Do Now! 1. Define the octet rule. 2. Give the number of valence electrons, charge of the ions and symbol of ions for the following elements: 1) 2) 3) 4) 5) 6) 7) Li O S F Se Ca Br The Octet Rule Atoms lose, gain, or share electrons in order to have the stable electron configuration of a noble gas (8 valence electrons) • The “goal” of most atoms (except H, Li and Be) is to have an octet or group of 8 electrons in their valence energy level. • They may accomplish this by either giving electrons away or taking them. • Metals generally give electrons, nonmetals take them from other atoms. • Atoms that have gained or lost electrons are called ions. Ions • When an atom gains an electron, it becomes negatively charged (more electrons than protons ) and is called an anion. • In the same way that nonmetal atoms can gain electrons, metal atoms can lose electrons. They become positively charged cations. Ions • Here is a simple way to remember which is the cation and which the anion: + This is Ann Ion. She’s unhappy and negative. + This is a cat-ion. He’s a “plussy” cat! Do Now! 1. Define the octet rule. 2. Give the number of valence electrons, charge of the ions and symbol of ions for the following elements: 1) 2) 3) 4) 5) 6) 7) Li O S F Se Ca Br Periodic Trends Elemental Properties and Patterns Periodic Trends 1. Atomic Radius 2. Ionization Energy 3. Electronegativity 1. Atomic Radius Atomic Radius •Since an electron cloud’s edge is difficult to define, scientists use define covalent radius, or half the distance between the nuclei of 2 bonded atoms. •Atomic radii are usually measured in picometers (pm) or angstroms (Å). An angstrom is 1 x 10-10 m. Covalent Radius Two Br atoms bonded together are 2.86 angstroms apart. So, the radius of each atom is 1.43 Å. 2.86 Å 1.43 Å 1.43 Å Atomic Radius Atomic Radius • What is the trend as you move down a group (column)? Atomic radii increase • What is the trend as you move across a period from left to right(row)? Atomic radii decrease Why atomic radii increase as one moves down in the Periodic Table? The number of energy levels (shells) increases as you move down a group. Why atomic radii decrease as one moves from left to right across a period? Although more electrons are being added to atoms, they are at similar distances to the nucleus; and the increasing number of protons "pulls" the electron clouds inwards, making the atomic radii smaller. Effective Nuclear Charge The effective nuclear charge is the net positive charge experienced by valence electrons Zeff = Z − S (Z=atomic number, S=the number of shielding electrons) Zeff = 11-10=1 12-10=2 13-10=3 14-10=4 15-10=5 16-10=6 17-10=7 18-10=8 © 2009, Prentice-Hall, Inc. Atomic radii 16 Atomic Radius • Here is an animation to explain the trend. Ionization Energy Ionization Energy •If an electron is given enough energy (in the form of a photon) to overcome the effective nuclear charge holding the electron in the cloud, it can leave the atom completely. •The atom has been “ionized” or charged. •The number of protons and electrons is no longer equal. Ionization Energy •The energy required to remove an electron from an atom is ionization energy. (measured in kilojoules, kJ) •Ionization energy is always endothermic, that is energy is added to the atom to remove the electron. •The larger the atom is, the easier its electrons are to remove. •Ionization energy and atomic radius are inversely proportional. Ionization Energy Electronegativity Electronegativity •Electronegativity is a measure of an atom’s attraction for another atom’s electrons. •It is an arbitrary scale that ranges from 0 to 4. •The units of electronegativity are Paulings. •Generally, metals are electron givers and have low electronegativities. •Nonmetals are electron takers and have high electronegativities. •What about the noble gases? Electronegativity I love e- • Your help sheet should look like this: 0 Periodic trends rap • https://www.youtube.com/watch?v=MGTKVaZcq8Y Ionic Radius • Cations are always smaller than the original atom because the entire outer energy level is removed during ionization. • Conversely, anions are always larger than the original atom because electrons are added to another energy level. Cation Formation Effective nuclear charge on remaining electrons increases. Na atom 1 valence electron 11p+ Valence elost in ion formation Result: a smaller sodium cation, Na+ Remaining e- are pulled in closer to the nucleus. Ionic size decreases. Anion Formation A chloride ion is produced. It is larger than the original atom. Chlorine atom with 7 valence e17p+ One e- is added to the outer shell. Effective nuclear charge is reduced and the e- cloud expands.
