Chemical Periodicity

Ch. 14

Periodic Table Revisited

14-1

Periodic Table Revisited

• In 1871 Russian chemist, Dmitri Mendeleev, created the periodic table organized by atomic mass

• Now it is organized by increasing atomic #

• Physical and chemical properties within a group are similar

Mendeleev’s Table

Periodic Trends

14-2

#1 Patterns in Atomic Size

•Atomic size of an element decreases across a period (from left to right)

–As the # of electrons in an energy level increases, they are held more tightly to the nucleus (attracted to the protons inside)

•Atomic size increases down a group (from top to bottom)

–As the # of energy levels increases the size of the atom increases

Draw on white blank periodic table

Size increases down a group!

Size decreases across a period

#2 Patterns in Ionic Size• Ions are charged atoms.

– Cation: positive ion, loses electrons; smaller than atom because lost electrons

• B+3 is smaller than B

• Lose an energy level + more protons than electrons so held tighter = smaller

– Anion: negative ion, gains electrons; larger than atom because gained electrons

• N-3 is larger than N

• More electrons than protons, not as attracted to nucleus = larger

• Ionic size decreases across a period

– (Larger) Li+ Be+2 B+3 (Smaller)

– (Larger) N-3 O-2 F- (Smaller)

• Ionic size increases down a group.

Ionic Size Decreases

across

Ionic Size Increases

Down

Draw on Blank

PeriodicTable

#3 Ionization Energy• Ionization energy: The energy required to

remove an electron from an atom– Measures the resistance to the loss of electrons

• Ionization energy increases as you move across a period (left to right)– Easy to remove electron from cations (L) not

anions (R)

• Ionization energy decreases as you move down a group (top to bottom)– Farther away from the nucleus, the easier it is to

remove an electron

Draw on white blank periodic table

CATIONS ANIONS

+1

+2 +3

-1

-3 -20

0Ionization Energy

decreases

#4 Electronegativity

• Electronegativity measures the ability of an atom to attract, or gain, electrons– Ex: Cation does not want to gain electrons while

anions do; anions have a higher electronegativity• Electronegativity increases across a period

– Anions (R) have greater ability to gain electrons than cations (L)

• Electronegativity decreases down a group– Less ability to attract electrons when the size is

larger (farther from the nucleus with positive protons)

Draw on white blank periodic tableD

ecreasing EN

Summary of Periodic Trends

Trend Across Period

(Left to Right)

Down Group

(top to bottom)

Atomic Size

Ionic Size

Ionization Energy

Electronegativity

Summary of Periodic Trends

Trend Across

(Left to Right)

Down

(top to bottom)

Atomic Size Decrease Increase

Ionic Size Decrease Increase

Ionization E Increase Decrease

Electronegativity Increase Decrease