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Atomic radii are also known as atomic radius. It is the measure of distance from the centre of the nucleus to the outermost electron shield when an atom is in the equilibrium state (stable state). As we go across the periodic table from left to right, the atomic radii decreases because the nuclear charge increases and therefore the attraction becomes stronger and decreases the radius. As we go down the the atomic radii increases because of there’s an increase in electron shell.

Atomic radii differ depending on what’s around it, unlike a ball that has a fixed radius. The left diagram shows when the atoms are bonded together so the radius would be less than when they are just touching (right diagram). This type of atom can be found on covalent bonded atom or metallic bonded atom. Therefore it is called either **Metallic Radii** or **Covalent Radii** depending on what type of bond it is. The right diagram shows when atoms are just touching each other. The forces present are less than the left diagram and this measure is called **Van Der Waals Radius.** There is of course another one to measure ionic bonded atom and it is called **Atomic-Ionic Radii.** This kind of atomic radii is measured from bond lengths in molecules. This kind of measure will also take in account the fact that the some of the atoms will be electrically charged. For example, the //atomic-ionic radii// of chlorine (Cl-) will be larger than the //atomic radii//. Unlike normal measurements like centimetres and metres, the SI unit for atomic radii is either picometre (pm) or nanometre (nm). However, picometre has became more popular nowadays. As shown on the periodic table above, the radius of atoms increases as it goes down the group and decreases as it goes across the period. The radius gets bigger as it goes down the group because the number of electron increases therefore adding more electron shells. And so the radius decreases as it goes across the period because the number of proton in the nucleus increases and pulling the electrons tighter towards nucleus resulting in stronger attraction(force) and smaller radius. Noble gases are exceptions since they don't form bonds, hence they are gas.

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Bibliography http://www.creative-chemistry.org.uk/alevel/module1/trends1.htm#explanation __[|http://en.wikipedia.org/wiki/Periodic_trends#]__ __[]__ __[]__ http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Table_of_the_Elements/Periodic_Trends#Periodic_Trends_for_Atomic_Radius __[]__