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 S 2- Sulfur Anion 12 protons 10 electrons Cation Mg 2+ 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 1s 2 2s 2 2p 6 3s 1 Sodium ion 1s 2 2s 2 2p 6 (Sodium atom lost 1 electron) Neon 1s 2 2s 2 2p 6 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?