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Periodic Table Trends Atomic Radius Atomic Radius (Radii) • A trend is a prevailing tendency or a general course. • We commonly discuss weather trends and fashion trends. • Trends also occur in the arrangement of the periodic table. • Atomic size is measured according to the radius of the atom, or the atomic radius. • Remember that atoms are spherically shaped and are made up of protons and neutrons within the nucleus and electrons in the electron cloud. The negatively-charged electrons are held within the atom by their attraction to the positively-charged nucleus. • An atom doesn’t have a well-defined boundary, and its radius cannot be directly measured. • *Atomic size can be estimated by measuring the distance between the nuclei of two atoms of the same type that are joined together. • Therefore, the atomic radius is defined as one half the distance between the nuclei of two atoms of the same element that are joined. Atomic Radius **LOOK AT THE ATOMIC RADIUS CHART ON THE BACK OF THE NOTESHEET** Question - What general trends or patterns do you see among the elements? • On the periodic table, atomic radii tend to decrease left to right along a period, so group 1 atoms are generally larger than group 18 atoms. • This is because, within a period, the number of principle energy levels (PELs) in each element generally remains constant. • For example, all elements in Period 3 have three energy levels. However, the nucleus gains protons as atomic number increases, so it becomes more attractive to the electrons and pulls them in tighter. • This growing attraction causes the atomic radii to decrease as protons are added. • Moving down a group on the periodic table atomic radii tend to increase. • Atomic number increases as protons are added; however, the number of occupied orbitals increases as layers of electrons are added. • As the number of electrons located between the nucleus and the outer shell increases, valence electrons are increasingly shielded from the full attraction of the positive nucleus. • This is called the SHIELDING EFFECT. • Electron shielding has a greater influence on the atom than the increased attraction of the nucleus; therefore, atomic radii generally increase moving down the periodic table. • The largest elements are in the lower left corner of the periodic table. The smallest elements are in the upper right corner. IONIC RADIUS • The radius of an ion is different from the radius of a neutral atom. • An ion is an atom with a charge. • A positive ion, called a cation, forms when an atom loses electrons. Metals, such as sodium, tend to lose electrons to form cations. Cations are always smaller than the neutral atom. • When an atom loses electrons, the ionic radius decreases for two reasons. • First, lost electrons will almost always be valence electrons, so they can leave an empty outer shell, which makes the radius smaller. • Second, the positively-charged nucleus exerts the same amount of pull but on fewer electrons, so it pulls the remaining electrons closer to the nucleus. • A negative ion, called an anion, forms when electrons are gained. • Nonmetals, such as chlorine, tend to gain electrons to form anions. • Negative ions are always larger than the neutral atom. • When an atom gains electrons, the ionic radius increases because of the identical charge of all electrons. • Remember that only opposites attract— negatively-charged electrons are attracted to the positively-charged nucleus. • Electrons are not attracted to each other but instead repel and push away from each other. • Adding electrons to the outer shell forces them to push farther apart, which increases the radius. • On the periodic table, the trend in ionic radii is similar to the trend in atomic radii. • Ionic radii generally decrease from left to right across a period and increase down a group. • However, groups that form cations will have smaller radii than groups that form anions. • Elements in groups 1, 2, 13 and (14) lose electrons to form cations, so the ionic radii are smaller than the neutral atom. • Within these groups, the ionic radii decrease left to right along a period and decrease down the group. • Most elements in groups 15, 16, and 17 gain electrons to form anions, so the ionic radii are larger than the neutral atom. • Within these groups, the ionic radii generally increase left to right along a period and increase down the group. • Recall that group 18, the noble gases, do not form ions. Noble gases have a full valence energy level so they do not gain or lose electrons.