ATOMS AND MOLECULES
Symbols of Atoms of Different Elements
Atomic Mass
Molecules
Ions
Mole Concept
Atom
An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a
central nucleus that is usually surrounded by one or more electrons. Each electron is negatively
charged. The nucleus is positively charged, and contains one or more relatively heavy particles
known as protons and neutrons.
A proton is positively charged. The number of protons in the nucleus of an atom is the atomic
number for the chemical element. A proton has a rest mass, denoted mp, of approximately 1.673 x
10
-27
kilogram (kg). A neutron is electrically neutral and has a rest mass, denoted mn, of
approximately 1.675 x 10
-27
kg. The mass of a proton or neutron increases when the particle attains
extreme speed, for example in a cyclotron or linear accelerator.
The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or
atomic weight.
Atoms having the same number of protons, but different numbers of neutrons, represent the same
element, but are known as different isotopes of that element. The isotope for an element is
specified by the sum of the number of protons and neutrons. Examples of different isotopes of an
element are carbon 12(the most common, non-radioactive isotope of carbon) and carbon 14 (a less
common, radioactive isotope of carbon).
Protons and electrons have equal and opposite charge, and normally an atom has equal numbers
of both. Thus, atoms are usually neutral. An ion is an atom with extra electrons or with a deficiency
of electrons, resulting in its being electrically charged. An ion with extra electrons is negatively
charged and is called an anion; an ion deficient in electrons is positively charged and is called a
cat ion.
Isotones- Nuclei of atoms with the same neutron number. Example: S-36, Cl-37, Ar-38, K-39, Ca-40. These nuclei contain 20 neutrons each, but a different number of protons: sulphur 16, chlorine
17, argon 18, potassium 19 and calcium 20 protons.
Isobars are nuclides having the same mass number; i.e. sum of protons plus neutrons; Carbon12 and
Boron -12.
SYMBOLS OF ATOMS OF DIFFERENT ELEMENTS
Dalton was the first scientist to use the symbols for elements in a very specific sense.
Symbol for some elements
ATOMIC MASS
The most remarkable concept that Dalton’s atomic theory proposed was that of the atomic mass.
According to him, each element had a characteristic atomic mass. One atomic mass unit is a mass
unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic
masses of all elements have been found with respect to an atom of carbon-12.
The relative atomic mass of the atom of an element is defined as the average mass of the atom, as
compared to 1/12th the mass of one carbon-12 atom
HOW DO ATOMS EXIST?
Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These
molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.
Molecule
A molecule is in general a group of two or more atoms that are chemically bonded together that is,
tightly held together by attractive forces. A molecule can be defined as the smallest particle of an
element or a compound that is capable of an 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 many
elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But
this is not the case with most of the nonmetals.
For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a
diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get
ozone. The number of atoms constituting a molecule is known as its atomicity. Molecules of 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.
Atoms of different elements join together in definite proportions to form molecules of compounds.
Molecules of Some Compounds
ION
Compounds composed of metals and nonmetals contain charged species. The charged species
are known as ions. An ion is a charged particle and can be negatively or positively charged. A
negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. For an example,
sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and
negatively charged chloride ions (Cl–). Ions may consist of a single charged atom or a group of
atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic
ion.
Some Iconic Compounds
Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition.
The combining power (or capacity) of an element is known as its valency. Valency can be used to
find out how the atoms of an element will combine with the atom(s) of anoth er element to form a
chemical compound. The valency of the atom of an element can be thought of as hands or arms of
that atom.
Rules to follow while writing a chemical formula are as follows:
• the valencies or charges on the ion must balance.
• when a compound consists of a metal and a non-metal, the name or symbol of the metal is
written first.
For example:
calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where
oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium,
iron and copper are metals, and are written on the left.
• in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the
number to indicate the ratio.
Some Common & Simple Ions
The rules to follow while writing a chemical formula are as follows:
• The valencies or charges on the ion must balance.
• when a compound consists of a metal and a non-metal, the name or symbol of the metal is
written first.
For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide
(CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas
calcium, sodium, iron and copper are metals, and are written on the left.
• In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the
number to indicate the ratio.
FORMULAE OF SIMPLE COMPOUNDS
The simplest compounds, which are made up of two different elements are called binary
compounds. While writing the chemical formulae for compounds, we write the constituent elements
and their valencies as shown below. Then we must crossover the valencies of the combining atoms.
Examples
1. Formula of hydrogen chloride
Thus, in magnesium chloride, there are two chloride ions (Cl
-) for each magnesium ion (Mg2
+
). The
positive and negative charges must balance each other and the overall structure must be neutral.
Molecular Mass and Mole Concept
MOLECULAR MASS:- The molecular mass of a substance is the sum of the atomic masses of all
the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed
in atomic mass units (u).
Example (a) Calculate the relative molecular mass of water (H2O).
(b) Calculate the molecular mass of HNO3.
Solution:
(a) Atomic mass of hydrogen = 1u,
oxygen = 16 u
So the molecular mass of water, which contains two atoms of hydrogen and one
atom of oxygen is = 2 x1+ 1×16
= 18 u
(b) The molecular mass of HNO3 = the atomic
mass of H + the atomic mass of
N+ 3 × the atomic mass of O =
1 + 14 + 48 = 63 u
FORMULA UNIT MASS
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit
of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular
mass.
For example,
Sodium chloride has a formula unit NaCl. Its formula unit mass can be
calculated as– 1 x23 + 1 x35.5 = 58.5 u
Example- Calculate the formula unit mass of CaCl2.
Solution:
Atomic mass of Ca
+ (2 x atomic mass of Cl )
= 40 + 2 x35.5 = 40 + 71 = 111 u
MOLE CONCEPT
Take an example of the reaction of hydrogen
and oxygen to form water: 2H2 + O2 –>2H2O.
The above reaction indicates that
(i) two molecules of hydrogen combine with one molecule of oxygen to form two molecules of
water, or
(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u of water
molecules. We can infer from the above equation that the quantity of a substance can be
characterised by its mass or the number of molecules. But, a chemical reaction equation indicates
directly the number of atoms or molecules taking part in the reaction.
Therefore, it is more convenient to refer to the quantity of a substance in terms of the number of its
molecules or atoms, rather than their masses. So, a new unit “mole” was introduced. One mole of
any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to
its atomic or molecular mass in grams
Note Blast 