Periodic Trends Elemental Properties and Patterns

Periodic Trends There are several important atomic characteristics that show predictable trends that you should know. Atomic properties— Deal with only single atoms

An atom’s size affects chemical and physical properties of an atom Enables us to gain information on atom’s size Outer electrons hard to locate Atom’s size is related to the radius of the atom Radius of atom = ½ distance between nuclei of 2 adjacent atoms of the same element Radius is the distance from the center of the nucleus to the “ edge ” of the electron cloud. half the distance between the nuclei of 2 bonded atoms. Atomic Radii

Atomic Radius Trend Decreases Increases

1. Metallic Radius Half the distance between nuclei of adjacent atoms in a metal Radius of metallic atoms

2. Covalent Radius Half the distance between the nuclei of 2 adjacent, bonded atoms in a chemical compound. Atoms MUST be identical Radius of nonmetallic atoms (Ex. Cl 2. Br 2 ) Smaller radii than metallic—covalent bonds hold the atoms close Ex. Br Å 1.43 Å

Ions Atoms that have gained or lost electrons ( anion, cation ) Electrons gained/lost from valence shell (outer electron shell) Defined by the distance between the nuclei of 2 ions Studies with crystal structures Radii related to original atomic radii 3. Ionic Radii

1) Anions LARGER size than original neutral atom Generally nonmetals Additional electrons cause the atom’s size to increase Electrons repel as increase in number, causes size to increase Ionic Radii (cont.)

2) Cations SMALLER size than original neutral atom Generally metals Removal of electrons causes the atom’s size to decrease more attractive force from nucleus, nucleus can pull remaining electrons towards it. Ionic Radii (cont.)

Atomic Radius The effect is that the more positive nucleus has a greater pull on the electron cloud. The nucleus is more positive and the electron cloud is more negative. The increased attraction pulls the cloud in, making atoms smaller as we move from left to right across a period.

Identify the atom with the largest radii 1) Ca, F 2) Cl, O 3) N, P

Electron Shielding Repulsive forces exist between electrons. Electrons are found at various distances from the nucleus Electrons closer to the nucleus (at lower energy levels) can “screen/shield” outer electrons Outer electrons do not feel the attractive force from the nucleus as a result Affects periodic trends such as ionization energy Explains why ionization decreases down a column

Ionization Energy If an electron is given enough energy (in the form of a photon) to overcome the electrical forces holding it, the electron can be removed from the atom completely. Amount of energy needed to remove ONE electron from a neutral atom Removal from ground state in neutral atom of gaseous state to form positive ion

Ionization Energy (cont.) The energy required t o remove an electron from an atom is ionization energy. Formation of positive ions ( cations ) The larger the atom is, the easier its electrons are to remove. Ionization energy and atomic radius are inversely proportional.

Ionization Energy (cont.) 1 st ionization energy Energy required to remove 1 st electron from atom Taken from highest energy level Easiest to remove Energy increases as more electrons are removed

Ionization Energy Trend Increases Decreases

Ionization Energy

Identify the highest ionization energy 1) F, Mg 2) Na, Rb 3) P, O

Classwork: Ionization Energy Circle the atom with the highest first ionization energy. 1) Al, B5) Fr, Li9) Se, Cl 2) Mg, Na6) Mg, Al10) Rb, Mg 3) P, As7) C, F 4) I, At8) K, Sc

What does affinity mean?

Electron Affinity energy change that occurs when electrons are added to a neutral atom Some atoms will release energy, others will absorb energy to “force” electron into atom Ex. Hot Potato Greater value with smaller atoms Energy released when electron added Exothermic, negative value An atom’s “desire/affinity” for more electrons, wants to get more electrons ! ! ! Metals—decrease electron affinity. Nonmetals—increase electron affinity, more reactive Stable atoms—full octet

Electron Affinity Trend Increases Decreases

Electronegativity trait or characteristic of an atom Ability of an atom to pull electrons toward it, how tightly an atom wants to hold onto its electrons Atom’s ability to attract electrons from another atom Periodic Trend: increases decreases

Practice: Which atom is more electronegative? 1) Lithium or fluorine 2) Chlorine or iodine 3) Magnesium or oxygen 4) Calcium or barium 5) Rubidium or Strontium

Metallic Character This is simply a relative measure of how easily atoms lose or give up electrons. Tendency to LOSE electrons Related to atomic radius/ionization energy, how easy to remove electrons

Metallic Character Atoms with a tendency to LOSE electrons Decreases Increases

Nonmetallic Character Atoms with a tendency to GAIN electrons Increases Decreases