Leaving the noble gases out atoms get smaller as you

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Leaving the noble gases out, atoms get smaller as you go across a period.If you think about it, the metallic or covalent radius is going to be a measure of the distance from the nucleus to the electronswhich make up the bond. (Look back to the left-hand side of the first diagram on this page if you aren't sure, and picture thebonding electrons as being half way between the two nuclei.)From lithium to fluorine, those electrons are all in the 2-level, being screened by the 1s2electrons. The increasing number ofprotons in the nucleus as you go across the period pulls the electrons in more tightly. The amount of screening is constant forall of these elements.In the period from sodium to chlorine, the same thing happens. The size of the atom is controlled by the 3-level bondingelectrons being pulled closer to the nucleus by increasing numbers of protons - in each case, screened by the 1- and 2-levelelectrons.IONIC RADIUSIons aren't the same size as the atoms they come from. Compare the sizes of sodium and chloride ions with the sizes ofsodium and chlorine atoms.Positive ionsPositive ions are smaller than the atoms they come from. Sodium is 2,8,1; Na+is 2,8. You've lost a whole layer of electrons,and the remaining 10 electrons are being pulled in by the full force of 11 protons.Negative ionsNegative ions are bigger than the atoms they come from. Chlorine is 2,8,7; Cl-is 2,8,8. Although the electrons are still all inthe 3-level, the extra repulsion produced by the incoming electron causes the atom to expand. There are still only 17 protons,but they are now having to hold 18 electrons.IONIC STRUCTURESThe structure of a typical ionic solid - sodium chloride
18How the ions are arranged in sodium chlorideSodium chloride is taken as a typical ionic compound. Compounds like this consist of a giant (endlessly repeating) lattice ofions. So sodium chloride (and any other ionic compound) is described as having agiant ionic structure.You should be clear that giant in this context doesn't just mean very large. It means that you can't state exactly how many ionsthere are.There could be billions of sodium ions and chloride ions packed together, or trillions, or whatever - it simply depends howbig the crystal is. That is different from, say, a water molecule which always contains exactly 2 hydrogen atoms and oneoxygen atom - never more and never less.A small representative bit of a sodium chloride lattice looks like this:If you look at the diagram carefully, you will see that the sodium ions and chloride ions alternate with each other in each ofthe three dimensions.This diagram is easy enough to draw with a computer, but extremely difficult to draw convincingly by hand. We normallydraw an "exploded" version which looks like this:Only those ions joined by lines are actually touching each other. The sodium ion in thecentre is being touched by 6 chloride ions. By chance we might just as well have centredthe diagram around a chloride ion - that, of course, would be touched by 6 sodium ions.

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