Organizing the Elements
How do you organize a collection of different things?
Elements are organized into groups based upon their chemical properties
Elements that have similar chemical behavior are grouped together
Mendeleev is the father of the modern periodic table (1869)
Organized elements by◦ increasing atomic
mass◦ Repeating pattern of
properties Was able to predict
unknown elements using his table
Reorganized periodic table based upon increasing atomic number
Resolved inconsistencies of Mendeleev’s table
When elements are arranged by increasing atomic number….
Their physical and chemical properties repeat in a periodic fashion (repeating pattern)
Properties of elements within a group are very similar. Why?
Properties of elements vary in a regular way across a period
Most elements are metals (~ 80%) Properties of metals
◦ Solid at RT (except for Hg)◦ Good conductors of heat◦ Electrical conductivity◦ Luster◦ Ductile◦ Malleable
Most nonmetals are gases at RT Some solids, one liquid (Br) Properties of Nonmetals
◦ Poor conductors of heat◦ Poor electrical conductivity (except C)◦ Brittle
Metalloids sometimes called “semi-metals” What does this tell you about metalloids? Seven Metalloids Properties of Metalloids
◦ Intermediate between metals and nonmetals◦ Often depends upon conditions◦ Example: Si is a poor electrical conductor, but is
a good conductor when mixed with small amounts of boron.
Groups◦ vertical columns◦ 1-18◦ IA – VIII A are
“representative elements” Periods
◦ horizontal rows◦ 1-7
Squares◦ element symbol and other
information◦ ~115
Inner Transition Metals
Group 1: ___________________? Group 2: ___________________? Groups 3-12: ___________________? Group 17: ___________________? Group 18: ___________________?
Elements are within a group have the same ending electron configurations
This is why elements within a group have similar chemical properties
Atomic Radius: half the distance between nuclei of two like atoms joined together
Increases from top to bottom down a group Decreases from left to right across a period
Arrow points toward increase
Down a group, added energy levels increase radius
Because atomic number increases across a period, there is greater nuclear (+) charge
No energy levels are added across a period This tends to draw electrons closer to
nucleus, decreasing atomic radius
Ions are atoms which have gained or lost electrons
Cations are positively charged because they have lost electrons (negative charges)
Anions are negatively charged because they have gained electrons (negative charges).
Na0 → Na+ + e-
Cl0 + e- → Cl-
Positive and negative ions form when electrons are transferred between atoms.
6.3
Positive and negative ions form when electrons are transferred between atoms.
6.3
Cations and Anions Relative Sizes
Cations are smaller than their parent atoms
Anions are larger than their parent atoms
Ionic radius follows the same pattern as atomic radius but remember…
Cations are smaller than their parent atoms
Anions are larger than their parent atoms
This is because of a change in the relative strength of the nucleus, i.e…. ◦ The ratio of protons to electrons changes
In the case of cations, an energy level is lost
Energy required to remove an e- from an atom
Decreases from top to bottom down a group Increases from left for right across a period
A0 + Ei → A+ + e-
Na0 + Ei → Na+ + e-
Energy required to remove 1st, 2nd, & 3rd e-
Al0 → Al+ + e- Ei1= 578 kJ/mol
Al+ → Al2+ + e- Ei2 = 1816 kJ/mol
Al2+ → Al 3+ e- Ei3 = 2744 kJ/mol
Why would it require more energy to remove the 2nd electron?
The 3rd electron?
Why are these important?◦ These explain trends in atomic size, ionic size, and
ionization energy Nuclear charge
◦ tends to draw e-s toward nucleus (reducing atomic radius)
◦ The bigger the atomic number, the more positive it is◦ Increases across a period
Electron Shielding:◦ Inner (core) electrons “screen” outer electrons from
attractive force of the nucleus◦ Allows outer e-s to move further from nucleus
(increasing atomic radius)◦ Decreases down a group◦ Does not change across a period
Tendency of an atom to attract electrons to itself when bonded to another atom
A very important property!
Increases left to right across a periodDecreases down a group
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