Periodic Trends

Atomic Radius• Defined as half of the distance between two

bonding atoms nuclei

Atomic Radius Across a Period• Atomic radius generally decreases in size as

you move left to right across the period– Increasing positive charge in the nucleus

pulls the electrons of the same energy level in.

Atomic Radius Down a Group• Atomic radius increases as you move down a group

– Orbital size increases as you move down a group with increasing energy level

– Larger orbitals means that outer electrons are farther from the nucleus. This increased distance offsets the greater pull of the increased nuclear charge.

– As additional orbitals between the nucleus and the outer electrons are occupied, the inner electrons shield the outer electrons from the pull of the nucleus this is called shielding.

Shielding The inner electrons shield the outer electrons from feeling the pull of the nucleus

Question Time• How do you define the atomic radius since the

electron cloud does not have a clearly defined edge?

• What happens to the atomic radius as you move left to right? Why?

• What happens to atomic radius as you move down a group? Why?

• What is shielding?

Cation and Anion• An ion is a positively or negatively charged

atom that gains or loses an electron.

• A cation loses electrons and produces a positive charge

• An anion gains electrons and produces a negative charge

16 protons

18 electrons

Anion

S2- Sulfur Anion

12 protons

10 electrons

Cation

Mg2+ Magnesium Cation

Ionic Radius - Cations• Groups 1A, 2A, 3A, and other metals lose

electrons and form cations.• When atoms lose electrons they become

smaller– The electron lost will be a valence electron leaving

a completely empty outer orbital– Protons in nucleus can pull fewer electrons tighter

These atoms lost electrons and became smaller cations.

Ionic Radius - Anions• Group 5A, 6A, and 7A tend to gain electrons

and form anions• When atoms gain electrons and form

negatively charged ions, they become larger. – Protons in nucleus have more electrons to pull

and cannot pull in as tight

These atoms gained electrons and became larger anions

Question Time• What is an ion?• What is a cation and an anion?• What happens to the size of the atom when it

becomes a cation? Why?• What happens to the size of the atom when it

becomes an anion? Why?

Ionization Energy• The energy required to

remove an electron from a gaseous atom

• Indication of how strongly an atom’s nucleus holds onto its valence electron

• Groups 1A, 2A, and 3A tend to have low ionization energies because they want to lose electrons.

Ionization Energy Trends – Across a Period

• Ionization energy generally increases as you move left to right– Across a period electrons are added to the same

energy level (same distance away from the nucleus), yet the nuclear charge is increasing across a period increasing the attraction to the electrons.

Ionization Energy Trends – Down a Group

• Ionization energy decreases as you move down a group– Down a group electrons are added to a higher

energy level (farther distance away from the nucleus), making it easier to remove an electron

Octet Rule• Sodium atom 1s22s22p63s1

• Sodium ion 1s22s22p6 (Sodium atom lost 1 electron)• Neon 1s22s22p6

• Sodium ion has the same electron configuration as neon

• Octet rule states that atoms gain, lose, or share electrons to acquire a full set of eight valence electrons (to be like a noble gas)

Electronegativity• Indicates an element’s ability to attract

electrons in a shared chemical bond• fluorine (F) is the most electronegative

element• Cesium (Cs) and francium (Fr)are the least

electronegative• Noble gases do not tend to have an

electronegativity number since they tend not to form compounds

Trends with Electronegativity• Electronegativity increases as you move left-

to-right across a period

• Electronegativity decreases as you move down a group

Question Time• What is ionization energy?• What is the trend in ionization energy as you move left

to right? Down a group?• What is the octet rule? What elements do atoms want

to be like?• What is electronegativity?• What are the trends in electronegativity?• What is the most electronegative element? What is the

least?• Why do noble gases tend not to have an

electronegativity value?• What is the difference between ionization energy and

electronegativity?