ICSE Class 7 Chemistry Chapter 2 Structure of an Atom

Structure Of An Atom

In This Chapter You Will Learn

Composition of an atom

Fundamental particles of an atom

Arrangement of electrons in orbits or shells

Structure of an atom

Atomic mass and relative atomic mass

Combination of atoms to form molecules

Definition of valency

Valence electrons and valence shell

Types of valency

Writing a chemical formula

Structure Of An Atom

Introduction

The word atom comes from the word 'atomos' meaning indivisible coined by a Greek philosopher Democritus (460-361 B.C.). He forwarded the idea that the universe was made up of tiny indivisible particles called atoms. In 1808, John Dalton an English scientist suggested that, an atom is the basic unit of matter. Each chemical element is made up of identical atoms and elements are different because they are each made up of different kinds of atoms. According to Dalton an atom is the smallest particle exhibiting all the properties of a particular element.

Originally it was thought that atoms of an element can not be divided further, but studies in the early twentieth century showed that, an atom is itself made up of even smaller particles known as fundamental particles or subatomic particles. They are:

Electrons,

Protons and

Neutrons.

Electrons: Electrons are the negatively charged particles present in an atom. An electron has one unit negative charge, i.e., 1.602×10\(^{-19}\) coulombs. An electron has a mass of 9.1×10\(^{-28}\) grams which is 1/1837 of the mass of one hydrogen atom. Therefore electrons are considered to have negligible mass. Evidences show that electrons are essential components of all atoms.

Protons: Protons are the positively charged particles present in an atom. They are equal in number to electrons present in an atom. They have one unit positive charge, i.e., 1.602×10\(^{-19}\) coulombs. A proton has mass equal to one atomic mass unit (amu) which is

1.6×10\(^{-24}\) grams. The number of protons in an atom of an element determines the properties of that element and distinguishes it from the atoms of other elements.

Neutrons: Neutrons are particles with no electrical charge, so they are neutral. A neutron has mass equal to one atomic mass unit, which is almost equal to the mass of a proton, i.e., 1.6 × 10\(^{-24}\) grams.

Sir J.J. Thomson in 1897 discovered the electrons.

Robert Millikan in 1909 determined the negative charge on an electron.

Goldstein discovered protons

Earnest Rutherford in 1911 discovered that, an atom has a nucleus.

Neil Bohr in 1913 discovered electron shells.

James Chadwick in 1932 discovered neutrons in an atom.

Do You Know?

The electrons, protons and neutrons that make up an atom are just three of more than 200 subatomic particles.

Atom - Its Structure

The three sub-atomic particles i.e., protons, electrons and neutrons, are of great importance in understanding the structure of an atom. Various models were put forth to arrange these particles in an atom.

According to the modern standard model, an atom consists of two structural parts:

(i) Nucleus: It is the central part of an atom which contains protons and neutrons.

(ii) Shells or orbits: It is the imaginary path in the empty space of an atom surrounding the nucleus in which electrons revolve around the nucleus. There are a number of orbits in an atom and each orbit is associated with a fixed amount of energy.

An atom of hydrogen (protium) contains only one proton and one electron but no neutron. All other atoms have all the three particles.

In comparison to the overall size of the atom the nucleus is very small. If we consider a circular stadium as an atom, then its nucleus is no more than a cricket ball.

The nucleus makes up most of the mass of an atom because it contains both protons and neutrons and they are much heavier than electrons.

The nucleus is positively charged due to the protons present in it. Neutrons are also located in the nucleus but being neutral, they do not affect the positive charge of the protons.

Protons and neutrons present in the nucleus are together called 'nucleons'.

An Atom Is Electrically Neutral

The number of protons and the number of electrons in an atom are found to be equal. Protons are positively charged and electrons are negatively charged particles, thus we can say that they have equal and opposite electrical charges. Therefore, an atom, as a whole, is electrically neutral.

The stability of an atom: We know that there exists a force of attraction between particles with opposite electrical charges. Thus, there is a force of attraction between the negatively charged electrons and the positively charged protons, present in an atom.

Electrons revolve rapidly round the nucleus in fixed circular paths called energy levels or shells. It is expected that electrons being charged, light and in motion, would gradually come closer to the nucleus and eventually fall into it, resulting in the structural collapse of the atom. But this does not happen, because electrons revolve around the nucleus at a very high speed, in the shells or orbits associated with a fixed amount of energy also called as energy levels.

The speed of an electron is about 1/10th the speed of light.

Atoms neither lose nor gain energy until an external force is applied to them. As a result, electrons do not fall into nucleus making the atom structurally stable.

Atomic number: Atomic number refers to the number of protons present in an atom. It is denoted by the alphabet Z.

Atomic number (Z) = number of protons

Example: An atom of oxygen contains 8 protons. Therefore, its atomic number is 8.

In an electrically neutral atom, the number of protons is equal to the number of electrons. Therefore, in such an atom

Z = number of protons = number of electrons

Mass number: Mass number is the sum of the number of protons and neutrons present in the nucleus of an atom. It is denoted by the alphabet A.

Mass number (A) = number of protons + number of neutrons.

Example: An atom of carbon contains 6 protons and 6 neutrons.

Therefore, its mass number is 6 + 6 = 12.

The speed of light is about 3 lakh km/sec

Teacher's Note

Understanding atomic structure helps us grasp why materials conduct electricity differently - copper wires work better than plastic because of how their electrons are arranged. This fundamental knowledge underlies all electronics we use daily.

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