Class 11 CBSE Ch 1 Some Basic Concept of Chemistry

3. In this part we are going to learn about

4. recapitulation

5. History of Dalton’s Atomic Theory
 Although the concept of the atom dates back to the ideas of Democritus, the English
meteorologist and chemist John Dalton formulated the first modern description of it as the
fundamental building block of chemical structures. Dalton developed the law of multiple
proportions (first presented in 1803) by studying and expanding upon the works of Antoine
Lavoisier and Joseph Proust.
 Dalton also believed atomic theory could explain why water absorbed different gases in
different proportions: for example, he found that water absorbed carbon dioxide far better
than it absorbed nitrogen. Dalton hypothesized this was due to the differences in the mass
and complexity of the gases’ respective particles.
 Dalton proposed that each chemical element is composed of atoms of a single, unique
type, and though they cannot be altered or destroyed by chemical means, they can
combine to form more complex structures (chemical compounds). Since Dalton reached his
conclusions by experimentation and examination of the results in an empirical fashion, this
marked the first truly scientific theory of the atom .

6. John Dalton’s A New System of Chemical Philosophy
This image from Dalton’s A New System of Chemical Philosophy, published in 1808,
depicts various atoms and molecules.
Dalton’s Atomic Theory
Atomic theory has been revised over the years to incorporate the existence of atomic isotopes and
the interconversion of mass and energy. In addition, the discovery of subatomic particles has shown
that atoms can be divided into smaller parts. However, Dalton’s importance in the development of
modern atomic theory has been recognized by the designation of the atomic mass unit as a Dalton.
Atom and their symbols given by Dalton

8.  An atom is the smallest constituent unit of ordinary matter that constitutes a chemical
element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.
Atoms are extremely small; typical sizes are around 100 picometers (1×10−10 m, a tenth-
millionth of a millimeter, or 1/254,000,000 of an inch).
 They are so small that accurately predicting their behavior using classical physics – as if
they were billiard balls, for example – is not possible.
 Every atom is composed of a nucleus and one or more electrons bound to the nucleus.
The nucleus is made of one or more protons and a number of neutrons. Only the most
common variety of hydrogen has no neutrons.
 Protons and neutrons are called nucleons. More than 99.94% of an atom's mass is in the
nucleus. The protons have a positive electric charge whereas the electrons have a
negative electric charge. The neutrons have no electric charge. If the number of
protons and electrons are equal, then the atom is electrically neutral. If an atom has
more or fewer electrons than protons, then it has an overall negative or positive
charge, respectively. These atoms are called ions.

9. Structure of an atom
containing Protons, Electrons
and Neutrons

10. Atomicity, the total number of atoms present in 1 molecule of a
substance Valence, sometimes referred to as atomicity.
•Monoatomic-composed of 1 atom e.g. He, Ne, Ar (all noble
gases are monoatomic)
•Diatomic-composed of 2 atoms e.g. H2 , N2 , O2
•Triatomic-composed of 3 atoms e.g. O3
•Polyatomic-composed of 3 or more atoms e.g. P4 , S8
 All metals and some other elements, such as carbon, do
not have a simple structure but consist of a very large and
indefinite number of atoms bonded together. Their
atomicity cannot be determined and is usually considered
as 1.
e.g. Na, Zn, Cu, C, Ag, etc.
 Atomicity may vary in different allotropes of the same
element.

11. A chemical element is a species of atom having the same number of protons in its atomic
nuclei (that is, the same atomic number, or Z). For example, the atomic number of oxygen is
8, so the element oxygen describes all atoms which have 8 protons.
In total, 118 elements have been identified. The first 94 occur naturally on Earth, and the
remaining 24 are synthetic elements. There are 80 elements that have at least one stable
isotope and 38 that have exclusively radionuclides, which decay over time into other
elements. Iron is the most abundant element (by mass) making up Earth, while oxygen is the
most common element in the Earth's crust.

12. Some Examples

14.  A molecule is an electrically neutral group of two or more atoms held together
by chemical bonds. Molecules are distinguished from ions by their lack of
electrical charge. However, in quantum physics, organic chemistry, and
biochemistry, the term molecule is often used less strictly, also being applied to
polyatomic ions.
 A molecule may be homonuclear, that is, it consists of atoms of one chemical
element, as with oxygen (O2); or it may be heteronuclear, a chemical compound
composed of more than one element, as with water (H2O).

19. The atom which loses electron are called Cations
The atom which gains electron are called anions

20. Ions