New Year

The Periodic table

• In the periodic table the elements are arranged in order if increasing atomic number.
• The lements are arranged in periods and groups.

Period:
• A period is a horizontal raw of elements.
• There are seven periods.
• The period number refers to the number of shells present in an element.

For example: The element carbon and sodium.

Carbon has two shells, therefore it belongs to period two. Like wise sodium has three shells so it belongs to period three.

• First period consists of two elements. They are hydrogen and Helium.
• The second and the third period consists of eight elements.
• The fourth and the fifth period consists of eighteen elements.

Giant Molecular structure

Some substances consists of extremely large molecules or giant molecules. The molecules contains many hundreds and thousands of atoms covalently bonded with each other. We call this substances macromolecules.
E.g.: - Diamond, graphite, silicon (IV) oxide (silica)

Structure of Diamonds
• Each single crystal if a diamond is one giant molecule. carbon atom is bonded to other carbon atoms tetrahedrally.
• All the four outermost electrons in carbon atom are involved in the bond formation and there are no free electrons to move. therefore diamond does not conduct electricity.
• A lot if energy is required to break apart the strong covalent bonds between the carbon atoms, hence diamond has very high melting & boiling point.
• Diamond is the hardest substance known since the carbon atoms are not able to slide over each other due to strong covalent bonds.
• Diamond is a transparent, colourless crystal.

Structure of Graphite
• Graphite has a layered structure each carbon is bonded to three other carbon atoms in a hexagonal arrangement to form rings of regular hexagons. the different layers are held together by weak vanderwaal's force of attraction.
• Graphite has very high melting & boiling point. therefore a lot of energy is required to break apart the strong covalent bonds between the carbon atoms.
• It is a good conductor of electricity because each carbon atom uses only 3 out of 4 outer electrons for bonding with one free electron to conduct electricity.
• The weak vanderwaal's force between the layers enables layers to slide over each other. hence graphite is a soft substance.
• It is black, opaque and shiny solid.

Structure of Polythene
• Polythene is a plymer. It is made up of thousands of molecules called monomers joined together in long chains. The monomer used in making polythene is called ethene.
• Polythene has high melting point and does not conduct electricity.
• Polythene can be softened on heating and melting and set again when cooled.

Simple Molecular structure

• A simple molecular structure contains small molecules.
• Simple molecular structure are formed from only a few atoms.
• They have strong covalent bonds between the atoms with in a molecule, what have weak bonds between molecules is called as Vanderwaal’s force.
• These vanderwaal’s forces increase steadily with the increasing size of the molecules.
• Most of the simple molecular structures are liquids or gases under normal conditions.
• In simple molecular structures the forces between the molecules in the solid and liquid state are weak such that very little energy is needed to break up the structure.
E.g.: - Methane (CH4), Iodine (I2), Water (H2O), Carbon dioxide (CO2)

Structure of Methane
This is the tetrahedral shake of methane molecule. The carbon-hydrogen bond is a strong covalent bond. There is weak vanderwaal’s force is between the molecules of methane.

Structure of Iodine
Iodine has a crystal structure in which iodine molecules are packed together. The force between the molecules are weak so iodine is a flaky solid that sublimes if heated gently iodine crystals can be crushed easily.

Elements, Compounds & Mixtures

Elements:
• An element is made up of only one types of atom.
• Elements are substances that cannot be broken down chemically into smaller substances.
E.g.: - Carbon, Nitrogen, Oxygen, Calcium, Helium etc.

Compounds:
• A compound is made up of two or more different kinds of atoms chemically combined together.
E.g.: - Water, Hydrochloric acid, Carbon monoxide etc.

Mixtures:
Mixtures are the substances that are simply mixed together without any chemical reaction taking place between them. A mixture is made up of two or more elements or compounds physically combined together. The individual substances can be separated by physical means.
E.g.: - Air, Sea water etc.

Electronic configuration of Ions

• Positive ions are formed when atoms loose electrons.

• Ions with single positive charge are formed when an atom loose one electron.

E.g.: - Sodium atom (Na) loose one electron to form sodium ion (Na+)

• Ions with two positive charges are formed when an atom loose two electrons.

E.g.: - Beryllium atoms (Be) loose 2 electrons to form beryllium ion (Be²).

• Therefore the number of positive charges carried by an ion depends of the number of electrons lost by a particular atoms.

• Negative ions are formed when atoms gain electrons.

• Ions with single negative charge are formed when an atom gains one electrons

E.g.: - Chlorine (Cl) atom gains one electron to form chloride ion (Cl−).

• Ions with 2 negative charges are formed when an atom gains 2 electrons.

E.g.: - Oxygen (O) atom gain 2 electrons to form oxide ion (O²−).

• Therefore the number of negative charges carried by an ion depends on the number of electrons gained by a particular atom.

Electron Arrangement in Atoms

• Electrons are arranged in shells or energy levels around the nucleus of an atom.
• This arrangement of electrons around the nucleus of an atom is called electronic configuration.
• Shells are numbered starting from the shell which is nearest to the nucleus of the atom.
• Hence the shell which is furthest away from nucleus is called the outermost shell and the electrons in the shell are referred to as the outermost electrons.
• The shell nearest to nucleus is called the first shell and this is also the lowest energy level.
• Each shell can hold a fixed number of electrons. The formula helps us to find the number of electrons in each shell. And the formula is 2n² where ‘n’ is the shell number.

For the first 20 elements (hydrogen to calcium)
» The first shell (lowest energy level) can hold maximum of 2 electrons.
» The second shell can hold a maximum of 8 electrons.
» The third shell can hold a maximum of 8 electrons.
» The remaining electrons are placed in the fourth shell.
» The energy levels must be filled in order of increasing energy. The first level is filled first before going to the second level and subsequent higher levels.