Atoms the basic building block of all matter the smallest particle of an element that retains the properties of that element electrically neutral: the number of positive charges (protons) equals the number of negative charges (electrons) composed of 3 subatomic particles: protons (p + ), electrons (e - ) and neutrons (n o )
Models of the Atom - History Democritus a fifth century B.C. Greek philosopher proposed that all matter was composed of indivisible particles called atoms (“atoma” - Greek for indivisible).
John Dalton (1803) Billiard Ball Model viewed the atom as a small solid sphere. He really got the "ball" rolling for modern chemistry! Each element was composed of the same kind of atoms. Compounds are composed of atoms in specific ratios. Chemical reactions are rearrangements of atoms (mass is conserved).
J. J. Thomson (1897) Plum Pudding Model proposed that the atom was a sphere of positive electricity with negative particles imbedded throughout after discovering the electron a discovery for which he was awarded the Nobel Prize in physics in 1906.
Ernest Rutherford (1908) discovered that the atom is mostly empty space with a dense positively charged nucleus surrounded by negative electrons. Rutherford received the Nobel Prize in chemistry in 1908 for his contributions into the structure of the atom.
Electron Cloud Model (1920's) an atom consists of a dense nucleus composed of protons and neutrons surrounded by electrons that exist in different clouds at the various energy levels. Erwin Schrodinger and Werner Heisenburg developed probability functions to determine the regions or clouds in which electrons would most likely be found.
Drawing Bohr Diagrams Atomic number: identifies the element equal to the number of protons in the nucleus since atoms are electrically neutral, # of protons = # of electrons Mass number: # of protons + # of neutrons # neutrons = atomic mass (rounded off) – atomic # Protons and neutrons account for most of the mass of the atom
Bohr Diagrams (continued) an energy level represents a specific value of energy of an electron and corresponds to a general location around the nucleus the number of occupied energy levels in any atom is normally the same as the period number in which an atom appears
for the first 3 energy levels, the maximum number of electrons that can be present are 2, 8 and 8 in order of increasing energy (increasing distance from the nucleus) a lower energy level is filled with electrons to its maximum level before the next level is started. The electrons in the highest (outermost) occupied energy level are called valence electrons. Number of valence electrons is the same as the group number for group A elements (1,2,13-18).
Bohr Diagrams (continued) Draw a circle for the nucleus Put in the number of protons and neutrons inside the circle. Determine the number of electrons and place them in energy levels starting closest to the nucleus and filling them in the order of 2, 8, 8.
Stable Atoms have low chemical reactivity include noble gases, all of which have 8 valence electrons (except He, which has 2) stability is a function of having a full complement of valence electrons. Atoms that do not have full electrons energy levels are unstable and must gain, lose or share electrons to become stable. other atoms can become more stable by reacting and changing the number of their electrons
Stable Atoms atoms can follow one of 2 rules: a) Octet rule -atoms attempt to obtain 8 valence electrons -includes most atoms b) Duet rule-atoms attempt to obtain 2 valence electrons -includes H, Li and Be Atoms can achieve a stable octet or duet by forming ions.
Ions an atom or groups of atoms that have a positive or negative charge, due to the loss or gain of one or more electrons. Single atoms form simple ions (monatomic ions); groups of atoms form complex ions (polyatomic ions)
Example: sodium metal and chlorine gas react to produce NaCl, a very stable and unreactive substance, compared to Na or Cl. The sodium atom loses 1 electron to the chlorine atom so both of their outer levels are filled. In doing so, the atoms form ions of opposite charge.