Transcript File
Periodic Trends Notes Starting Question Describe the periodic trends with respect to atomic mass and atomic radius. Reason for lesson • Today we are going add to our knowledge of the periodic table and introduce the important ideas of the periodic law, atomic radius, electronegativity, ionization energy, electron affinity and shielding effects. Periodic Trends • The number of protons and electrons affects the properties of an atom. • The periodic table is organized according to groups (up and down) and periods (right to left). Periodic Trends • There are a number of periodic characteristics that either increase or decrease along the periodic table: Periodic Trends • 1. Atomic Mass: – increases as you move across the periodic table (from left to right) – Increases as you move down a group • 2. Atomic Radius: the size of the atom. If you compare two atoms that are directly above and below each other on the table, the one underneath will be larger because it has another shell added. Periodic Trends Atomic radius (cont.) Lithium Rubidium 3+ 37+ • However, if you compare two atoms that are side-by-side on the table, the one on the left will be larger because when you add more electrons and protons, the electric force that pulls them together will increase, making the radius smaller. Atomic Radius (cont.) Atomic Radius (cont.) Lithium 3+ Fluorine 9+ Atomic Radius (cont.) Periodic Trends • 3. Ionization Energy--the amount of energy that is required to remove an electron from a gaseous atom. • If an atom will easily lose an electron (like the Alkali Metals), the ionization energy will be low. • If it will not lose electrons easily (noble gases, Halogens), the ionization energy will be high. Ionization Energy (cont.) • Ionization energy increases going left to right on the table, and decreases going down. • Larger atoms lose electrons more easily because the electric force is less because the distance from the nucleus is greater. Ionization Energy (cont.) Lithium Rubidium 3+ 37+ Ionization Energy (cont.) • Across a period the atoms get smaller because the larger electric force in the nucleus pulls the valence shell in tighter allowing for a better hold on the outer electrons and a greater ionization energy. Ionization Energy (cont.) Lithium 3+ Fluorine 9+ Ionization Energy (cont.) Multiple Ionization Energies • It is possible to remove all the electrons of an atom. To remove the second electron it will take more energy then to remove the first. To remove the third will take more energy then to remove the second. The table on the next slide gives first, second , third, etc ionization energies for different atoms. Multiple Ionization Energies (cont.) Ionization Energies Element 1st 2nd 3rd Lithium 0.5 7.3 11.8 Beryllium 0.9 1.8 14.8 21.0 Boron 2.4 3.7 25.0 0.8 4th 5th 32.8 Review of Last Lesson http://wps.prenhall.com/wps/media/objects/4 39/449969/Media_Portfolio/index.html Chapter 7 Ionization energy Periodic Trends • 4. Electron Affinity-the energy change when an electron is added to a gaseous atom. This is closely related to ionization energy, and increases going left to right, and decreases going down. Electron Affinity (cont.) • Increases because as you move across the row, atoms become increasingly stable when they receive an electron. In chemical terms, energy is always released when stability is achieved. That is because when something is stable it is at a lower energy level. Electron Affinity (cont.) Lithium 3+ Fluorine 9+ Electron Affinity (cont.) • Decreases going down a group due to increasing number of electrons and shells therefore the effect is not as great when an atom gains one electron Electron Affinity (cont.) Lithium Rubidium 3+ 37+ Electron Affinity (cont.) Periodic Trends • 5. Electronegativity- - Ability of an atom to attract electrons when in a molecule. Electronegativity (cont.) • This increases going left to right, and decreases going down (with the exception of noble gases, which don’t make molecules with other atoms). Electronegativity (cont.) • Linus Pauling assigned a value of 4.0 to the most electronegative element fluorine and all other elements were measured relative to F. Electronegativity (cont.) • Why is Fluorine the most electronegative element? • Which should be the least electronegative? Electronegativity (cont.) • How does electronegativity effect molecules? • Let’s use water as an example. • Electronegativity is defined as the ability of an atom to attract an electron in a molecule or bond. Electronegativity (cont.) Water naturally has a bend in its structure. O H H If hydrogen and oxygen had the same electronegativity we would expect the electrons to be shared in the middle. Electronegativity (cont.) But they have different electronegativities. Oxygen has a larger electronegativity then hydrogen O H H Electrons are more attracted to the oxygen and spend more time closer to the oxygen. Electronegativity (cont.) This gives the oxygen a partial negative charge and the hydrogen a partial positive charge. dd+ O d+ H H We call these forces “dipoles”. Depending on the shape of the molecule, some dipoles cancel out. If they do not cancel out, we call the bonds/molecule “polar”.* Review of Electonegativity http://wps.prenhall.com/wps/media/objects/4 39/449969/Media_Portfolio/index.html Chapter 7 Electronegativity Periodic Trends Reviewed Periodic Trends. Up and to right, electrons are held more strongly. Shielding effect • The idea that the inner electron shells insulate the valence electrons from the positive charge of the nucleus so the outer electrons are not held as strongly as they would be without the presence of those inner electron shells.