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Periodic Trends A periodic trend is a trend that repeats. Which of these actions is periodic? A. School starts in August. B. A hurricane causes massive power outages and shuts down the schools for two weeks. All the periodic trends can be understood in terms of three basic rules: 1. Electrons are attracted to the protons in the nucleus. a. The closer an electron is to the nucleus, the more strongly it is attracted. B. The more protons in the nucleus, the more strongly an electron is attracted. 2. Electrons are repelled by other electrons in an atom. If there are other electrons between a valence electron and the nucleus, the valence electron will be less attracted to the nucleus. That’s called shielding. Definition: Shielding – the effect of inner electrons on the attraction between the valence electrons and the nucleus 3. Completed shells are very stable. Atoms prefer to add or subtract valence electrons to get 8 electrons in the outer energy level. Exception: Helium is stable with 2 electrons in its outer (and only) energy level. Trend Number One: Atomic radius Trend Number One: Atomic radius Moving from left to right across a period, atomic radius decreases. Why? As you move across a row, the effective nuclear charge increases. Effective nuclear charge is the charge “felt” by the valence electrons after you have taken into account the number of shielding electrons that surround the nucleus. Which is larger, an atom of sulfur or an atom of argon? Trend: Atomic radius Moving down a group, atomic radius increases. Why? As you move down a group, there are more energy levels occupied by electrons. Which is larger, an atom of oxygen or an atom of sulfur? Easy way to remember the size trends: Helium atoms are the smallest! Therefore, atoms must get larger as you go down a group, and larger as you go to the left. Practice Questions Use the periodic table to choose the largest atom: a. Al, Ga, Ge b. N, F, P Trend Number Two: Ionization Energy But, wait! What is ionization energy? Ionization energy is the amount of energy required to remove an electron from an atom. Trend Number Two: Ionization Energy Moving across a period, ionization energy increases. Why? As you move across a period, the effective nuclear charge increases, so the electrons are more attracted to the nucleus. Which has a higher ionization energy, oxygen or fluorine? Trend: Ionization Energy Moving down a group, ionization energy decreases. Why? As you move down a group, the valence electrons are farther from the nucleus and held more loosely. It takes less energy to remove a “loose” electron. Trend: Electronegativity Electonegativity refers to how strongly the nucleus of an atom attracts the electrons of other atoms in a bond. Trend: Electronegativity Moving from left to right across a period, electronegativity increases. Why? The effective nuclear charge is increasing as you move from left to right. The atoms not only attract their own electrons more strongly, they attract electrons from other atoms more strongly. Electronegativity Moving down a group, electronegativity decreases. Why? As you move down a group, the atoms get larger and larger, so the nucleus is farther away from the bonding electrons. Which element is the most electronegative? Fluorine