Chapter 2:Unit 3. Concept of Atom

Concept of atom

All matter, such as solids, liquids and gases, is composed of atoms. Any material that is composed of only one type of atom is called a chemical element, a basic element, or just an element. An atom is the smallest particle of any element that still retains the characteristics of that element. A piece of an element that we are able to see or handle is made of many, many atoms and all atoms are the same…they all have the same number of protons.   Protons and other subatomic particles will be discussed a little later.  The atoms of different elements are different from each other because they have different numbers of protons. The graphic below illustrates this point by showing the atoms of two elements in the containers of oxygen and hydrogen.

The atoms in oxygen are identical to each other. The atoms in hydrogen are identical to each other. However, the atoms of oxygen are different from the atoms of hydrogen. Elements can be monoatomic like Al, Na, diatomic like H₂, N₂ or polyatomic like S₈ or P₄.

Compounds, like water, are formed by combining the atoms of different elements together according to some chemical formula. The fundamental unit if compound is a molecule. Molecule is a tightly bound combination of two or more atoms that acts as a single unit.

According to modern scientific theory, any matter is made of atom that consists of subatomic particles. The theory of atom was developed over the time. The word “atom” comes from the ancient Greek word “atomos,” which means “indivisible.”

Atoms visible through spectroscope

Watch the video:

GPB: Development of Atomic Theory

The modern atomic Theory was proposed by English Chemist John Dalton is a fundamental concept that that state

1)Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.

2)An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element.

3)Atoms of one element differ in properties from atoms of all other elements.

4)A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the number of atoms of each of its elements are always present in the same ratio.

5)Atoms are neither created nor destroyed during a chemical change, but instead rearrange to yield a different type(s) of matter.

Some elements exist as diatomic molecules such as Hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂), fluorine (F₂), Chlorine (Cl₂), Bromine (Br₂) and Iodine(I₂). Three fundamental laws of science 1) law of conservation of matter 2) law of definite proportion and 3) Law of multiple proportion can be explained using Dalton’s atomic theory.

Up to this point, atoms were believed to be the smallest units of matter. In 1897, J.J. Thomson discovered the electron. He believed atoms could be divided. Because the electron carried a negative charge, he proposed a plum pudding model of the atom, in which electrons were embedded in a mass of positive charge to yield an electrically neutral atom.

Ernest Rutherford, one of Thomson’s students, discarded plum pudding model in 1909. Rutherford found that the positive charge of an atom and most of its mass were at the center, or nucleus, of an atom. He described a planetary model in which electrons orbited a small, positive-charged nucleus. Modern structure of atoms was established from Rutherford’s experiment.

Observation from Rutherford’s experiment:  A very thin metal foil was aimed a beam of positively charged particles from a radioactive source toward the foil. Most of the particles traveled straight through the foil, but some alpha particles were deflected off one side. Some were even deflected toward the source. He proposed the planetary model of atom.

Please find below the diagrams for Rutherford’s Observation

Top: Predicted, Bottom: Actual

Activity:

Go to the following link and Perform Rutherford’s experiment as well as Plum Pudding model experiment. What difference do you notice when you perform both experiments?

Questions

1. What postulate of atomic theory is partially correct and needs to be modified? Why?
2. What is the main discovery of Rutherford’s experiment?

Ans: 1. Isotopes, atoms of same elements but not identical

2.Nucleus in an atom