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Updated: 9/18/2018
Unknown Story
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  • But teacher, we don't know what is atom radius yet.
  • Today we are going to talk about atom radius and atomic size.
  • Atom radius is one half of the distance between the nuclei of two atoms of the same element when they join together.
  • Oh, so atom size is decided by the atom radius?
  • Partly yes. And if we look at the periodic table, we can find out a pattern of the atom size.
  • By looking at the periodic table?!
  • Yes, and this called the periodic trend. By studying the periodic trend, we can easily find out the atom size is getting bigger or smaller.
  • So look at the board. The periodic trend for the stom size is that in a period,the size of atom decreases; in a group, the size of atom increases.
  • And i am going to explain the reason for you now.
  • In a period, the number of protons andelectrons is increasing, and the number of shells stays the same
  • Once the nuclei has more protons, it means that the nuclei has more force to pull the electrons closer,therefore the size of atom decreases.
  • In a group, the number of protons and electrons is increasing while the number of shells is also increasing.
  • Okay, got it!
  • The nuclei does not have much force to pull the electrons of the outest shell closer, therefore the size of atom increases.
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