 Size is expressed in terms of atomic radius.  Atomic radius is ½ the distance between the nuclei of two atoms of the same element when the atoms are joined.  In general, atomic radius increases from top to bottom within a group and decreases from left to right across a period.

 Two changes as atomic number increases as you go down a group…  The charge on the nucleus increases, drawing electrons and closer to the nucleus, and…  The number of occupied energy levels increases, with occupied orbitals shielding electrons in the highest occupied energy level from the attraction of protons in the nucleus.

 The shielding effect is greater than the attractive force of the nucleus, so atomic size increases.

 Within a period, size decreases from left to right.  As protons are added across the period, the electrons in the same principal energy level are pulled closer to the nucleus.  Since the shielding effect is constant for all elements in a period, atomic size decreases across the period.

 An ion is an atom or group of atoms that has a positive or negative charge.  Atoms are electrically neutral ….the number of protons equals the number of electrons.  Positive and negative ions form when electrons are transferred between atoms.  A cation is a positively charged ion. It has lost one or more electrons, so it has more protons than electrons, and a net positive charge. Na 1+

 An anion is a negatively charged ion. It has gained one or more electrons. With more electrons than protons, it has a negative charge. Cl 1-

 Ionization energy is the energy required (measured in the gaseous state) to remove an electron from an atom.  The energy required to remove the first electron from an atom, forming a cation with a 1+ charge, is called the first ionization energy.  First ionization energy tends to decrease from top to bottom within a group, and increase from left to right across a period.

 First ionization energy generally decreases from top to bottom within a group.  Since atomic size increases down the group, the nucleus’ charge has a smaller effect on the electrons in the highest occupied energy level.

 First ionization energy generally increases from left to right across a period.  As the nuclear charge increases, the shielding effect remains constant. The electrons have a greater attraction for the nucleus, so it takes more energy to remove an electron from an atom.

 Cations are always smaller than the atoms from which they are formed.  Anions are always larger than the atoms from which they are formed.

 Electrons are involved in the bonding of ions to form compounds.  Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound.  In general, electronegativity values decreases from top to bottom within a group, and, for representative elements, the values tend to increase from left to right across a period.

 The least electronegative element is cesium, with the least tendency to attract electrons (0.7).  The most electronegative element is fluorine (4.0).