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Atomic Theory Atom, Subatomic Particles, and Isotopes The Atom • All the elements listed on the periodic table are made up of atoms • An atom is the smallest part of an element that retains the characteristics of that element. • The word atom comes from the Greek word atomo. Dalton’s Atomic Theory • Scientist, John Dalton (1766-1844) developed an atomic theory that proposed that atoms were responsible for the combinations of elements found in compounds Dalton’s Atomic Theory • Theory States: • All matter is made up of tiny particles called atoms • All atoms of a given element are similar to one another and different from atoms of other elements • Atoms of two or more different elements combine to form compounds. A particular compound is always made up of the same kinds of atoms and always has the same number of each kind of atom. • A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are never created nor destroyed during a chemical reaction. The Atom • Atoms are the building blocks of everything we see around us; yet we cannot see an atom or even a billion atoms with the naked eye. • When billions and billions of atoms are packed together, the characteristics of each atom are added to those of the next until we can see the characteristics we associate with the element. Parts of the Atom • Dalton had imagined atoms as solid spheres; but by the early twentieth century growing evidence indicated that atoms were not solid spheres. • Subatomic Particles • • Smaller bits of matter that compose an atom Three Subatomic Particles • Proton (+)...(p+) • Electron ( - )...(e-) • Neutron (neutral-no electrical charge)...(n) Remember!! • Like charges repel (they push away from each other) and opposite or unlike charges attract Mass • All of the subatomic particles are extremely small compared with the things you see around you. • One (p+) has a mass of 1.7x10-24g and the (n) is about the same. • The mass of the (e-) is even less! Mass • Because the mass of the subatomic particles is so minute (small), chemists use a small unit called the Atomic Mass Unit • Atomic Mass Unit (amu) • Is one-twelfth of the mass of the carbon atom (with 6 (p+) and 6 (n); the standard for which every other atom is compared Mass • The amu is the same as the Dalton in Biology. • The (p+) and the (n) have a mass of about 1 amu. Because the mass of the (e-) is so small (0.0005) it is usually ignored in the atomic mass calculations Structure of the Atom • In 1911, an experiment led Ernest Rutherford to propose that most of the mass of the atom was contained in a small region at the center of the atom. • The Nucleus is the dense core of the atom where (p+) and (n) are located; it has a (+) charge and contains most of the mass of the atom. • Most of the atom is empty space, which is occupied by only by fast moving (e-) Structure of the Atom Atomic Number • Atomic Number (AN) • is equal to the number of (p+) in the nucleus of an atom • It is used to identify each element. • AN = Number of (p+) in an atom • On the Periodic Table the AN is the whole number above the symbol. Atomic Number • An atom is electrically neutral (there is no charge) • The number of (p+) is equal to the number of (e-) • This electrical balance gives the atom an overall charge of zero • In every atom, the AN also gives the number of (e-) Mass Number • We know that (p+) and (n) determine the mass of an atom • Mass Number or Atomic Mass (AM) • the sum of the number of (p+) and (n) in the nucleus. • AM = (p+) + (n) Atomic Symbol Isotopes • Isotopes • Atoms of the same element that have different numbers of neutrons. • The different number of neutrons causes the atom to have a different mass number but not a different chemical behavior Isotopes • To distinguish between the different isotopes of an element, we can write the Atomic Symbol that indicates the mass number and the AN of the atom Isotopes of Magnesium 24 Mg 12 (n)=12 26 25 Mg 12 (n)=13 Mg 12 (n)=14 2 Ions • An atom with a positive or negative charge. • Atoms of the same element have a different number of electrons • Can also occur with groups of bonded atoms.