There are things called group and period. the groups are basically used for the electron dot diagram. the period is used for the cloud model. there are 7 periods. for example, Sodium is in period 3, so sodium has 3 energy levels. draw a circle. the circle is your nucleus. then draw another circle around your nucleus. that is your first energy level. there are 2 electrons in the first level, 8 in the second and 18 in the third. for sodium the atomic number is 11. first draw your nucleus. then draw your first energy level. the first energy level has 2 electrons. then draw your second energy level. it had 8 electrons. so far your atomic number is 8. sodium has 3 energy levels because it's in period 3. draw your third energy level with 1 electron. For sodium you only draw 1 electron because you already have 2 electrons on the first level and 8 on the second. the atomic number for sodium is 11 so it has to equal 11. that is why you only draw one electron on the third energy level. 8+2+1=11. it is the same number as your atomic number. and there you have your electron cloud model.