Structure of the Periodic Table

Structure of the Periodic Table Aim PT2 – what are some of the characteristics of elements we study using the Periodic Table?

Groups of the Periodic Table Elements in the same group have similar chemical and physical properties They tend to react the same way in chemical reactions Examples Group 1 elements all react violently in water Group 17 elements all occur in nature only combined with other elements Group 18 elements are all unreactive gases Mendeleev did that on purpose.

Groups of the Periodic Table Mendeleev put elements in the same group that had the same number of valence electrons These valence electrons are lost or gained during chemical reactions Elements in Period 1 and the first half of Period 2 gain or lose electrons in order to become like HELIUM with 2 electrons in the valence shell (Duet Rule) Everyone else gains or loses electrons to become like Group 18 Noble Gases and have 8 electrons in the valence shell (Octet Rule)

Periods in the Periodic Table Rows in the Periodic Table are called PERIODS Each period has elements in it with the same number of energy levels or shells The characteristics of the elements change as you move from left to right across each period, forming trends This trend repeats itself in the next row This is called PERIODICITY

Periodicity = Trends There are many properties we look at in chemistry, each having specific patterns or trends in the PT Most of the values for these properties are given, along with other elemental properties, in Table S of your Chem Reference Tables

Atomic Radius The distance from the nucleus to the edge of the electron cloud Measured as ½ the internuclear distance between two bonded atoms Table S gives atomic radii values in picometers (pm), or trillionths of a meter Trends: As you move from top to bottom in a group the atomic radius increases as you move from left to right across a period, the atomic radius decreases Radius of bonded atoms

Group Atomic Radius Trends As you move down a group You increase the number of shells or energy levels Making the atoms bigger in size Example – Period 1 of the table Li Na K Rb Li - Two energy levels Na - Three energy levels K - Four Energy levels Rb – Five Energy levels

Period Atomic Radius Trends As you move across a period You don’t increase the number of shells, but the number of electrons and protons increase Therefore the attractive forces between them increases, pulling the electrons closer to the nucleus and decreasing the atomic radius Be B N F Li

Ionic Radius The radius of the ion formed from an atom’s gain or loss of electrons Based on whether an atom becomes a cation or anion Cations - positive ions Lose their valence electrons, losing a whole shell Become smaller than their atoms Anions - negative ions Gain valence electrons, whose negative charges push against each other, making them larger than their atoms Cl Na Cl- Atom Cation Na + Atom Anion

First Ionization Energy Amount of energy needed to remove the first valence electron from a given element Measured in kilojoules / mole A mole – represents 6.02 x 1023 particles in a sample More later Example of first ionization energy from Table S: K + First ionization energy  K+ + e- K + 418kilojoules/mole K+ + e-

Electronegativity A relative measure of an atom’s ability to attract electrons Measured on a scale of 0 – 4 Pauling Units (named after chemist Linus Pauling) Values given on Table S HIGH electronegativity higher attraction for electrons usually nonmetals – flourine has the highest LOW electronegativity lower attraction for electrons usually metals – francium has the lowest

How do I quickly find the trends in the Periodic Table? REMEMBER THIS With three elements only All trends in the periodic table can be determined Lithium vs Francium – the trends in groups can be determined Lithium vs Flourine – the trends in periods can be determined FLOURINE – the most active nonmetal FRANCIUM – the most active metal

Structure of the Periodic Table