Chapter 4 Section 2 and 3

Periodic Trends How are elements arranged on the table? Order of increasing atomic number What is a trend? Predictable change in a particular direction Trends on the periodic table allow us to make predictions about chemical behavior

Atomic Radii Do we know the exact size of an atom? No it is difficult to determine It is determined by the volume occupied by the electrons surrounding the nucleus Radii are usually determined for atoms that are chemically bonded or close together in the solid state

Atomic Radii What is bond radius? Van der Waals radius: ½ the distance between the nuclei of the atoms in each molecule Line A represents twice the bond radius Van der Waals radius: Distance between the nuclei in adjacent nonbonded molecules, which is equal to twice the distance called the Van der Waals radius

Atomic Radii Which radius is used in this book? Bond radius Van der Waals are not very precise Measuring bond radius is a useful way to compare the size of atoms

Atomic Radius What is the trend for atomic radius on the periodic table? The radii increases as you go down a group Due to the addition of another energy level from one period to the next

Atomic Radius Electron shielding: Electrons in the inner energy level are between the nucleus and the outermost electrons The inner electrons shield the outer electrons from the full charge of the nucleus Since they are not subject to the full charge of the nucleus they are not held as close to the nucleus

Atomic Radius What happens to the radius as you go across a period? The radius decreases as you go across a period The radius decreases because the positive charge of the nucleus increases This causes the nucleus to pull the electrons closer This reduces the size of the atom Eventually the electrons will not come any closer and the size of the atom tends to level off

Ionization Energy, Electron Affinity, and Melting Points Are atoms neutral or do they have a charge? Electrically neutral Protons = Electrons Ionization energy: Energy used to remove an outer electron

Ionization Energy, Electron Affinity, and Melting Points If an atom loses an electron it becomes a _________ ion Positive ion Alkali metals lose 1 electron If you lose an electron the ion is smaller than the atom Figure 4-25 page 138

Ionization Energy, Electron Affinity, and Melting Points Which way does ionization energy increase? Increases across a period The electrons are held more tightly to the nucleus Decreases down a group The outer electrons are held less tightly to the nucleus and less energy is required to remove one Attractive forces between the nucleus and the electrons decreases and the energy required to remove an electron decreases

Electron Affinity Electron Affinity: Ability of an atom to attract and hold an electron An approaching element could feel the net pull and enter the vacant orbital When Fluorine gains an extra electron it becomes a ___________ ion. Negative ion Figure 4-28

Electron Affinity What does a negative electron affinity indicate? Energy is released when an atom gains an electron What does a positive electron affinity indicate? Energy is needed to add the electron to an atom

Electron Affinity What is the trend for electron affinity? It becomes more negative across a period Decreases from top to bottom in a group There are exceptions to these general trends

Trends in Melting and Boiling Point What is the trend for melting and boiling point? At first, as the # of electrons increases the melting and boiling points increases This indicates stronger bonding However, when the d orbital becomes half-filled the melting and boiling points decrease

Homework Page 142 1-14 due before notes

Section 2 - Metals What one property distinguishes metals from nonmetals? Metals are excellent conductors of electricity The most widely used metal to conduct electricity is copper

Metals In general, poor electrical conductors are poor heat conductors The mechanism by which electricity is conducted must be closely connected with the mechanism for conduction of heat

Other Properties of Metals Tungsten has the highest melting point Most are strong and durable Aluminum and magnesium have a high strength-to-weight ratio making them useful in construction Some are ductile and can be squeezed into a wire Some are malleable and can be hammered into sheets Gold 2 or more can be mixed together to make an alloy