Lesson 4: Periodic Trends

Periodic trends are

patterns in properties of elements that change regularly across periods and down groups in the periodic table  

Examples of periodic trends include: 

Atomic radius 

Ionic radius 

Ionization energy 

Electronegativity 

 

Across a period: 

from left to right of the periodic table

Down a group:

 

 from top to bottom of a given group in the periodic table 

 

How does it seem scientist have been measuring the radius of an atom? 

X-ray diffraction, XRD, is used to measure the distance between the nuclei of the bonded atom of the same type and it is divided by 2, assuming the distance between the nuclei of the two atoms is equivalent to the diameter. 

AR: So “diameter”

divided by 2 

Assuming the distance is 120 pm between the nuclei of the bonding atom then the radius is: 

r = d/2 = 120 pm /2 = 60 pm 

1 pm =

10^-12 m 

Therefore, atomic radius is defined as

one-half of the distance between the nuclei of identical bonding atoms 

Atomic radius may be defined as 

one-half the distance between the nuclei of identical atoms that are bonded together 

AR is expressed using

picometer, 10^-12m 

Atomic radius generally _____ across a period from left to right

decreases

AR decrease

due to increasing nuclear charge (more protons in the nucleus)  

As nuclear charge increases, the attraction between the nucleus and the electrons

increases, pulling electrons in the same energy level closer to the nucleus 

 

 

Atomic radius generally ___ down a group from top to bottom 

increases

AR Increase

because the outermost electron occupies higher energy levels, which are farther from the nucleus. 

When atoms form ions, they become

 charged, and their radius changes. 

 

If an atom loses an electron, it

becomes positively charged, they become smaller 

 

And if atoms  gains an electron, it

becomes negatively charged, they become larger 

Cationic Radii are smaller than

their respective atomic radii 

Anionic Radii are larger

 

than their respective atomic radii 

The octet rule states that

atoms tend to gain, lose, or share electrons to attain a full outermost energy level (

eight valence electrons;

 complete s and p sublevels

The first-period elements and the second-period metals are an

exception to the octet rule.

First period and second period elements are stable when they have

just 2 valence electrons, in the 1s orbital. 

 

The ionic radii of positive ions generally

decrease from left to right. 

The ionic radii of negative ions generally

decrease from left to right, beginning with group 15 or 16. 

Both positive and negative ions increase in

size moving down a group. 

Cations become smaller because:

 loss of outer energy level or less electron-electron repulsion nucleus pulls remaining electrons closer 

 

Anions become larger because: 

increases electron-electron repulsion 

 

An electron can be removed from

an atom, if enough energy is supplied. 

First ionization energy, IE₁, is the

energy needed to remove one electron from one mole of a gaseous atom 

First ionization energy,

first ion made

IE₁ generally____  across a period 

 

increases

IE Increase

because the effective nuclear charge increase while electrons removed are of the same energy level. 

IE₁ generally_______  down a group 

decreases

IE decreases because:

because outer electrons are farther from the nucleus and there is more shielding 

shielding

the reduction of the attractive force between the nucleus and outer-shell electrons, caused by the repulsive, intervening inner-core electrons

When two atoms form a bond, each atom

attracts the other atom’s electrons in addition to its own.

Electronegativity indicates the

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

Electronegativity generally ____ across a period

increases

Electronegativity generally ___ down a group

decreases