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Chemistry Mid-Term Study Guide, January 2012

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Chemistry Practice Problems

Chemistry Midterm Study Guide


Steps of the Scientific Method:

  1. Observation
  2. Hypothesis
  3. Experiment
  4. Data
  5. Conclusion
  6. Retest         


Experimental Variables:

  • Independent Variable-the factor that is changed
  • Dependent Variable-the factor that is measured or observed
  • Control-the factor that is not being tested or changed.  It’s used for comparison.




homogenous mixtures

  • have uniform composition
  • components cannot be easily identified or separated
  • example:  sugar water or vanilla ice cream


heterogeneous mixtures

  • do not have the same appearance or composition throughout
  • components can be easily identified and separated
  • example:  fruit salad or chocolate chip cookies


Separating Mixtures

  • two methods used in lab to separate mixtures
  • filtration- use filter paper to filter out larger components like sand
  • distillation-evaporate liquid until the solid is left behind.  Evaporate water and salt is left behind in flask. 



Physical & Chemical Properties:

physical property

  • characteristic that is observed with the senses and can be determined without destroying the object.
  • examples-color, shape, length, density, melting point




chemical property

  • characteristic of matter that can only be observed when one substance changes into a different substance, such as iron into rust.
  • examples-flammability, reactivity with vinegar, reacts with acid to form hydrogen gas.   



Physical & Chemical Changes:


Physical Change

  • change in which the identity of the substance does not change
  • examples:  density, melting point, cutting paper, dissolving salt in water


Chemical Change

  • transforms one type of matter into another kind, which may have different properties
  • examples:  burning leaves, bleaching your hair, frying an egg




Atomic History & Models:


Dalton’s Atomic Theory

  1. an element is composed of tiny, indivisible, indestructible particles called atoms.
  2. all atoms of an element are identical and have the same properties.
  3. billiard ball model-atoms are indivisible spheres with constant density throughout




  1. plum pudding model-atoms pictured as spheres of positive charge with small negative particles dispersed throughout the postive mass.



gold foil experiment-

  1. alpha particles were shot at a thin piece of gold foil.
  2. the particles were expected to pass through the foil, but instead some particles were deflected and some bounced back.
  3. because some of the particles were deflected and bounced back, this proved the existence of a dense nucleus in the center of the atom.
  4. since most of the particles passed right through, this indicated that the atom is mostly empty space. 



planetary model-

  1. first developed by rutherford and later adapated by bohr.
  2. electrons orbit around the nucleus like planets around the sun.
  3. the orbitals were described by rutherford as overlapping circles surrounding the nucleus.
  4. the model was later changed by bohr, and he described the orbitals as concentric circles that do not overlap and move out from the nucleus.






Atomic Structure:


atom-the smallest particle that represents an element


subatomic particles:


  • much larger and heavier than electrons
  • postive electrical charge (+)
  • located in the nucleus of the atom



  • large and heavy like a proton
  • no electrical charge (neutral)
  • located in the nucleus of the atom


  • tiny, very light particles
  • negative electrical charge (-)
  • moves around the outside of the nucleus


atomic number

  • describes the number of protons in an atom
  • in a neutral atom the number of protons also equals the number of electrons


atomic mass number

  • describes the number of protons + neutrons in an atom


isotopes-atoms of the same element that vary in the number of neutrons in the nucleus



calculating atomic mass

  1. convert %abundance into a decimal by moving the decimal point over two places to the left.
  2. multiply the decimal by the mass of each isotope
  3. add the new masses together & round to two decimal places
  4. do not forget units of AMU



calculating PEN for neutral atoms:

  1. the number of protons is equal to the atomic number of the element
  2. the number of electrons equals the number of protons if the atom is neutral
  3. the number of neutrons is equal to (the atomic mass – the atomic number)


ions-charged particles that are formed when an atom loses or gains electrons


cation-positively charged ion that results from the loss of electrons


anion-negatively charged ion that results from the gain of electrons




Calculating PEN For Ions:

  1. each electron has a charge of (-1)
  2. the number of protons is equal to the atomic number
  3. the number of electrons is equal to the number of protons (+/-) any extra electrons to account for the charge
  4. the number of neutrons equals the (atomic mass – atomic number)



  • if an atom has a charge of (-3), three electrons must be added to the normal electron count
  • if an atom has a charge of (+2), two electrons must be subtracted from the normal electron count



The Periodic Table:


most elements on the table are metals


elements are classified into three major groups

  • metals
  • metalloids
    • found on the “step” of the table between the metals and nonmetals
    • have characteristics in between those of metals and nonmetals
  • nonmetals




  • horizontal rows on the periodic table are called periods
  • elements in a period are not alike in properties



  • vertical columns on the periodic table are called groups or families
  • elements in the same group exhibit similar chemical characteristics because they have the same number of valence electrons


valence electrons

  • are electrons found in the outermost shell of an atom
  • these are the electrons that are free to bond with other elements
  • the number of valence electrons is determined by the group number at the top of the periodic table

important groups of the periodic table:


Alkali Metals

  • group one on the table
  • very reactive metals
  • they react violently with water
  • they are shiny and malleable
  • form cations by donating one or two electrons from their outer shell



Alkaline Earth Metals

  • group two on the table
  • reactive, but not as reactive as the alkali metals
  • always combine with nonmetals in nature
  • form cations by donating one electron from their outer shell



Transition metals

  • found in the center block of the table
  • reffered to as “D” block
  • form positive ions
  • charge is indicated in parentheses as roman numerals



  • group seven on the table
  • very reactive and volatile nonmetals
  • only need one electron to have a full shell so they are likely to gain an electron and form anions


Noble Gases

  • group eight on the table
  • not very reactive
  • have full electron shells so they do not gain or lose electrons
  • they do not form ions


Periodic Trends

  • octet rule-atoms bond with other atoms until they have a full octet, which is 8 electrons.
  • atoms with fewer valence electrons (left side of table, groups 1 & 2) will lose electrons making them more positive, cations)
  • atoms with more valence electrons (right side of table, groups 5-7) will gain electrons making them more negative, anions)


Atomic Radius: size of the atom

  • atoms increase in size as you move down a group
  • atoms decrease in size as you move from left to right across the table


  • why?  Going across the table, the number of protons increases, making the electrical attraction between the nucleus and the electrons stronger and that pulls in the electrons closer to the nucleus making the atomic radius smaller


Ionic Radius:  size of an ion

  • ions are charged atoms.  a cation is a positively charged ion and an anion is a negatively charged ion
  • trend is the same as atomic radius:
  • atoms increase in size as you move down a group
  • atoms decrease in size as you move from left to right across the table
  • Cations are smaller than their neutral atom because they have lost electrons, the remaining electrons are pulled in more by the strength of the protons in the nucleus.


  • Anions are larger than their neutral atom because they have gained electrons, so there is an increased repulsion between the nucleus and the orbital with the electrons. 



Ionization Energy:  energy required to remove an electron from an atom


  • ionization energy decreases as you move down a group because the outer electrons are farther from the nucleus so they do not experience as much pull.  since they are not pulled with a lot of strength, they are easier to remove, thus requiring less energy.


  • ionization energy increases as you move across from left to right because elements to the right of the table are close to being full with 8 electrons.  since they are so close to full, they are not very likely to want to give up their electrons, thus requiring more energy.



  • The ability of an atom to atrract electrons towards itself.


  • electronegativity decreases as you move down a group because the electrons in the outer orbital are father away from the nucleus and do not experience high attraction forces so additional electrons would be difficult to attract.  Also, elements to the left of the table have extra electrons that they give away, so they would not need to atrract any additional electrons.


  • electronegativity increases as you move across from left to right because atoms to the right of the table are close to being full and need electrons, so they would need high electronegativity in order to attract the electrons they need to be full and become stable. 


Electron Configurations

  • Groups 1 & 2 (s sublevels)
  • Groups 13-18 (p sublevels)
  • Groups 3-12 d block (d sublevels)
  • Bottom Two (f sublevels)


  • large number- row number
  • letter-sublevel area
  • superscript-how many spaces have you moved in the sublevel area


noble gas notation

  • Find the noble gas closest to the element, that comes BEFORE
  • Put the noble gas in brackets, continue notation from the next area


steps for writing an electron configuration:

  1. find the element
  2. start at the beginning of the periodic table and indicate row number, letter section, and how many times you count across until you reach your element.