|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 ).
ATOMICITY OF ELEMENTS
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.
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:
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”.
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”.
By suggesting that compounds contained characteristic atom-to-atom ratios, Dalton effectively explained the law of definite proportions.
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, “..it must be presumed to be a binary one, unless some cause appear to the contrary” . 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