Atoms And Molecules

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Atoms and Molecules - The tiny particles that make up elements are called atoms. An atom is the smallest unit of an element that retains or keeps the properties of that element. For example, the atoms that make up oxygen can never be changed. The inside of an atom contains three different kinds of particles as well. There are three parts to an atom. Protons have a positive electrical charge and are found together.

What is your brain made of?

Everything you can see, touch, smell, feel, and taste is made of atoms. Atoms are the basic building-block of all matter (including you and me, and everyone else you’ll ever meet), so if we want to know about what Earth is made of, then we have to know a few things about these incredibly small objects.

Atoms and molecules concept map


Everyday experience should convince you that matter is found in myriad forms, yet all the matter you have ever seen is made of atoms, or atoms stuck together in configurations of dizzying complexity. A chemical element is a substance that cannot be made into a simpler form by ordinary chemical means. The smallest unit of a chemical element is an atom , and all atoms of a particular element are identical.

Parts of an Atom

There are two parts to an atom ( Figure below ):

  • At the center of an atom is a nucleus made up of two types of particles called protons and neutrons.
    • Protons have a positive electrical charge. The number of protons in the nucleus determines what element the atom is.
    • Neutrons are about the size of protons but have no charge.
  • Electrons , much smaller than protons or neutrons, have a negative electrical charge, move at nearly the speed of light, and orbit the nucleus at exact distances, depending on their energy.

Major parts of an atom. What chemical element is this? (Hint: 3 protons, 3 electrons)

Atomic Mass

Because electrons are minuscule compared with protons and neutrons, the number of protons plus neutrons gives the atom its atomic mass . All atoms of a given element always have the same number of protons, but may differ in the number of neutrons found in the nucleus.


Atoms are stable when they have a full outermost electron energy level. To fill its outermost shell, an atom will give, take, or share electrons. When an atom either gains or loses electrons, this creates an ion . Ions have either a positive or a negative electrical charge. What is the charge of an ion if the atom loses an electron? An atom with the same number of protons and electrons has no overall charge, so if an atom loses the negatively charged electron, it will have more protons, therefore, a positive charge. Ions with a positive charge are referred to as a cation.What is the charge of an ion if the atom gains an electron? If the atom gains an electron, there will be more electrons and will have a negative charge. Anions are ions that have a negative charge.


In the previous section we said that many atoms are more stable when they have a net charge: they are more stable as ions. When a cation gets close to an anion, they link up because of their different net charges — positive charges attract negative charges and vice versa. When two or more atoms link up, they create a molecule . A molecule of water is made of two atoms of hydrogen (H) and one atom of oxygen (O). The molecular mass is the sum of the masses of all the atoms in the molecule. A collection of molecules is called a compound.


  • An atom has negatively-charged electrons in orbit around its nucleus, which is composed of positively-charged protons and neutrons, which have no charge.
  • An atom that gains or loses electrons is an ion. Positively charged ions are cations, negatively charged ions are anions.
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Atoms And Molecules Ppt

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  • Around 500 BC, an Indian Philosopher Maharishi Kanad, first time postulated the concept of indivisible part of matter and named it ‘pramanu.’

  • In 1808, John Dalton used the term ‘atom’ and postulated the atomic theory to the study of matter.

Dalton’s Atomic Theory

  • According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

  • According to Dalton’s atomic theory, all matters, whether they are elements, compounds, or mixtures, are composed of small particles known as atoms.

Salient features of Dalton’s Atomic Theory

  • All matter is made of very miniscule particles known as atoms.

  • Atom is an indivisible particle, which cannot be created or destroyed through chemical reaction.

  • All atoms of an element are identical in mass and chemical properties whereas, atoms of different elements have different masses and chemical properties.

  • To form a compound, atoms are combined in the ratio of small whole numbers.

  • In a given compound, the relative number and kinds of atoms are constant.

Atomic Mass

  • The mass of an atom of a chemical element; it is expressed in atomic mass units (symbol is u).

  • The atomic mass is roughly equivalent to the number of protons and neutrons present in the atom.

  • One atomic mass unit is a mass unit equal to the exactly one-twelfth (1/12th) the mass of one atom of carbon-12 and the relative atomic masses of all elements have been calculated with respect to an atom of carbon-12.


  • The smallest particle of an element or a compound, which is capable to exist independently and shows all the properties of the respective substance.

  • A molecule, normally, is a group of two or more atoms which are chemically bonded together.

  • Atoms of the same element or of different elements can join (with chemical bond) together to form molecules.

  • The number of atoms that constitute a molecule is known as its atomicity.


  • A charged particle is known as ion; it could be either negative charge or positive charge.

  • The positively charged ion is known as a ‘cation’.

  • The negatively charged ion is known as an ‘anion.’

Chemical Formulae

  • A chemical formula of a compound demonstrations its constituent elements and the number of atoms of each combining element.

Atoms And Molecules Pdf

  • The chemical formula of a compound is the symbolic representation of its Composition.

  • The combining capacity of an element is known as its ‘valency.’

Molecular Mass

  • The molecular mass of a substance is calculated by taking the sum of the atomic masses of all the atoms in a molecule of respective substance. For example, the molecular mass of water is calculated as −

    • Atomic mass of hydrogen = 1u

    • Atomic mass of oxygen = 16 u

  • The water contains two atoms of hydrogen and one atom of oxygen.

  • Molecular Mass of Water is = 2 × 1+ 1×16 = 18 u (u is the symbol of molecular mass).

Formula Unit Mass

  • The formula unit mass of a substance is calculated by taking the sum of the atomic masses of all atoms in a formula unit of a compound.

Avogadro Constant or Avogadro Number

  • Avogadro was an Italian scientist who had given the concept of Avogadro Number (also known as Avogadro Constant).

  • The number of particles (atoms, molecules, or ions) present in 1 mole of any substance is fixed, and its value always calculated as 6.022 × 1023.

  • In 1896, Wilhelm Ostwald had introduced the concept of ‘mole;’ however, mole unit was accepted to provide a simple way of reporting a large number in 1967.

Law of Conservation of Mass

  • During a chemical reaction, sum of the masses of the reactants and products remains unchanged, which is known as the ‘Law of Conservation of Mass.’

Law of Definite Proportions

  • In a pure chemical compound, its elements are always present in a definite proportion by mass, which is known as the ‘Law of Definite Proportions.’