Periodic Trends

Can Studying Chemistry Be Trendy?  As you look at the periodic table and focus in on the elements and their characteristics, you can see there are noticeable patterns (trends) that go across a period (horizontal row) on the periodic table.  These quantitative (able to measured with numbers and units) characteristics that follow distinct patterns across the periodic table are called periodic trends.  Remember…Mendeleev didn’t get all the fame and fortune for nothing – he was crazy smart!  The periodic trends that we will be studying are:  Atomic Radius  Ionization Energy  Electron Affinity  Electronegativity

Atomic Radius  The atomic radius of an element is an estimate of the size of an atom from its nucleus to its outer perimeter.  The Trend of Atomic Radius…  Atomic radius gets smaller as you move left to right on the periodic table.  As you go across a row, you add more protons to the nucleus and more electrons to the orbits – this means more attraction between the opposing charges and the orbits are pulled in even closer to the nucleus.  Atomic radius gets larger as you go top to bottom on the periodic table.  Each time you go down a spot on one of the columns on the periodic table, you are adding another orbit – this additional orbit increases the size of the atom.  Understanding atomic radius will actually help us understand some of the other periodic trends.

Atomic Radius

Ionization Energy  The trend of ionization energy deals with ions – atoms that have lost or gained electrons. (Anions are negative ions; cations are positive ions).  Ionization energy is the energy needed to remove an electron from a gaseous atom.  The Trend of Ionization Energy…  Ionization energy increases as you go left to right on the periodic table.  As you go let to right, the radius of the atom is smaller because of the greater attraction between the protons and electrons. The electrons are being held more tightly and closely by the nucleus. You have to fight to get one free.  Ionization energy gets weaker as you move down a column on the periodic table.  As you go down a column, you add another orbit so the negative electrons are further away from the positive protons and the attractive force between them is not as strong. It’s easier for anyone to come by an rip off an electron.

Multiple Ionization Energies  The term “multiple ionization energies” refers to the taking of more than one electron from a gaseous atom.  The trend here is that it gets a lot tougher to take more and more electrons from an atom.  The first electron taken will seem easy compared to the second. The second electron will be tougher than the first, but, will come away easier than the third and so on…  As an electron is taken away, the protons are acting on fewer electrons and can pull them in even tighter.

Ionization Energy

Electron Affinity  Electron affinity is the energy released when an electron is added to a gaseous atom.  There is no clear pattern for the periodic trend of electron affinity although there tends to be a general increase in electron affinity as you go from the left to the right on the periodic table.  Note that you must spend energy to rip an electron off of an atom and energy is released, or given off, when an electron in added to an atom.

Electron Affinity

Electronegativity  Electronegativity is the relative strength of attraction an atom has for electrons while it is in a chemical bond.  Remember that chemical bonds can involve the sharing of pairs of electrons. The atoms are literally fighting to gain possession of those electrons – the amount “electron-grabbing muscle” they have is called electronegativity.  The Trend of Electronegativity…  Electronegativity increases as you go left to right across the periodic table.  There are more protons in the atoms as you go across the periodic table and this means there is more positive charge to attract the negative electrons.  Electronegativity decreases as you go down a column on the periodic table.  You are adding more orbits so the electrons are further away from the protons and there is less attractive force to grab the electrons.

Electronegativity

THE END