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Atoms Building Blocks of Matter Beginning of Atomic Theory • What Is an Element? Around 440 BCE, a Greek philosopher named Democritus thought that you would eventually end up with a particle that could not be cut. He called this particle an atom. Philosophers Debate: • Aristotle, another Greek philosopher, disagreed with Democritus’s ideas. He believed that you would never end up with a particle that could not be cut. Democritus was right, though: Matter is made of particles, which we call atoms. An atom is the smallest particle into which an element can be divided and still be the same substance. John Dalton: A new view Dalton’s Theory • Dalton’s Theory John Dalton published his atomic theory in 1803. His theory stated that all substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances. • Not Quite Correct The atomic theory was then changed to describe the atom more correctly. J.J. Thompson: • Negatively Charged Particles Thomson experimented with a cathode-ray tube like the one shown on the next slide. He discovered negatively charged particles that are now known as electrons. • Like Plums in Pudding After learning that atoms contain electrons, Thomson proposed a new model of the atom. Thomson thought that electrons were mixed throughout an atom, like plums in a pudding. Ernest Rutherford • Negatively Charged Particles In 1909, Ernest Rutherford aimed a beam of small, positively charged particles at a thin sheet of gold foil. The next slide shows his experiment. • Surprising Results Rutherford expected the particles to pass right through the gold in a straight line. To Rutherford’s great surprise, some of the particles were deflected. Where are the Electrons? • Far from the Nucleus Rutherford proposed that in the center of the atom is a tiny, positively charged part called the nucleus. • Bohr’s Electron Levels In 1913, Niels Bohr proposed that electrons move around the nucleus in certain paths, or energy levels. • The Modern Atomic Theory According to the current theory, there are regions inside the atom where electrons are likely to found. What are atoms? • The Nucleus Protons are positively charged particles in the nucleus. Neutrons are the particles of the nucleus that have no electrical charge. • Outside the Nucleus Electrons are the negatively charged particles in atoms. Electrons are found around the nucleus within electron clouds. All the structures of the atom can be seen on the next slide. Modern Atom: Electron Cloud Model Electron energy level groups: • Each “level” of the electron cloud in an atom has a set of orbitals. • The orbitals are three-dimensional probability “shapes” in which electrons from that set may be found. • The number of orbitals increases as electrons are farther away from nucleus (they have more room there…) • The orbitals are assigned letters: s= “sharp”, p= “principal”, d= “diffuse”, and f= “fundamental”. (corresponds to the azimuthal quantum number…”l”) • The maximum number of electrons possible within an orbital increases from s to f. s=2, p=8, d=16, f=32 s orbitals p orbitals d orbitals f orbitals How do Atoms of Elements Differ? • Starting Simply The hydrogen atom has one proton and one electron. • Now for Some Neutrons The helium atom has two protons, two neutrons, and two electrons. • Building Bigger Atoms For bigger atoms, simply add protons, neutrons, and electrons. • Protons and Atomic Number The number of protons in the nucleus of an atom is the atomic number of that atom. All atoms of an element have the same atomic number. How many of each particle? • Atomic number = the number of protons in an atom • Atomic mass (mass number) = the average mass of the nucleus from all of the isotopes of a certain element. (remember that the mass of the electron is very small) • To find the number of neutrons in any typical atom, subtract the atomic number from the atomic mass. • Remember that in a neutral atom, the number of electrons is equal to the number of protons. • Link to TERC atom builder. Isotopes • Isotopes—atoms of the same element with different numbers of neutrons. • Properties of Isotopes Some isotopes are radioactive. • Telling Isotopes Apart You can identify each isotope of an element by its mass number. • Naming Isotopes To identify a specific isotope of an element, write the name of the element followed by a hyphen and the mass number of the isotope. • Calculating the Mass of an Element The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element. Hydrogen Isotopes: Various atoms and isotopes: Forces in Atoms: • Four Basic Forces Four basic forces are at work everywhere, even within the atom. These forces are gravitational force, electromagnetic force, strong force, and weak force. These forces work together to give an atom its structure and properties.