[Music] atoms and molecules this is an anthill the ants are busy building it it takes many small grains of sand to make an anthill if you take a closer look at the grain of sand under a scanning tunneling microscope you can see that even the small grain of sand consists of millions of particles which are miniscule in nature these particles are called atoms atoms are the building blocks of all material atoms are made up of three types of particles called protons neutrons and electrons an atom has a central nucleus with protons and neutrons in it protons are positively charged and neutrons have no charge so the nucleus is positively charged protons and neutrons are collectively called nucleons electrons are negatively charged and are located around the nucleus an atom has the same number of negatively charged electrons as it has positively charged protons in the nucleus therefore atoms are electrically neutral electrons are distributed in a definite path around the nucleus and are called shells or orbits this concept was introduced by names BA according to Bohr different electrons revolve around the nucleus in their own different orbits the atom has been compared to the solar system in an atom the nucleus is the Sun and the electrons are the planet just like planets in the solar system each electron revolves around the nucleus in its own different orbit the orbits of shells are represented by the letters K L M etc the maximum number of electrons present in a shell is given by the formula - N squared where n is the orbit number of energy cells the first orbit or K shell can have two the second orbit or L shell can have eight and M shell can have 18 electrons etc the electrons present in the outermost shell of an atom are called valence electrons and the shell which valence electrons occupy is called the valence shell a molecule is in general a group of two or more atoms that is chemically bonded together atoms combine chemically by external forces such as heat and light this leads to the formation of molecules during the formation of a molecule atoms lose the valence electrons or gain electrons from other atoms for instance if two pieces of wax are to be joined they are heated after heating the two pieces are pressed together to form a single structure here heat is a chemical factor which is responsible for joining the molecules of wax pieces there are two main types of bonds covalent and ionic if a molecule is made up of atoms of the same element it is called a homo atomic molecule if the molecule is made up of atoms of different elements it is called the hetero atomic molecule things to remember building blocks of matter that we see in the universe are atoms protons neutrons and electrons constitute an atom protons have a positive charge and neutrons have no charge and electrons are negatively charged the maximum number of electrons present in a shell is given by the formula - n square a molecule is a chemical bonding of two or more atoms bonds are classified as covalent and ionic there are two types of molecules homo atomic and hetero atomic [Music] you