Periodic Trends

Periodic trends Periodic trends – properties of elements that change in a predictable way as you move through the periodic table

Atomic Radius What happens to atomic radii as you move across a period or down a group?

Atomic radius increases down a group Atomic radius decrease across a period

As you move down a group you have an increasing number of electrons in orbitals further away from the nucleus

As you move across a period there are an ever increasing number of protons in the nucleus that exert an attractive force on electrons in orbitals with the same principle quantum number. This stronger force pulls them closer.

Ionic Radii Ionic radii are a bit more complex

Cations When an atom loses electrons it shrinks Outermost orbital becomes empty Reduced repulsive forces between electrons allow them to be drawn closer to the nucleus

Anions When atoms gain electrons they grow electrons repel each other requiring more space forcing them further from the nucleus

Ionic Radius Trends Ionic radii increase as you move down a group Ionic radii of cations decreases from left to right across a period Ionic radii of anions decreases from left to right across a period

Ionization Energy Ionization energy – the energy needed to remove electrons in the gaseous state measure of how strongly held electrons are higher IE indicates atoms hold valence electrons more strongly low IE indicates weak hold on valence electrons atoms with high IE unlikely to form cations

Ionization Energy

Ionization Energy Trends IE decrease down a group IE increase moving left to right across a period

Ionization Energy Trends Each additional electron removed costs more energy Once all valence electrons are removed the energy cost of removing electrons will be dramatically higher atoms hold inner core electrons much more strongly than valence electrons Theoretically valence electrons from the right side of the periodic table can be removed but this is unlikely

Electronegativity electronegativity – ability of an atom to attract electrons in a chemical bond

Trends in Electronegativity Increases across a period (left to right) Decreases down a group Fluorine is the most electronegative element Least electronegative, Cs & Fr Electronegativities are not generally considered meaningful for noble gases since they do not react

Electronegativity and Chemical Bonds Covalent bonds – when atoms share electrons they will be more attracted to the atom with a higher electronegativity when this difference is high the bond is said to be polar The atom with higher electronegativity will have a slightly negative charge – δ- The atom with less electronegativity will have a slightly positive charge – δ+

Water: example of a polar covalent bond

Nonpolar covalent bond When the electrons are shared equally the bond will be a nonpolar covalent bond best examples are bonds between atoms of the same element (eg H2) Ionic bonds – when a cation and anion are held together by electrostatic attraction instead of electron sharing

Bonding spectrum Bonding spectrum can be defined based on differences in electronegativity