CLASS 6TH | PHYSICS | ELEMENTS, COMPOUNDS, SYMBOLS AND FORMULAE | REVISION NOTES

The name atom comes from the Greek word ‘atomos’, which means uncuttable or indivisible.

An atom is the smallest particle of an element having the same chemical properties of the element. Atoms are the basis for everything in the universe, including us.

Atoms are tiny particles that are too small to see, even with a microscope. They are so tiny that if people were the same size as atoms, the entire population of the world would fit into a small box.

Just like letters are the basic building blocks of any language, atoms are the primary building blocks for things around us. The chair you sit on, the lunch box you carry to school, pizza you eat  – just everything is made up of atoms.

When atoms combine together they form molecules.

Let us study about the atoms and molecules in detail. 

 Dalton’s Atomic Theory:

• Matter is made of atoms which cannot be broken down further.
• Atoms cannot be created or destroyed in a chemical reaction.
• Atoms of the same element are identical to each other while atoms of different elements differ in mass and chemical properties.
• Atoms combine in definite ratios of whole numbers to form compounds.
• The number and composition of atoms are constant for a given compound.

A few important definitions

1. Atom: Atom is the smallest unit of matter and is further made up of protons, neutrons, and electrons. Atomic radius is measured in nanometres (10−910−9 m)
2. Atomic mass: A mass unit equal exactly to 112th112th the mass of one atom of Carbon-12
3. Gram atomic mass: The atomic mass of an element expressed in grams is called gram atomic mass
4. Molecule: The smallest unit of an element or a compound which can exist independently and show all its physical and chemical properties is called molecule. A molecule may have atoms of the same kind(element) or of different kinds (compound)

We come across many chemical substances and compounds daily. Everything around us consists of chemical substances. These may be man-made chemical substances and substances extracted from nature

Examples of chemical products at home are dishwashing and cleaning detergents, glue, common salt, baking soda, etc

 These chemicals are made up of atoms of a fixed proportion and these atoms are arranged in a certain way. In order to find out what these proportions are and how they are arranged for any given substance, we need to know the chemical formula of the substance or compound.

In order to make it easier to describe elements and molecules, chemical formulae are used. They are used to describe the number of atoms in each element or compound. It contains the symbols of the atoms of the elements present in the compound, as well as how many there are for each element in the form of subscripts.

For example, ‘H’ represents one atom of hydrogen and ‘O’ represents one atom of oxygen.  If we want to represent two atoms of hydrogen, instead of writing H H, we write H₂. The subscript “2” means that two atoms of the element hydrogen have joined together to form a molecule.

We will study about the formulae in this chapter.

Importance of chemical formulae:

• It tells us about the types of elements present in the compound
• It tells us the number of atoms of each kind present in the molecule of the compound
• It gives us the mass of one molecule of the compound

 Chemical Formulae

• Every chemical substance is known by a specific name.
• Each chemical compound is represented by a chemical formula that gives its composition (constituent elements present) and the number of elements of each type present. 

Molecular Formula

• A chemical formula that represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule is also called molecular formula.
• For example, the molecular formula of water is H₂O, which means that one molecule of water contains two atoms of hydrogen and one atom of oxygen.

Significance of Molecular Formula

• It represents name of the substance
• It represents one molecule of substance
• It gives the names of all the elements present in one molecule of the substance. It also gives the number of atoms of each element present.
• It represents a definite mass and refers to the presence of 6.023 x 10²³ molecules (known as the mole concept).

Ancient people like the Greeks, Romans, Indians, the Chinese, knew about elements like iron, copper, gold, silver, lead etc. The first element humans knew was most probably the carbon, in the form of charcoal.

There are about 115 elements out of which there are 92 elements that occur naturally. Most of them were discovered by the late 1800’s.

 The modern arrangement of the elements is known as the Periodic Table of Elements and is arranged according to the atomic number of elements. The scientist Dmitri Mendeleev, a Russian chemist, proposed an arrangement of know elements based on their atomic mass.

• The periodic table of the chemical elements is a tabular display of the known chemical elements. Each element is listed by its atomic number and chemical symbol.
• Symbols are used to represent elements. A symbol is a letter or picture used to represent something. One or two letters are used to represent elements. Example: The symbol for aluminum is Al and the symbol for oxygen is O.
•  A chemical formula is used as a quick way to show the composition of compounds. Letters, numbers, and symbols are used to represent elements and the number of elements in each compound.

Let us study in detail about elements, compounds, symbols, and formulae in this chapter.

Substances

 Substances:

Substances are classified into two types.

• Pure Substances – A substance which is made of one kind of particle.

Examples – iron, gold

• Impure substances or Mixtures – A substance which contains two or more different particles is known as a mixture.

Example: a salt solution is made up of two components, salt, and water.

On the basis of their chemical composition, pure substances are classified as elements and compounds.

Elements :

A pure substance which is made up of only one kind of atom and cannot be broken into two or more simpler substances by physical or chemical means is referred to as an element. Example: Zinc, Sulphur.
 

Characteristics of elements:

• An element is homogeneous in nature; it is a pure substance, made up of only one kind of atoms. For example, iron and silver are made of only iron and silver atoms.
• An element cannot be broken down into simpler substances by any physical or chemical methods such as heat, light electricity, or chemical reactions with other substances. For example, if you were to smash a piece of iron into smaller pieces or heat it, the piece still remains as the element iron.
• An atom is the smallest unit that shows all the properties of an element. For example, an atom of iron shows all the properties of that metal.
• Elements have sharp melting and boiling points.

Symbols to represent elements:

Each element is represented by a symbol. It is usually derived from the first letter of its name in English or Latin. The first letter of the symbol is always in upper case. Some elements are represented by the starting alphabet of its name. Example: Carbon is represented as C. Symbols of some elements are the first two alphabets, where the second alphabet is in the lower case. Example: Sodium is represented as Na, where Natrium is the Latin name of sodium.

Compounds:

• They are made up of one or more elements and are denoted by chemical formulae.
• A compound contains atoms of different elements chemically combined in a fixed ratio.The smallest unit of a compound is a molecule
• Compounds contain different elements in a fixed ratio arranged in a defined manner through chemical bonds.
• A compound can be broken down into simpler substances by chemical methods/reactions 
• A compound is represented using a formula. Example: Water (H₂O), Sodium chloride (NaCl),Magnesium oxide (MgO)