PERIODIC TRENDS
You can make predictions about certain behavior patterns of an atom and its electrons based on the
position of the atom in the periodic table. All the periodic trends can be understood in terms of three
basic rules.
1. Electrons are attracted to the protons in the nucleus of an atom.
a. The closer an electron is to the nucleus, the more strongly it is attracted.
b. The more protons in a nucleus, the more strongly an electron is attracted.
2. Electrons are repelled by other electrons in an atom. So, if other electrons are
between a valence electron and the nucleus, the valence electron will be less
attracted to the nucleus. That's called shielding.
3.. Completed shells (and to a lesser extent, completed subshells) are very stable.
Atoms prefer to add or subtract valence electrons to create complete shells if
possible. , ,
The atoms in the left-hand side of the periodic table are called metals. Metals give up electrons
when forming bonds. Most of the elements in the table are metals. The elements in the upper right-
hand portion of the table are called nonmetals. Nonmetals generally gain electrons when forming
bonds. The metallic character of the elements decreases as you move from left to right across the
periodic table. The elements in the borderline between metal and nonmetal, such as silicon and
arsenic, are called metalloids.
ATOMIC RADIUS
The atomic radius is the approximate distance from the nucleus of an atom to its valence electrons.
Moving from Left to Right Across a Period (li to Ne, for Instance), Atomic
Radius Decreases
Moving from left to right across a period, protons are added to the nucleus, so the valence electrons
are more strongly attracted to the nucleus; this decreases the atomic radius. Electrons are also being
added, but they are all in the same shell at about the same distance from the nucleus, so there is not
much of a shielding effect.
Moving Down a Group (lito Cs, for Instance), Atomic Radius Increases
Moving down a group, shells of electrons are added to the nucleus. Each shell shields the more distant
shells from the nucleus and the valence electrons get farther away from the nucleus. Protons are also
being added, but the shielding effect of the negatively charged electron shells cancels out the added
positive charge.
Cations (Positively Charged Ions) Are Smaller than Atoms
Generally, when electrons are removed from an atom to form a cation, the outer shell is lost, making
the cation smaller than the atom. Also, when electrons are removed, electron-electron repulsions are
reduced, allowing all of the remaining valence electrons to move closer to the nucleus.
Anions (Negatively Charged Ions) Are larger than Atoms
When an electron is added to an atom, forming an anion, electron-electron repulsions increase, caus-
ing the valence electrons to move farther apart, which increases the radius.
26 Ill CRACKING THE AP CHEMISTRY EXAM
You can make predictions about certain behavior patterns of an atom and its electrons based on the
position of the atom in the periodic table. All the periodic trends can be understood in terms of three
basic rules.
1. Electrons are attracted to the protons in the nucleus of an atom.
a. The closer an electron is to the nucleus, the more strongly it is attracted.
b. The more protons in a nucleus, the more strongly an electron is attracted.
2. Electrons are repelled by other electrons in an atom. So, if other electrons are
between a valence electron and the nucleus, the valence electron will be less
attracted to the nucleus. That's called shielding.
3.. Completed shells (and to a lesser extent, completed subshells) are very stable.
Atoms prefer to add or subtract valence electrons to create complete shells if
possible. , ,
The atoms in the left-hand side of the periodic table are called metals. Metals give up electrons
when forming bonds. Most of the elements in the table are metals. The elements in the upper right-
hand portion of the table are called nonmetals. Nonmetals generally gain electrons when forming
bonds. The metallic character of the elements decreases as you move from left to right across the
periodic table. The elements in the borderline between metal and nonmetal, such as silicon and
arsenic, are called metalloids.
ATOMIC RADIUS
The atomic radius is the approximate distance from the nucleus of an atom to its valence electrons.
Moving from Left to Right Across a Period (li to Ne, for Instance), Atomic
Radius Decreases
Moving from left to right across a period, protons are added to the nucleus, so the valence electrons
are more strongly attracted to the nucleus; this decreases the atomic radius. Electrons are also being
added, but they are all in the same shell at about the same distance from the nucleus, so there is not
much of a shielding effect.
Moving Down a Group (lito Cs, for Instance), Atomic Radius Increases
Moving down a group, shells of electrons are added to the nucleus. Each shell shields the more distant
shells from the nucleus and the valence electrons get farther away from the nucleus. Protons are also
being added, but the shielding effect of the negatively charged electron shells cancels out the added
positive charge.
Cations (Positively Charged Ions) Are Smaller than Atoms
Generally, when electrons are removed from an atom to form a cation, the outer shell is lost, making
the cation smaller than the atom. Also, when electrons are removed, electron-electron repulsions are
reduced, allowing all of the remaining valence electrons to move closer to the nucleus.
Anions (Negatively Charged Ions) Are larger than Atoms
When an electron is added to an atom, forming an anion, electron-electron repulsions increase, caus-
ing the valence electrons to move farther apart, which increases the radius.
26 Ill CRACKING THE AP CHEMISTRY EXAM
