Trends of the Periodic Table

BackgroundBackground

• Electrons can jump between shells (Bohr’s model supported by line spectra)

•The electrons can be pushed so far that they escape the attraction of the nucleus

Losing an electron is called ionization

IonsIons• An electrically charged particle formed when

an atom or molecule loses or gains an electron.

• Q: what would the charge be on an atom that lost an electron?

• A: +1 (because your losing a -ve electron) called a cation

• Q:Gained two electrons?• A: -2 (because you gain 2 -ve electrons) called an anion

Ionization energyIonization energy

• Ionization energy is the energy required to remove one outer electron from an atom.

11p+ 12n°

11p+ 12n°+ Ionization

Energy

Na

+ 1e-

+1Most weakly held

+ E ionization Na+1 (g) + 1e-

Predict…Is there an IE trend in the PT of E?• Down a group?

I.E. DECREASES because:o More energy levels, resulting in a decrease in

attractive forces between the nucleus and valence electrons. Removing one electron becomes easier.

• Across a period? I.E. INCREASES because:o Electrons are added along the same energy level,

increasing the attractive forces between protons and valence electrons requiring more IE to remove them.

Ionization energy vs. atomic numberIonization energy vs. atomic number

Ca

K

H

He

Li

B

Be C

N O

F

Ne

Na

MgAl

SiP S

Cl

Ar

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12 14 16 18 20

Element

Ioni

zatio

n en

ergy

(kJ

/mol

)

Electrostatic Force Theory Electrostatic Force Theory (EFT)(EFT)

• The degree of attraction or repulsion experienced by two charged particles depends on TWO main factors:

1. The distance between them 2. The magnitude of the charge.

• As the # of protons in the nucleus increases, the electrons will experience a greater electrostatic attraction for the nucleus.

• As electrons are positioned in energy shells further from the nucleus, they will experience less electrostatic attraction from the positive nucleus.

What does this mean?What does this mean?Look at the arrangement of electrons

Sodium

BUT is ‘shielded’ by 10 electrons in front of it

11p+ 12n°

Is being ‘pulled’ in by the 11 protons

Chlorine

BUT is ‘shielded’ by 10 electrons in front of it

17p+ 18n°

Is being ‘pulled’ in by the 17 protons

More protons means greater attraction between nucleus and outer electron thus higher ionization energy.

Electron AffinityElectron Affinity

• The energy released when a free electron is added to the lowest available energy shell of an atom in the gas phase.

+ Electron Affinity

Cl

+1e-

-1

+ E.A.Cl-1 (g) +

17p+ 18n°

17p+ 18n°

1e-

Predict…Is there an EA trend in the PT of E?• Down a group?

EA. DECREASES because:o More energy levels are added increasing the distance

between the nucleus and valence electrons.

• Across a period? EA. INCREASES because:o More energy is released as atoms become more

stable. For example, elements in group 7 are very stable with the addition of 1 e-, thus a higher EA. Elements in group 1 become less stable with the addition of 1 e-, thus a lower EA.

Atomic RadiusAtomic Radius

• the estimated distance from the centre of the nucleus to the outer edge of the atom.

• Typically radii = 100 pm (picometer)

» 1 pm = 1 x 10-12 m

Predict…Is there a trend in Atomic Radii in the PT of E?

Down a group?• Atomic size INCREASES because:

o Energy levels are added, therefore electrons are at a greater distance from the nucleus.

o There is an increase in shielding effect due to the inner electrons.

Across a period?• Atomic size DECREASES because:

o Electrons are added along the same energy level, so are protons, therefore overall attraction increases, atomic radius decreases.

Atomic radius vs. atomic numberAtomic radius vs. atomic number

Ca

K

H He

Li

B

Be

C N O FNe

NaMg

Al Si P S Cl

Ar

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

Element

Ato

mic

Rad

ius

(pm

)

• Handout• Follow directions on sheet and answer

questions (you can use textbook for help)

Hint: Ignore Hydrogen when looking at trends.

Look at many periods/groups when summarizing trends.

Periodic table Periodic table

trendstrends Answers Answers

AnswersAnswers2 a) – He, Ne, Ar, Noble gases2 b) – Li, Na, K, Alkali metals3 a) – Li, Na, K, Alkali metals3 b) – He, Ne, Ar, Noble gases4. As one increases, the other decreases5. Ionization energy increases

Atomic radius decreases6. Ionization energy decreases

Atomic radius increases

7.

11p+ 12n°

Na has 11 electrons

Na+ has 10 electrons

Ne has 10 electrons

11p+ 12n°

10p+ 10n°

electron configuration of Na+ resembles NeAlkali metals become like noble gases8. Radius increases because shells are added

Increased radius will make it easier to lose an electron because of greater distance between positive and negative charges

9.Proton # increases. More protons means greater attraction between nucleus and outer electron thus higher ionization energy.The greater attraction also means that outer electrons are brought closer to the nucleus, thus smaller atomic radius results.

Li (enc = 1)

+++

Be (enc = 2)

++++

B (enc = 3)

+++++

10.Noble gases are ignored11.Electron affinity is energy associated with an

atom gaining an electron. It is highest in the top right where atoms are smallest with the greatest number of protons

12.Electronegativity is a number that describes the relative ability of an atom (when bonded) to attract electrons. The trend is the same as affinity for the same reason