The periodic table/Periodic Trends

Periodic trends are evident when closely evaluating the periodic table based on a score of chemical/physical properties. This short page aims to sufficiently explain these properties and the trend posed in the table.

Valence Electrons
Valence Electrons are electrons in the outer shells. These electrons are important because they're the only ones involved in chemical bonding:
 * Across a period: Increases
 * Down a group: Stays the same

Metallic Character
Metallic character are characteristics present in metals, such as luster, ability to conduct electricity, ductility (able to spread into thin lines), and malleability (able to be separated into thin sheets). Metalloids are elements right-along the zig-zag... they can have characteristics of both metals and nonmetals.
 * Across a period: Decreases
 * Down a group: Increases

Atomic Radius
Atomic Radius (size) is the distance from the center of the nucleus to the edge of the electron cloud.

Atomic radius decreases across a period due to nuclear charge. Each successive elements gains an additional proton and electron--this added electron is increasing the attraction between the protons inside the nucleus and the electrons, causing the atom to become smaller [due to the pull].

Atomic radius increases down a group because of the additional energy level, increasing the size of the atom.
 * Across a period: Decreases
 * Down a group: Increases

Ionic Radius
The ionic radius is the measure of the distance between the nucleus and the valence electrons in an ion. Remember, ionic radius is NOT a periodic trend.
 * Cations - Cations are positive ions and are formed by metals. They are smaller than the neutral atom since an electron has been lost, therefore increasing the nuclear charge.
 * Anions - Anions are negative ions and are formed by nonmetals. They are larger than the neutral atom since an electron has been gained, therefore decreasing the nuclear charge. There is no increased force to pull inwards.

Shielding Effect
The shielding effect is when the inner energy level electrons shield the outer electrons from the positive charge of the nucleus. Atoms with more energy levels exhibit more shielding because there are more inner electrons to block the pull of the positive nucleus. Elements across a period have the same shielding effect as the number of inner electrons always stays the same.
 * Across a period: Stays the same
 * Down a group: Increases

Ionization Energy
Ionization energy is the energy required to remove an electron from a neutral atom. The general trend is that metals are cations since they want to lose electrons and nonmetals are anions since they want to gain electrons.
 * Across a period: INCREASES/HIGH
 * Down a group: DECREASES/LOW

Electronegativity
Electronegativity is the attraction an atom has for an additional electron. The general trend is that metals increase while non-metals decrease (down a group). Electronegativity goes along with ionization energy. Across a period, they increase since metals want to lose rather than gain electrons while non-metals want to gain than lose electrons. Down a group, it decreases since electrons gained would be farther from the nucleus and not have a want for any more electrons.
 * Across a period: INCREASE
 * Down a group: DECREASE