National Geographic : 1989 Dec
LIGHT TO HEAVY The weight of atoms gener ally increases in relation to their atomic number. At right, , heavier elements receive ' , darker shading. SOLID, LIQUID, GAS °+r. + Each element assumes one of the three states of matter at room temperature, 77F. Here solids appear white, liquids Silicon, the stuff of electronic turquoise, gases magenta. chip, reacts with carbon toMEAVRSSN MTL fornm a hard ceramic and a ce- EA ESSNNEA rami fiber used in compos- Metals can usually be recog ites.It is a component of nized by an ability to conduct s pringy silicone elastomer. electricity. Here metals are 4.v bright blue, nonmetals deep red. Carbon is one of the most AN ELEMENT OF vrsratile ingredients of struc. PREDICTABILITY THE NOBLE GASES tural materials: inplastics, in To the practiced eye the peri- Six naturally occurring gases Ste fiber and matrix of com- odic table suggests how ele- (purple) have filled outer elec posites, in heat-resistant ce- mens will react with one tron shells, giving chemical rmics, and in carbon steels. another. The key lies in their stability that discourages the electrons, which can orbit the formation of compounds. nucleus in as many as seven Nitrogen atoms go into nylon layers, called shells. Each TRANSITION ELEMENTS a nd other polymers and can shell can hold a limited num- Two blocks of metals occupy give hardness to ceramics ber of electrons: the inner, the center of the table (green nd metallic surfaces. two; the second, eight; and and turquoise); most possess so forth (diagram below). The useful catalytic, magnetic, number in the outer shell dic- electric, and structural ticsgand coosites, os ell tates whether an atom will properties. ticsand ompoites as ell donate electrons to another, as cement and most ceram- RADIOACTIVITY ics.It s esental o hgh- them, and how tightly the two With the exception of techne temperature superconductors. will bond. If the outer shell is tium and promethium, only !1 filled, the atom is elements having an atomic inert, as with helium and the nubr hge n8 r other noble gases. naturally radioactive. As the atoms emit radiation, they - _ transmute into other elements. THE MAKING OF A MOLECULE 6 Atoms may join one or more other atoms to form molecules, the smallest stable particles ofanel ement or compound. This chem ical process takes place between the electrons in m ar 0each atom's outer shell. Here beryllium loses two loose elec trons to oxygen, whose outer shell lacked two "' x~' 0 electrons. This transfer process generates a positive SHELLS SHAPE THE TABLE BeO charge on the beryllium and a nega The horizontal structure of the tive on the oxygen, uniting the two periodic table reflects the num- atoms in a process called ionic bond ber of electron shells of each ele- ing. The result is beryllium oxide, which ment. Hydrogen and helium, each forms a useful, heat-tolerant ceramic. Atoms with a single shell, set the pattern can also unite by sharing electrons, a pro as row one. cess known as covalent bonding. DIAGRAMS BY1ALLENCARRFPO LL, NATIONALGEOGRAPHICSTAFF, AND DALED. GLASGOWN CONSULTANT;VICTOR F. ZACKAYr,MATERIALSAND METHODS7', INC.