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Atom And Molecules

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, 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; Carbon-12 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.
Molecules
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.

MOLECULES OF COMPOUNDS
Atoms of different elements join together in definite proportions to form molecules of 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.

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 another 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.

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.
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+ 1x16 = 18
(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
RORMULA 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 x 23 + 1 x 35.5 = 58.5 u
Example- Calculate the formula unit mass of CaCl2.
Solution:Atomic mass of Ca + (2 x atomic mass of Cl)
= 40 + 2 x 35.5 = 40 + 71 = 111 u
Mole Concept
we can express the quantity of a substance is expressed in terms of mole. 
Mole is a fundamental unit in the System International d' Unités, the SI system, and it is used to measure the amount of substance In Latin mole means a "massive heap" of material 
Mole is defined as the amount of substance that contains as many specified elementary particles as the number of atoms in 12g of carbon-12 isotope.

One mole is also defined as the amount of substance which contains Avogadro number (6.023 x 
 1023) of particles. 
Avogadro number: Number of atoms or molecules or ions present in one mole of a substance is called Avogadro number. Its value is 6.023 x 1023
Therefore, one mole of any substance contains Avogadro number of particles. The particles may be atoms, molecules, ions etc
E.g. 1 mole of oxygen atoms = 6.023 x 1023 atoms, molecules, ions etc  of oxygen
                                               = 32g of O2

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