A molecule is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. Molecules are made up of atom s that are held together by chemical bonds. These bonds form as a result of the sharing or exchange of electron s among atoms.

The atoms of certain elements readily bond with other atoms to form molecules. Examples of such elements are oxygen and chlorine. The atoms of some elements do not easily bond with other atoms. Examples are neon and argon.

Molecules can vary greatly in size and complexity. The element helium is a one-atom molecule. Some molecules consist of two atoms of the same element. For example, O 2 is the oxygen molecule most commonly found in the earth’s atmosphere; it has two atoms of oxygen. However, under certain circumstances, oxygen atoms bond into triplets (O 3 ), forming a molecule known as ozone. Other familiar molecules include water, consisting of two hydrogen atoms and one oxygen atom (H 2 O), carbon dioxide, consisting of one carbon atom bonded to two oxygen atoms (CO 2 ), and sulfuric acid, consisting of two hydrogen atoms, one sulfur atom, and four oxygen atoms (H 2 SO 4 ).


1.MONOATOMIC ELEMETS:these are elements with only one atom e.gthe noble gases(neon,argon,helium),sodium magnesium, in short all metals are monoatomic.

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2.DIATOMIC ELEMENTS:these elements contains two atoms .most non metals fall under this category.e.g N2.F2.Cl2,O2,Br2.I2.H2.

3.POLYATOMIC ELEMENTS: are those elements with more than two atoms.e.g phosphorus 5,sulphur 8

Foundations of Dalton’s atomic theory

Dalton’s atomic theory makes the following assumptions:

  1. All matter consists of tiny particles. The existence of atoms was first suggested more that 2000 years before Dalton’s birth. Atoms remained pure speculation through most of this time, although Newton used arguments based on atoms to explain the gas laws in 1687. (Newton’s speculations about atoms in the Principia were carefully copied by hand into Dalton’s notebooks.)
  2. Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass and on centuries of experimental evidence.

With the discovery of subatomic particles after Dalton’s time, it became apparent that atoms could be broken into smaller parts. The discovery of nuclear processes showed that it was even possible to transform atoms from one element into atoms of another. But we don’t consider processes that affect the nucleus to be chemical processes. The postulate is still useful in explaining the law of conservation of mass in chemistry. A slightly more restrictive wording is “Atoms cannot be created, destroyed, or transformed into other atoms in a chemical change”.

  1. Elements are characterized by the mass of their atoms. All atoms of the same element have identical weights, Dalton asserted. Atoms of different elements have different weights. (Dalton used the word “weight” rather than mass, and chemists have called atomic masses “atomic weights” ever since).

We now know that atoms of the same element sometimes have slightly different masses, but always have identical nuclear charge. In modern atomic theory, the postulate has been amended to read: “Elements are characterized by the nuclear charge of their atoms”.

  1. When elements react, their atoms combine in simple, whole-number ratios. This postulate suggested a practical strategy for determining relative atomic weights from elemental percentages in compounds. Experimental atomic weights could then be used to explain the fixed mass percentages of elements in all compounds of those elements!

By suggesting that compounds contained characteristic atom-to-atom ratios, Dalton effectively explained the law of definite proportions.

  1. When elements react, their atoms sometimes combine in more than one simple, whole-number ratio. Dalton used this postulate to explain why the weight ratios of nitrogen to oxygen in various nitrogen oxides were themselves simple multiples of each other. Even Dalton’s critics were impressed by the power and simplicity of his explanation, and it persuaded many of them that his atomic theory was worthy of further investigation.

Unfortunately, Dalton included an additional postulate that prevented his theory from being accepted for many years. When atoms combine in only one ratio, Dalton said, “ must be presumed to be a binary one, unless some cause appear to the contrary” [2]. He had no experimental evidence to support this postulate, and it lead him to mistakenly assume that the formula of water was OH and the formula of ammonia was NH. As a result, Dalton’s atomic weights for oxygen and nitrogen were incorrect and his experimental data did not support many of the conclusions he drew from it.A consistent set of atomic weights was absolutely essential before the theory could be accepted and applied. Next, we’ll see how Dalton’s postulates can be used to estimate atomic weights from experimental data, and how they explain three basic laws of chemistry.



1.Define molecules and Atomicity.

2.State with examples, the types of Atomicity.

3.State Dalton’s Atomic theory. Describe its modification


See also



Chemistry as a Subject



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