12/25/21, 12:43 PM Atoms and Molecules
Laws of Chemical Combination
Chemical Reactions
In a chemical reaction,two or more molecules interact to produce new compounds
and are called reactants whereas the newly formed compounds are called products.
In a chemical reaction, a chemical change must occur which is generally observed
with physical changes like precipitation, heat production, color change etc.
Law of conservation of mass
According to the law of conservation of mass matter can neither be created nor
destroyed in a chemical reaction. It remains conserved.
Mass of reactants will be equal to the mass of products.
Law of constant proportions
A pure chemical compound contains same elements combined together in a fixed
proportion by mass is given by law of definite proportions.
For e.g. If we take water from river or from an ocean, both has oxygen and hydrogen in
the same proportion.
Atoms
An atom is the defining structure of an element, which cannot be broken by any
chemical means.
The atomic symbol has three parts:-
The symbol X: the usual element symbol
The atomic number A: equal to the number of protons
The mass number Z: equal to the total number of protons and neutrons in an element.
Dalton's Atomic Theory
1. Matter is made up of indivisible particles known as atoms.
2. The properties of all the atoms of a given element are same including mass.This can
also be stated as, all the atoms of an element have identical mass and chemical
properties; atoms of different elements have different masses and chemical
properties.
3. Atoms of different elements combine in fixed ratios to form compounds.
4. Atoms are neither created nor destroyed. The formation of new products (compounds)
results from the rearrangement of existing atoms (reactants) in a chemical reaction.
5. The relative number and kinds of atoms are constant in a given compound.
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Dalton's Atomic Theory
Atomic Mass
Atomic mass and atomic mass unit
Atomic mass is the total of the masses of the electrons, neutrons, and protons in an
atom, or in a group of atoms, the average mass.
Mass of an atomic particle is called the atomic mass.
This is commonly expressed as per the international agreement in terms of a unified
atomic mass unit (amu).
It can be best defined as 1/12 of the mass of a carbon-12 atom in its ground state.
Molecular mass
Molecular mass of an element is defined as the sum of the masses of the
elements present in the molecule.
Molecular mass is obtained by multiplying the atomic mass of an element with the
number of atoms in the molecule and then adding the masses of all the elements in the
molecule.
Mole Concept
Mole concept & Avogadro Number
In a substance, the amount of entities present for e.g.atoms, molecules, ions, is defined
as a mole. A mole of any substance is 6.022 × 10 molecules.
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Mole concept is one of the most convenient ways of expressing amount of reactants
and product in the reaction.
The value of Avogadro’s number is approximately 6.022 × 10 .The definition of Avogadro’s
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number is that it tells us the number of particles in 1 mole (or mol) of a substance. These
particles could be electrons or molecules or atoms.
Mole and Mass
Molar mass
A substance is something which has mass and occupies space. The molar
mass/molecular weight is actually the sum of the total mass in grams of the
atoms present to make up a molecule per mole. The unit of molar mass is
grams/mole.
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Atomic Valency
Molecules and Atomicity
A molecule is defined as the smallest unit of a compound that contains the
chemical properties of the compound.
The atomicity of an element is the number of atoms in one molecule of the element.
For e.g:- Hydrogen, nitrogen, oxygen, chlorine, iodine, bromine all have two atoms in
each of their molecules. So, the atomicity of hydrogen, nitrogen, oxygen, chlorine,
iodine, bromine is two each.
Structure of atom
Atom is made of three particles; electron, proton and neutron.
The centre of atom is called nucleus. Nucleus of an atom contains the whole mass of
an atom.
Electrons in an atom are arranged in shells/orbitals.
Atomic structure of carbon
Valency
Valence electrons are those electrons which are present in the outermost orbit
of the atom.
The capacity of an atom to lose, gain or share valence electrons in order to complete
its octet determines the valency of the atom.
Writing Chemical Formulae
Compounds
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When two or more elements chemically combine in a fixed ratio by mass, the obtained
product is known as a compound.
Compounds are substances consisting of two or more different types of elements in a
fixed ratio of its atoms.
Ions
An ion is defined as an atom or molecule which has gained or lost one or more of its
valence electrons, giving it a net positive or negative charge.
A negatively charged particle is called an anion and a positively charged particle is
called cation.
Ionic compounds: chemical formula
Ionic compounds are chemical compounds in which ions are held together by
specialised bonds called ionic bonds.
An Ionic compound always contains an equal amount of positive and negative charge.
For example: In Calcium chloride, the ionic bond is formed by oppositely charged
calcium and chloride ions.
Calcium atom loses 2 electrons and attains the electronic configuration of the nearest
noble gas (Ar). By doing so, it gains a net charge of +2.
The two Chlorine atoms take one electron each, thus gaining a charge of -1 (each) and
attain the electronic configuration of the nearest noble gas (Ar).
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