Ch. 6.3 Periodic Trends (pp.170-178)
ATOMIC SIZE
Trends
When atoms of the same element are joined together they form molecules. The atoms in each molecule are identical; therefore the distance between the nuclei of these atoms can be used to determine estimation for its size. This size is referred to as the atomic radius .
Atomic radius = ½ the distance between the nuclei of two atoms of the same element
when they are joined.
The distances are extremely small and are therefore measured in picometers (there are one trillion or 10
12
picometers in a meter). Generally, atomic size increases from top to bottom in a group and decreases from left to right in a period.
Group Trends
As the atomic number in a group increases, the charge on the nucleus increases and the number of occupied energy levels increases; however, atomic size is affected in opposite ways.

Increase in positive charge brings electrons closer to the nucleus

Increase in the number of occupied orbitals shields electrons in the highest energy level from attracting protons in the nucleus. Shielding is greater than the increase in charge so the atomic size increases
Periodic Trends
Generally the atomic size decreases from left to right across the period. Each element has one more proton and electron than the element before it. The shielding effect remains constant across the period. The increase in nuclear charge pulls electrons in the highest occupied energy level closer to the nucleus and decreases the atomic size.
IONS
An ion is an atom or group of atoms that has a positive or negative charge. An atom is electrically neutral because it has equal numbers of protons and electrons. Positive and negative ions form when electrons are transferred between atoms.
An ion with a positive charge is a cation . Atoms of metals form ions by losing electrons from their highest energy level.
Ex. Na + or Na 1+ – Sodium .
The # of electrons (10) is not equal to the # of protons (11) when the atom loses one electron. Because there are more positively charged protons, the
Na ion has a positive charge.
An ion with a negative charge is an anion . Atoms of nonmetals form ions by gaining electrons.
Ex. Cl or Cl 1 - Chlorine . The # of electrons (18) is not equal to the # of protons (17) when the atom gains one electron. Because there are more negatively charged electrons than protons, the Cl ion has a negative charge.

Trends in Ionization Energy
Atoms can move to higher energy levels when atoms absorb energy. The energy required to remove an electron from an atom is called ionization energy . This energy is measured when the element is in a gaseous state. First ionization energy tends to decrease from top to bottom within a group and increase left to right across a period. The second ionization energy is the energy needed to remove an electron from 1 + charged ion and the third is the energy needed to remove and electron from a 2
+
charged ion. That ion produced has a 3
+
charge.
Group Trends in I.E.
As the size of an atom increases, nuclear charge has a smaller effect on the electrons in the highest occupied energy level. Less energy is required to remove an electron from this energy level and the first ionization energy is lower.
Periodic Trends in I.E.
The nuclear charge increases and the shielding effect remains constant, therefore the ionization generally increases from left to right across the periods. There is an increase in the attraction of the nucleus for an electron. It takes more energy to remove an electron from an atom.
Trends in Ionic Size
During reactions, metal atoms lose electrons and nonmetal atoms gain them. This transfer makes it possible to predict the size of the atoms that form. Cations are always smaller than the atoms from which they form and anions are always larger . In a group, size generally increases from top to bottom. In a period, the size of cations and anions decreases from left to right.
Trends in Electronegativity
Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound. Noble gases are generally omitted because they rarely form compounds. In general, electronegativity values decrease from top to bottom within a group and for representative elements (i.e. not noble gases), the values increase from left to right across a period .

The least electronegative element is cesium (0.7). It has the least tendency to attract electrons

The most electronegative element is fluorine (0.4). Because it has such a high tendency to attract electrons, it either attracts the shared electrons or forms a negative ion when it is bonded to any other element.
Summary of Trends
Variations in atomic structure can explain the trends that exist among the properties of groups and periods.
See handout summary of trends.