Periodic TrendsSection 6.3

Objectives:AOD C.3.1 Define atomic radii, ionization

energy, electronegativity, and energy levels.

AOD C.3.2 Recognize periodic trends of elements, including the number of valence

electrons, atomic size, and reactivity.

Atomic Radius• Def: The distance between an atom’s

nucleus and its valence electrons.

• Size of the atom varies from substance to substance

Trends within Periods

• Atomic radius INCREASES as you move to the left across a period.– Due to decreased nuclear charge (total

charge in the nucleus --- which would be based on WHAT?)

– Decreased nuclear charge has less pull on the outermost (highest energy, valence) electrons so they can move further away from each other (and the nucleus).

Trends within Groups

• Atomic radius INCREASES as you move down a group.

• Nuclear charge increases, but as electrons are added to higher energy levels, they are further from the pull of the nuclear protons, AND they are “shielded” by the electrons between them and the nucleus.

• Summary: Increases left and down!

Examples• Which element has the smallest atomic

radius? Largest atomic radius?Iodine (I)

Bromine (Br)

Fluorine (F)

Chlorine (Cl)

• Fluorine

• Iodine

Ionic Radius• Ion: an atom that gains or loses electrons

• When atoms lose electrons they form positive ions and become smaller.– The electron lost will always be a valence

electron.– Loss of valence electrons may leave an

empty outer orbital which results in a smaller radius. (Example: Na+1)

– Even if the orbital is not emptied, repulsion between fewer electrons decreases allowing them to be pulled closer to the nucleus.

• When atoms GAIN electrons they form negative ions and they always become larger.– The addition of an electron to an atom

increases the repulsion between the valence electrons, forcing them to move farther apart (Example: O-2).

– The result is a larger radius.

Ionic Radius, continued….….

Ionic Radius within Periods• Size of the positive ions gradually

increases from right to left across a period.

• The size of the much larger negative ions also gradually increases from right to left, until you reach the smaller positive ions.

• Ionic radius GENERALLY INCREASES to the left across periods.

Ionic Radius within Groups• As you move down a group an ion’s outer

electrons are in higher principal energy levels resulting in a gradual increase in ionic size.

• Ionic radius INCREASES as you move down a group.

• Summary: Increases left and down.

Ionization Energy

• Def: the energy required to remove an electron from an atom in the gaseous state

• “How strongly an atom’s nucleus holds on to its valence electrons”

• High IE indicates atom has a strong hold on its electrons

• Low IE indicates an atom loses its outer electron more.easily

Ionization Energy

• Energy required to remove the 1st electron is the first ionization energy.

• Energy required to remove the 2nd electron is the second ionization energy.

• 1st IE is ALWAYS highest.

Ionization Energy within Periods

• INCREASES as you move from left to right across a period

• The increased nuclear charge of each successive element produces an increased hold on the valence electrons, as they are all in the same principal energy level.

Ionization Energy within Groups

• INCREASES as you move up a group because the valence electrons are closer to the nucleus.

• Summary: Increases up and right.

Octet Rule

• Atoms tend to gain, lose, or share electrons in order to acquire a full set of eight valence electrons (resembling which elements?).

• Elements on the right side of the periodic table (nonmetals) tend to gain electrons in order to acquire the 8 valence electrons, forming negative ions)

• Elements on the left side of the periodic table (metals) tend to lose electrons and form positive ions.

Electronegativity of an Element

• Indicates the relative ability of an atom to attract electrons in a chemical bond.

• Noble Gases are not assigned values

• Fluorine is the most electronegative element.

• Fr & Cs are the least.

• In a chemical bond the atom with the greater electronegativity more strongly attracts the shared electrons.

Electronegativity Trends within Periods & Groups

• INCREASES as you move up a group and across a period (up and right).

• The lowest electronegativities are found at the lower left side

• Highest are found at the upper right side

Example Problems• Which element has the highest

electronegativity? Lowest?

• N- Nitrogen• P- Phosphorus• As-Arsenic• Sb-Antimony• Bi- bismuth• N=highest• Bi= Lowest

Homework Problems• Pg. 175

• #56, 57, 59, 62, 63, 65-67