to describe all matter in terms of atoms and their properties. 
• All matter is made of small particles called atoms.      

• Atoms can neither be created, destroyed, nor divided into smaller particles.     
• All atoms of the same element are identical in mass and size. 
• Compounds are created when atoms of different elements link together in definite proportions. 
• Atoms of same or different elements combine in different ratios to form a compound 

Based on Dalton’s theory, he divided it into two laws: 

1.  The law of conservation of mass 

2. The law of constant composition 

The law of conservation of mass says that matter can neither be created nor destroyed. That means the total mass of the reactant side is equal to the sum of masses of the products side and the masses of unreacted reactants. 

The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass. All compounds, have same elements that are present in the same ratio by mass. 

Summary:

• Atoms consists of protons, neutrons, and electrons.
• The number of protons in an atom is called the atomic number.
• Empedocles made one of the first attempts to explain the matter, claiming that matter was composed of four ‘elements’-earth, air, wind, and fire-in varying quantities.
• Democritus argued that matter was subdivided into indivisible piece. He called this piece ‘Atomos’, from which we get the word ‘atom’.
• Aristotle believed all substances like air, water, earth and fire were made of small amounts of these four elements of matter.
• Dalton’s atomic theory was the first to describe all matter in terms of atoms and their properties.
• The law of conservation of mass says that matter can neither be created nor destroyed.
• The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass.

Frequently asked questions 

1. What is the law of conservation of mass? 

Ans: The law of conservation of mass states that matter can neither be created nor destroyed. That means the total mass of the reactant side is equal to the sum of masses of the products side and the masses of unreacted reactants. 

2. What is the law of constant proportions? 

Ans: The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass. All compounds, have same elements that are present in the same ratio by mass. 

3. Why wad Dalton’s atomic theory rejected? 

Ans: Dalton’s atomic theory was the first to describe all matter in terms of atoms and their properties. It stated that atoms were indivisible. But the discovery of subatomic particles- the electrons, protons, and neutrons disapproved his postulate.