The Atom

 Matter –Anything that takes up space and has weight  Physical Forms of Matter –Solids –Liquids –Gases  Chemical Forms of Matter –Elements –Compounds –Mixtures

The Atom  Elements –63 elements arranged in Periodic Table by Mendeleev in 1869 –92 elements now recognized as natural –117 total elements recognized if we include man-made

The Atom  Compounds –Substances made from two or more elements combined together –The smallest particle of a compound, which retains the properties of the compound, is called a “molecule”  Mixtures –Substances which contain a mixture of different elements and compounds

The Atom  Atom –Democritus (530 BC) “All matter is formed from atoms.” –John Dalton (1808) “Some substances cannot be broken down into other substances. These pure substances are elements. The atom is the smallest particle of an element.”

The Atom  Are atoms real? –Indirect evidence Behavior of gases Chemical combinations Brownian motion –Direct evidence X-ray crystallography (1912) Atomic scale microscopy (1980)

The Atom  Periodic Table –Arranged in columns and rows which show the relationship between the different elements

The Atom  Elements on left side of periodic table tend to be solids –Sodium, potassium, magnesium, calcium  Elements on right side of periodic table tend to be gases –Oxygen, helium, neon, chlorine  Only one element is a liquid at normal atmospheric temperatures and pressures –mercury

The Atom  Subatomic Particles SizeMassCharge Protons Same as neutron 1+ Electrons 1 / 1000 of the proton or neutron 0 - Neutrons Same as proton 1 no charge

The Atom  Atomic Structure –Nucleus Dense central region Contains protons and neutrons –Shells Energy levels Contain electrons  Rutherford model vs. Bohr model

The Atom  Rutherford Model –Similar to planets in our solar system, fixed positions –Not mathematically possible  Bohr Model –Electrons can move between higher and lower energy levels –Drop in level releases photon of energy in form of light –Light emitted or absorbed determines spectrum (an atom’s “fingerprint”) –Mathematically possible

The Atom  Construction of atoms –Atomic number Number of protons in atomic nucleus Determines which element is present Equals the number of electrons in a neutral atom –Atomic mass Reflects the mass of protons and neutrons in the atomic nucleus Is not influenced by the number of electrons He 4 2 symbol atomic number atomic mass

The Atom  Energy shells –Number of electrons equals the number of protons –Innermost (1 st ) can hold 2 electrons –2 nd shell can hold 8 electrons –3 rd shell can hold 18 electrons –Capacity increases in additional shells –Outer shell can only hold 8 no matter what capacity

The Atom helium carbon sodium He 2 4 C 6 12 Na 11 23

The Atom  Atoms in combination –Covalent compounds Composed of 2 or more elements joined together by covalent bonds Sharing of electrons –Ionic compounds Composed of 2 or more elements joined together by ionic bonds Transfer of electrons from one atom to another

The Atom  Elements in vertical columns have same number of electrons in their outer shells  Outer shell electrons called “valence” electrons  Valence electrons determine which elements react with each other  Chemical bonds link atoms together –Covalent –Ionic

The Atom  Columns IA and VIIA –Gives up and takes on one electron (ionic bonds)  Columns IIA and VIA –Gives up and takes on two electrons (ionic bonds)  Column IVA –Shares four electrons (covalent bonds)  Columns VIIIA (0) –Does not react because outer shell is full (no bonding)

The Atom  Ions –Charged derivative of an atom –Positive if the atom loses electrons –Negative if the atom gains electrons  Fourth state of matter = Plasma –Ionized gases in which electrons have been stripped from atoms –Formed at high temperatures

The Atom  Isotopes –Multiple forms of the same atom –Same atomic number –Different mass number Differs in neutron content Many are radioactive (e.g.. Carbon-14)

The Atom  Radioactivity –Spontaneous release of energetic particles Becquerel (late 1800’s) Marie Curie (early 1900’s) –Types of radioactive decay Alpha Beta Gamma –Half-life Time it takes for ½ of radioactive isotopes to decay

The Atom  Atomic energy –Nuclear fission Splitting of a large atom’s nucleus –Atomic bomb –Nuclear reactors –Nuclear fusion Joining two small atomic nuclei together to form a larger atom –Hydrogen bomb –Energy for the future?