13.4 Periodic Trends in 13.4 Periodic Trends in Atomic SizeAtomic Size

Remember that, according to the Remember that, according to the quantum mechanical model, an atom quantum mechanical model, an atom does not have a specifically defined does not have a specifically defined boundary that sets the limit of its boundary that sets the limit of its size.size.

13.4 Periodic Trends in 13.4 Periodic Trends in Atomic SizeAtomic Size

However, there are ways to estimate However, there are ways to estimate the relative sizes of atoms.the relative sizes of atoms. X-ray diffraction – estimates the size of X-ray diffraction – estimates the size of

atoms in crystalline solidsatoms in crystalline solids The distance between the nuclei of The distance between the nuclei of

diatomic molecules (examples: Odiatomic molecules (examples: O22 or Br or Br22) ) can be used to estimate the can be used to estimate the atomic atomic radiusradius of an atom. of an atom.

atomic radiusatomic radius – half the distance between – half the distance between the nuclei of two like atomsthe nuclei of two like atoms

13.4 Atomic Size – Group 13.4 Atomic Size – Group TrendsTrends

Atomic size generally increases as Atomic size generally increases as you move down a group of the you move down a group of the periodic tableperiodic table

The size increases because electrons The size increases because electrons are added to higher principle energy are added to higher principle energy levels levels The added charge of nucleus pulls The added charge of nucleus pulls

electrons inward, but the electrons inward, but the net effectnet effect is an is an increase in size because electrons are increase in size because electrons are further from nucleusfurther from nucleus

13.4 Atomic Size – Periodic 13.4 Atomic Size – Periodic TrendsTrends

Atomic size generally decreases as you Atomic size generally decreases as you move from left to right across a period move from left to right across a period

The size decreases because electrons are The size decreases because electrons are added to the same principle energy level, added to the same principle energy level, but the added charge of nucleus pulls but the added charge of nucleus pulls electrons inward; the electrons inward; the net effectnet effect is a is a decrease in sizedecrease in size This trend is less pronounced in periods where This trend is less pronounced in periods where

there are more electrons in the occupied there are more electrons in the occupied principle energy levels between the nucleus principle energy levels between the nucleus and the outermost electrons; this is referred to and the outermost electrons; this is referred to as the as the shielding effectshielding effect

13.5 Periodic Trends in 13.5 Periodic Trends in Ionization EnergyIonization Energy

When an atom gains or loses an electron When an atom gains or loses an electron it forms an ion.it forms an ion.

The energy that is required to overcome the The energy that is required to overcome the attraction of the nuclear charge and remove attraction of the nuclear charge and remove an electron from a gaseous atom is called the an electron from a gaseous atom is called the ionization energyionization energy The first ionization energy is the amount needed The first ionization energy is the amount needed

to remove the first outermost electron to remove the first outermost electron The second ionization energy is the amount The second ionization energy is the amount

needed to remove the next outermost electron needed to remove the next outermost electron The third ionization energy is the amount to The third ionization energy is the amount to

remove the third and so on (Table 13.1 page 362)remove the third and so on (Table 13.1 page 362)

13.5 Periodic Trends in 13.5 Periodic Trends in Ionization EnergyIonization Energy

Ionization energies can be used to Ionization energies can be used to predict how many electrons an atom predict how many electrons an atom will gain or lose in a chemical will gain or lose in a chemical reaction reaction 1A vs. 2A – Table 13.1 page 3621A vs. 2A – Table 13.1 page 362

Two factors affect ionization energy: Two factors affect ionization energy: nuclear chargenuclear charge and and distance from distance from the nucleusthe nucleus

13.5 13.5 Ionization Energy – Group Ionization Energy – Group TrendsTrends

In general, the first ionization In general, the first ionization energy decreases as you move energy decreases as you move down a group on the periodic down a group on the periodic table.table. The size of the atoms increases as you The size of the atoms increases as you

move down; thus the outermost move down; thus the outermost electron is farther from the nucleus and electron is farther from the nucleus and more easily removed more easily removed This results in a lower ionization energyThis results in a lower ionization energy

13.5 13.5 Ionization Energy – Ionization Energy – Periodic TrendsPeriodic Trends

For the representative elements, For the representative elements, the first ionization energy the first ionization energy generally increases as you move generally increases as you move from left to right across a period.from left to right across a period. The nuclear charge is increasing and The nuclear charge is increasing and

the atomic size is decreasing, therefore the atomic size is decreasing, therefore there is more of an attraction between there is more of an attraction between the nucleus and the outermost electronthe nucleus and the outermost electron This results in a higher ionization energyThis results in a higher ionization energy