Lewis structure of lewis electron dot formula of a molecule can be written easily written by following few steps.These steps will not only inspect you when u write lewis structure , you will also save time by following them:
1) Count the number of valence electrons : For the given molecule simple sum up the valence electrons of each atom present ( valence electrons are the electrons in the outermost principal energy level of atom like for Flourine 1s2 2s2 2p5, the last seven electrons in its outermost principal energy level are valence electrons, you can easily identify number of valence electrons of atom by the group number in which it is present like Flourine is in 7A group and so have 7 valence electrons).For polyatomic anion,one electron is added for each unit of negative charge.For polyatomic cation,a number of electrons equal to the positive charge must be subtracted.
2) Draw a skeleton structure for the species,joining atoms by single bonds : To draw the lewis structure correctly,we must know the atoms are connected.The guidlines help us to do this.The lewis structure does not have to match the skeleton structure so we can arrange the skeleton sturcture or framework in any convienient way.For example: In PCl3 the three chlorine atom being more electronegative than phophorus are the outer or terminal atoms and phosphorus being less electronegative is the central atom.Each chlorine atom can be shown bonded to phosphorus atom using straingth lines.
3) Distribute electrons to the atoms surrounding the central atom : to satisfy the octet rule for these surrounding atoms,except H.Hydrogen always associated with two electrons.All other outer atoms always have eight electrons.
4) Assign the remaining valence electrons to inner atoms: This is a straight step when there is only one inner atom.If the molecule has more than one inner atom,place non bonding pairs around the most electronegative untill it has an octet of electrons.If there are still unassigned electrons,do the same for the next most electronegative atom.