Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Bell-ringer Who were the two scientists we discussed yesterday that helped shape our modern atomic theory? What were the three laws we discussed yesterday that helped formulate the atomic theory? Section 3-2 The Structure of the Atom Coach Kelsoe Chemistry Pages 72–76 What is an atom? Even though not all of John Dalton’s ideas about the atom were true, many of them were. An atom is the smallest particle of an element that retains the chemical properties of that element. Atomic Pioneers Joseph John Thomson concluded that electrons were found in all atoms. Robert Millikan confirmed the negative charge of the electron and suggested a possible mass. Ernest Rutherford determined that most of the mass of an atom is found in the nucleus and that the nucleus occupies very little space within the atom. Regions of the atom Nucleus There are two basic regions of the atom: Electron The nucleus – found near the center of the atom and contains protons and neutrons. Surrounding the nucleus are the electrons Composition of the Atomic Nucleus All nuclei (except for one hydrogen) have two types of particles: protons and neutrons. The positive charge of a proton is equal to the negative charge of an electron. Atoms are electrically neutral because they contain equal numbers of protons and electrons. A neutron is electrically neutral. Masses of Atomic Particles A proton has a mass of 1.673 x 10-27 kg. The mass of a neutron is 1.675 x 10-27 kg. The mass of an electron is about 1836 times less than that of a proton, so they are usually neglected when calculating mass. Forces in the Nucleus Normally particles with the same charge repel each other, but there are strong attractions between protons. The short-range forces between protons and neutrons that hold the nucleus together are called nuclear forces. The Sizes of Atoms The radius of an atom is the distance from the center of the nucleus to the outer portion of the “electron cloud.” Since atomic radii are so small, they are expressed in picometers (10-12 m).