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s8pe-20701-ca 12/19/05 4:39 PM MAZER Page 201 The Structure of an Atom A key discovery leading to the current model of the atom was that atoms contain particles that have electric charges. A particle’s charge can be either positive or negative. Particles with the same charge repel each other—they are pushed apart. Particles with different charges attract each other—they are drawn toward each other. RESOURCE CENTER CLASSZONE.COM Learn more about the atom. Atoms are composed of three types of particles—protons, neutrons, and electrons. A proton is a positively charged particle. A neutron is an uncharged particle. The neutron has approximately the same mass as a proton. Protons and neutrons are grouped together in the atom’s center. This combination of protons and neutrons is called the nucleus of the atom. Because protons are the only particles in the nucleus that have an electric charge, the nucleus is positively charged. Electrons are negatively charged particles that move around outside the nucleus. VOCABULARY Remember to make a frame for neutron, proton, and electron and for other vocabulary terms. We can only guess where electrons are at any time. Because we can’t locate any one electron with certainty, we say that electrons move about in an electron cloud. We do know that each electron in the electron cloud has a certain amount of energy. This energy keeps it moving about the nucleus. The energy keeps the negative electron from falling into the positive nucleus. We also know that electrons with similar amounts of energy are grouped together in shells. Each shell can hold only a certain number of electrons that have about the same energy. An electron may jump from one shell to the next if enough energy is added to it. The Atomic Model Atoms are made of protons, neutrons, and electrons. proton + + + + + + + Particle Charges and Mass + + + neutron FPO Particle Relative Relative Mass Charge nucleus Electron 1 –1 The nucleus has an overall positive charge. Proton 2000 +1 Neutron 2000 0 electron cloud The electron cloud has a negative charge. Which part of the atom has a negative charge? Chapter 7: Atomic Structure and the Periodic Table 201 PDF