The attractive forces are much less, and the atoms are essentially "unsquashed". The right hand diagram shows what happens if the atoms are just touching. The type of atomic radius being measured here is called the metallic radius or the covalent radius depending on the bonding. This is what you would get if you had metal atoms in a metallic structure, or atoms covalently bonded to each other. The atoms are pulled closely together and so the measured radius is less than if they are just touching. ![]() ![]() The left hand diagram shows bonded atoms. The radius of an atom can only be found by measuring the distance between the nuclei of two touching atoms, and then halving that distance.Īs you can see from the diagrams, the same atom could be found to have a different radius depending on what was around it. Unlike a ball, an atom doesn't have a fixed radius. Important! If you aren't reasonable happy about electronic structures you should follow this link before you go any further. It assumes that you understand electronic structures for simple atoms written in s, p, d notation. Join my 10,000+ subscribers on my YouTube Channel for new A Level Chemistry video lessons every week.This page explains the various measures of atomic radius, and then looks at the way it varies around the Periodic Table - across periods and down groups. Please LIKE this video and SHARE it with your friends! Topic: Periodicity, Inorganic Chemistry, A Level Chemistry, Singaporeīack to other previous Inorganic Chemistry Video Lessons.įound this A Level Chemistry video useful? Overall the effective nuclear charge increases, there is a stronger attraction between nucleus and valence electrons, the nucleus can pull the electrons closer to itself, hence ionic radius decreases. Within the cations and anions, the explanation will go back to effective nuclear charge.Īll cations (and anions) are isoelectronic with the same number of electrons and electronic configuration, hence shielding effect is the same. Therefore all anions with 3 electron shells will be larger than all cations with only 2 electron shells. The next 3 elements (P, S, Cl) gain electrons to form anions with 18 electrons, hence will fill up 3 principal quantum shells. ![]() ![]() The first 4 elements (Na, Mg, Al, Si) lose electrons to form cations with 10 electrons, hence will fill up 2 principal quantum shells. Decrease in radii within cations and anions There are 2 observations for ionic radii:ī. Overall the effective nuclear charge increases, there is a stronger attraction between nucleus and valence electrons, the nucleus can pull the electrons closer to itself, hence atomic radius decreases. Note that effective nuclear charge is also used to explain first ionisation energy trend across Period 3.Īcross Period 3, proton number increases, the nucleus is more positively charged hence nuclear charge increases.Įlectrons are added to the same principal quantum shell, do not shield each other since they are the same distance from the nucleus, hence shielding effect is the same. We can use the concept of effective nuclear charge (ENC) to explain the atomic radii. The atomic radii decreases across Period 3. In this video we want to dicuss the atomic and ionic radii of Period 3 elements.
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