 Unlike a ball, an atom doesn't have a fixed radius.  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. As you can see from the diagrams,  the same atom could be found to have a different radius depending on what’s around it. The left hand diagram shows bonded atoms. The right hand diagram shows what happens if the atoms are just touching.

 The atoms get bigger as you go down groups.  The reason - you are adding extra layers of electrons.  Leaving the noble gases out, atoms get smaller as you go across a period.  The increasing number of protons in the nucleus as you go across the period pulls the electrons in more tightly.

Positive ions: Positive ions are smaller than the atoms they come from. Sodium’s configuration 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 ions Negative ions are bigger than the atoms they come from. Chlorine’s configuration is: 2,8,7; Cl - is: is 2,8,8. Although the electrons are still all in the 3rd-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.

Definition:  The amount of energy it takes to detach one electron from a neutral atom.  Ions are atoms which have gained or lost electrons.  Some elements actually have several ionization energies.  When this is the case, we refer to them as the "first ionization energy" or 'I', "second ionization energy" or 'I 2 ', and so on.

Increases as you go across a period Decrease as you go down a group

 The ionization energy of an atom is equal to the amount of energy needed to remove the outermost, or highest energy, electron from a neutral atom in the gas phase.  Electron affinity is the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion.  Electron affinities follow the same trends as the ionization energy across the periodic table as seen below:

Definition:  A measure of the tendency of an atom to attract a bonding pair of electrons.  Fluorine (F) (the most electronegative element) is assigned a value of 4.0, and values range down to cesium (Cs) and francium (Fr) which are the least electronegative at 0.7.  Trend: across a period  down a group

 Both have the same tendency to attract the bonding pair of electrons.  Bonding pair will be found “ on average” half way between the two atoms. (They share)  To get a bond like this, they usually have to be the same atom. Ex. H 2 or Cl 2 molecules. This sort of bond could be thought of as being a "pure" covalent bond - where the electrons are shared evenly between the two atoms. “Nonpolar Covalent Bond”

 That means that one end of the bond has more than its fair share of electron density and so becomes slightly negative.  At the same time, the other end (rather short of electrons) becomes slightly positive. B will attract the electron pair more than A does. This is described as a polar bond. This a covalent bond in which there is a separation of charge between one end and the other.

 The electron pair is dragged right over to one end of the bonding pair.  For all intents and purposes one atom has lost control of its electron, and the other atom has complete control over both electrons.  Ions have been formed.

 Atoms will gain or lose electrons to form chemical bonds. They do this in order to obtain a stable octet (8 valence electrons).  When an atom loses electrons there are now more protons (+ charge) than electrons (- charge).  This difference causes the overall charge on the atom to go from being neutral to positive.  Now that it is charged it’s an ION or CATION.  Metals form cations.  Hint: CATION (See a + Ion)  Ex. Na + : Sodium gives off 1 e- when bonding. Therefore, it has 1 less negative e - than positive p +.

 Sometimes atoms will gain electron to form that stable octet.  Now there are more electrons (- charge) than protons (+ charge).  This causes the overall charge on the atom to go from neutral to positive.  This charge atom is now called an ION or ANION.  Nonmetals form anions.  The prefix “an” means not or negative.  Hint: ANION : A – N egative – Ion  Ex. Cl- : Chlorine gains 1 e - when bonding. Therefore, it has 1 more negative e - than positive p +

 1834 – 1907  Created 1 st periodic table  Arranged element by atomic weight  Arranged 63 elements in groups based on common properties  Predicted unknown elements  1887 – 1915  Updated Mendeleev’s periodic table  Arranged elements by their atomic numbers  Considered to be “Modern Periodic Table”  Died at 27 in WWI