Periodic Trends Chemistry 5(C)

Learning objectives Use the periodic table to identify and explain trends in – Atomic and ionic radii – Electronegativity – Ionization energy Periodic Trends

Periodic Trends – patterns of properties seen across periods and within groups on the periodic table – Atomic Radius – the size of an atom measured from the nucleus to the outermost electron – Ionic Radius – the size of an ion measured from the nucleus to the outermost electron – Electronegativity – the tendency of an atom to pull electrons towards itself when bonding with another atom – Ionization energy – the energy required to remove an electron from a neutral gaseous atom Trends are shown most clearly and consistently in main group elements Periodic Trends

Decreases Increases Atomic Radii Atomic radius increases moving down a group and decreases moving to the right across a period Observed Atomic Radii Trend Increases

Atomic radius increases moving down a group – Each period represents an energy level Elements further down on the periodic table have atoms with more energy levels – Each new energy level is further away from the nucleus – Electrons in inner energy levels contribute to shielding Inner layers of electrons shield outer layers of electrons from the pull of the positively charged nucleus Atomic Radii Down a Group

Atomic radius decreases moving to the right across the periodic table – Each atom in a period uses the same energy level to hold its outermost electrons, while moving to the right increases the positive charge of the nuclei Attraction of outer electrons to nuclei increases, drawing electrons in and decreasing the radius Atomic Radii Across a Period

Ion – charged atom formed by losing or gaining electrons in the outermost energy level – Ions generally increase in size moving down a group Outer electrons are in higher energy levels Observed Ionic Radii Trend Atomic Radii & Ionic Radii Increases

Cation – positively charged ion formed when an atom loses electrons – Radius will decrease when a cation is formed Fewer electrons present decreases the size of the electron cloud – Cations generally decrease in size moving left-to-right across a period Cations increase in positive charge moving left-to-right – Pull on electrons is stronger, decreasing radius Ionic Radii Trend – Cations Group Period

Anion – negatively charged ion formed when an atom gains an electron – Radius will increase when an anion is formed More electrons are present increasing the size of the electron cloud – Anions generally decrease in size moving from left-to-right across a period Moving left-to right, anions decrease in negative charge because elements have more protons – Electrons more tightly held Ionic Radii Trend – Anions Group Period

Electronegativity decreases moving down a group and increases moving to the right across a period Observed Electronegativity Trend Electronegativity Increases Decreases Increases

When atoms bond they do not always share electrons equally – Atoms with larger electronegativities pull bonding electrons closer to themselves Atoms with greater electronegativities have smaller atomic radii – Atoms with smaller radii have less shielding Positively charged nucleus is more exposed and can better attract shared electrons in the bond – Fluorine is the most electronegative element Electronegativity Trend

Ionization energy decreases moving down a group and increases moving to the right across a period Observed Ionization Energy Trend Increases Decreases 1 st Ionization Energy Increases He

Large ionization energy value means that electrons are difficult to remove – Requires a large amount of energy Atoms with large ionization energy values have smaller radii – Fewer energy levels of electrons between the nucleus and the outer electrons Positively charged nucleus strongly attracts electrons and a large amount of energy is required to remove them – Helium has the largest ionization energy Ionization Energy

Summary of Trends Electronegativity & Ionization Energy Increase Atomic Radii Increase Ionic Radii Increase Cation Radii Increase Anion Radii Increase

Learning objectives Use the periodic table to identify and explain trends in – Atomic and ionic radii – Electronegativity – Ionization energy Periodic Trends