Electron Configuration and Periodic Trends

Na: 1s2 2s2 2p6 3s1

Na: [Ne] 3s1

• Electron configurations tells us in which orbitals the electrons for an element are located.

• Three rules:– electrons fill orbitals starting with lowest n and

moving upwards;– no two electrons can fill one orbital with the same

spin (Pauli);– for degenerate orbitals, electrons fill each orbital

singly before any orbital gets a second electron (Hund’s rule).

Electron Configurations

Filling Diagram for Sublevels

Aufbau Principle

Electron Configurations• The electron configuration of an atom is a

shorthand method of writing the location of electrons by sublevel.

• The sublevel is written followed by a superscript with the number of electrons in the sublevel.

– If the 2p sublevel contains 2 electrons, it is written 2p2

Writing Electron Configurations• First, determine how many electrons are in the

atom. Iron has 26 electrons.

• Arrange the energy sublevels according to increasing energy:

– 1s 2s 2p 3s 3p 4s 3d …

• Fill each sublevel with electrons until you have used all the electrons in the atom:

– Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d 6

• The sum of the superscripts equals the atomic number of iron (26)

• The periodic table can be used as a guide for electron configurations.

• The period number is the value of n.• Groups 1A and 2A have the s-orbital filled.• Groups 3A - 8A have the p-orbital filled.• Groups 3B - 2B have the d-orbital filled.• The lanthanides and actinides have the f-orbital filled.

Electron Configurations Electron Configurations and the Periodic Tableand the Periodic Table

Blocks and Sublevels• We can use the periodic table to predict which

sublevel is being filled by a particular element.

Noble Gas Core Electron Configurations

• Recall, the electron configuration for Na is:

Na: 1s2 2s2 2p6 3s1

• We can abbreviate the electron configuration by indicating the innermost electrons with the symbol of the preceding noble gas.

• The preceding noble gas with an atomic number less than sodium is neon, Ne. We rewrite the electron configuration:

Na: [Ne] 3s1

Condensed Electron Configurations• Neon completes the 2p subshell.• Sodium marks the beginning of a new row.• So, we write the condensed electron configuration for sodium

as

Na: [Ne] 3s1

• [Ne] represents the electron configuration of neon.• Core electrons: electrons in [Noble Gas].• Valence electrons: electrons outside of [Noble Gas].

Electron ConfigurationsElectron Configurations

Valence Electrons• When an atom undergoes a chemical reaction,

only the outermost electrons are involved.

• These electrons are of the highest energy and are furthest away from the nucleus. These are the valence electrons.

• The valence electrons are the s and p electrons beyond the noble gas core.

Predicting Valence Electrons• The Roman numeral in the American convention

indicates the number of valence electrons.

– Group IA elements have 1 valence electron

– Group VA elements have 5 valence electrons

• When using the IUPAC designations for group numbers, the last digit indicates the number of valence electrons.

– Group 14 elements have 4 valence electrons

– Group 2 elements have 2 valence electrons

Electron Dot Formulas• An electron dot formula of an elements shows the

symbol of the element surrounded by its valence electrons.

• We use one dot for each valence electron.

• Consider phosphorous, P, which has 5 valence electrons. Here is the method for writing the electron dot formula.

Ionic Charge• Recall, that atoms lose or gain electrons to form

ions.

• The charge of an ion is related to the number of valence electrons on the atom.

• Group IA/1 metals lose their one valence electron to form 1+ ions.

– Na → Na+ + e-

• Metals lose their valence electrons to form ions.

Predicting Ionic Charge• Group IA/1 metals form 1+ ions, group IIA/2

metals form 2+ ions, group IIIA/13 metals form 3+ ions, and group IVA/14 metals from 4+ ions.

• By losing their valence electrons, they achieve a noble gas configuration.

• Similarly, nonmetals can gain electrons to achieve a noble gas configuration.

• Group VA/15 elements form -3 ions, group VIA/16 elements form -2 ions, and group VIIA/17 elements form -1 ions.

Ion Electron Configurations• When we write the electron configuration of a

positive ion, we remove one electron for each positive charge:

Na → Na+

1s2 2s2 2p6 3s1 → 1s2 2s2 2p6

• When we write the electron configuration of a negative ion, we add one electron for each negative charge:

O → O2-

1s2 2s2 2p4 → 1s2 2s2 2p6

Recap

• We can Write the electron configuration of an element based on its position on the periodic table.

• Valence electrons are the outermost electrons and are involved in chemical reactions.

• We can write electron dot formulas for elements which indicate the number of valence electrons.

Recap • We can predict the charge on the ion of an

element from its position on the periodic table.

General Periodic TrendsGeneral Periodic Trends• Atomic and ionic sizeAtomic and ionic size• Ionization energyIonization energy• ElectronegativityElectronegativity

Higher effective nuclear chargeElectrons held more tightly

Larger orbitals.Electrons held lesstightly.

Atomic SizeAtomic SizeAtomic SizeAtomic Size

• Size goes UPSize goes UP on going down a group. on going down a group. • Because electrons are added further Because electrons are added further

from the nucleus, there is less attraction. from the nucleus, there is less attraction. This is due to additional energy levels This is due to additional energy levels and the and the shielding effectshielding effect. Each additional . Each additional energy level “shields” the electrons from energy level “shields” the electrons from being pulled in toward the nucleus.being pulled in toward the nucleus.

• Size goes DOWNSize goes DOWN on going across a on going across a period.period.

• Size goes UPSize goes UP on going down a group. on going down a group. • Because electrons are added further Because electrons are added further

from the nucleus, there is less attraction. from the nucleus, there is less attraction. This is due to additional energy levels This is due to additional energy levels and the and the shielding effectshielding effect. Each additional . Each additional energy level “shields” the electrons from energy level “shields” the electrons from being pulled in toward the nucleus.being pulled in toward the nucleus.

• Size goes DOWNSize goes DOWN on going across a on going across a period.period.

Atomic SizeAtomic SizeAtomic SizeAtomic Size

Size Size decreasesdecreases across a period owing across a period owing to increase in the positive charge from the to increase in the positive charge from the protons. Each added electron feels a protons. Each added electron feels a greater and greater + charge because the greater and greater + charge because the protons are pulling in the same direction, protons are pulling in the same direction, where the electrons are scattered.where the electrons are scattered.

LargeLarge SmallSmall

Which is Bigger?Which is Bigger?

• Na or K ?Na or K ?

• Na or Mg ?Na or Mg ?

• Al or I ?Al or I ?

Ion SizesIon SizesIon SizesIon Sizes

Li,152 pm3e and 3p

Li+, 60 pm2e and 3 p

+Does the size goDoes the size goup or down up or down when losing an when losing an electron to form electron to form a cation?a cation?

Does the size goDoes the size goup or down up or down when losing an when losing an electron to form electron to form a cation?a cation?

Ion SizesIon SizesIon SizesIon Sizes

• CATIONSCATIONS are are SMALLERSMALLER than the than the atoms from which they come.atoms from which they come.

• The electron/proton attraction has The electron/proton attraction has gone UP and so size gone UP and so size DECREASESDECREASES..

Li,152 pm3e and 3p

Li +, 78 pm2e and 3 p

+Forming a Forming a cation.cation.

Forming a Forming a cation.cation.

Ion SizesIon SizesIon SizesIon Sizes

F,64 pm9e and 9p

F- , 136 pm10 e and 9 p

-Does the size go up or Does the size go up or down when gaining an down when gaining an electron to form an electron to form an anion?anion?

Does the size go up or Does the size go up or down when gaining an down when gaining an electron to form an electron to form an anion?anion?

Ion SizesIon SizesIon SizesIon Sizes

• ANIONSANIONS are are LARGERLARGER than the atoms from than the atoms from which they come.which they come.

• The electron/proton attraction has gone DOWN The electron/proton attraction has gone DOWN and so size and so size INCREASESINCREASES..

• Trends in ion sizes are the same as atom sizes. Trends in ion sizes are the same as atom sizes.

Forming an Forming an anion.anion.

Forming an Forming an anion.anion.F, 71 pm

9e and 9pF-, 133 pm10 e and 9 p

-

Trends in Ion SizesTrends in Ion SizesTrends in Ion SizesTrends in Ion Sizes

Figure 8.13Figure 8.13

Which is Bigger?Which is Bigger?

• Cl or ClCl or Cl-- ? ?

• KK++ or K ? or K ?

• Ca or CaCa or Ca+2+2 ? ?

• II-- or Br or Br-- ? ?

Mg (g) + Mg (g) + 738 kJ738 kJ ---> Mg ---> Mg++ (g) + e-(g) + e-

This is called the FIRST This is called the FIRST ionization energy because ionization energy because

we removed only the we removed only the OUTERMOST electronOUTERMOST electron

MgMg+ + (g) + (g) + 1451 kJ1451 kJ ---> Mg ---> Mg2+2+ (g) + e- (g) + e-This is the SECOND IE.This is the SECOND IE.

IE = energy required to remove an electron from an atom (in the gas IE = energy required to remove an electron from an atom (in the gas phase).phase).

Ionization EnergyIonization EnergyIonization EnergyIonization Energy

Trends in Ionization Trends in Ionization EnergyEnergy

Trends in Ionization Trends in Ionization EnergyEnergy

• IE increases across a IE increases across a period because the positive period because the positive charge increases.charge increases.

• Metals lose electrons more Metals lose electrons more easily than nonmetals.easily than nonmetals.

• Nonmetals lose electrons Nonmetals lose electrons with difficulty (they like to with difficulty (they like to GAIN electrons).GAIN electrons).

Trends in Ionization Trends in Ionization EnergyEnergy

Trends in Ionization Trends in Ionization EnergyEnergy

• IE increases UP a IE increases UP a group group

• Because size Because size increases (Shielding increases (Shielding Effect)Effect)

Which has a higher 1st ionization energy?

• Mg or Ca ?

• Al or S ?

• Cs or Ba ?

Electronegativity, Electronegativity, is a measure of the ability of an atom in a is a measure of the ability of an atom in a

molecule to attract electrons to itself.molecule to attract electrons to itself.

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Periodic Trends: Electronegativity

• In a group: Atoms with fewer energy levels can attract electrons better (less shielding). So, electronegativity increases UP a group of elements.

• In a period: More protons, while the energy levels are the same, means atoms can better attract electrons. So, electronegativity increases RIGHT in a period of elements.

ElectronegativityElectronegativity

Which is more electronegative?

• F or Cl ?

• Na or K ?

• Sn or I ?