Atoms and Molecules

Drawbacks

1. Today, we know atoms are not indivisible, i.e., they themselves are made up of protons, electrons and neutrons.
2. Acc. to Dalton, all atoms of an element have exactly the same mass. It is, however, now known that atoms of the same element can have slightly different masses.eg .
3. Acc. to Dalton, the atoms of different elements have different masses. It is, however,not known that even atoms of different elements can have the same masses.

Atoms: Atom is smallest particle which takes part in physical and chemical processes. It may or may not exist freely. Atoms form molecules and ions.

Atomic radius is measured in nanometers. 1 / 10⁹ m = 1nm
1 = 109nm

Dalton was the first scientist to use the symbols for elements in a very specific sense. The symbol also meant a definite quantity of that element ,i.e., one atom of that element. Earlier the names were derived from the places where they were found , specific colors , etc. Other symbols have been taken from the names of elements in Latin.

Significance of symbol:

1. It represents name of the symbol.
2. It represents an atom of the element.
3. It represents one mole of element.
4. It represents atomic mass of element.

Atomic mass: Each element has a characteristic atomic mass. Since determining the mass of an individual atom was a difficult task, relative atomic masses were determined using laws of chemical combinations and the compounds formed.

Atomic mass unit : Earlier it was (amu). Scientists took 1/16 of the mass of an atom of naturally occurring oxygen as the unit because -

1. Oxygen reacted with large no. of elements and formed compounds.
2. This atomic mass gave masses of most of the elements as whole numbers

At present - In 1961 Universally accepted atomic mass unit, carbon – 12 isotope was chosen as the standard reference for measuring atomic masses.

Atomic mass unit (u) is a mass unit equal to exactly 1/12th the mass of one atom of carbon-12.

Relative atomic mass:

It is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom.(Hydrogen is the lightest element as its At. mass is 1)

Molecule: It is the smallest particle of an element or a compound that is capable of independent existence and shows all the properties of that substance.

Atoms of the same element or of different elements can join together to form molecules.

Molecules of elements:

The molecules of an element are constituted by the same type of atoms.  Molecules of Argon(Ar), helium (He), etc. are made up of only one atom of that element. But it is not so in case of non-metals. Eg. A molecule of oxygen consists of two atoms of oxygen; so it is called  diatomic molecule, O₂. If 3 atoms of oxygen form  one molecule, we get ozone.

Atomicity-  The number of atoms constituting a molecule .

Molecules of compounds – Atoms of different elements join together in definite proportions to form molecules of compounds.

Ions-  Compounds composed of metals and non-metals contain charged species known as ions. It can be negatively (anion) or positively ( cation)  charged.

Polyatomic ions – A group of atoms carrying a charge .

Writing Chemical Formulae:

The chemical formula of a compound is a symbolic representation of its composition.For writing a chemical formula we use symbols and the combining capacity (valency)  of an element.

Rules for writing a chemical formula

1. The valencies or charges on the ions must balance.
2. When a compound consists of a metal and a non-metal, the name or symbol of metal is written first. ( CaO , FeS, etc.)
3. In compounds with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.If no. of polyatomic ion is one, bracket is not needed (NaOH)