Chemistry 1st Semester Exam

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pomea1
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Chemistry 1st Semester Exam
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2012-01-18 17:29:24
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  1. State if the following are elements, compounds or mixtures. If they are a mixture state if it is a mixture of an element with another element, a mixture of a compound with another compound or a mixture of a compound with an element. (Each circle represents an atom).
    .
  2. Complete the following table:
  3. What defines what type of element an atom is?
    The number of protons defines what type of element an atom is. Two atoms may differ in the number of neutrons but they are still the same element. (ex. One atom Hà 1 proton, 0 neutron, 1 electron and another H atom à 1 proton, 1 neutron, 1 electron. These two hydrogen atoms are ISOTOPES) Two particles may differ in the number of electrons but they are still the same element. (ex. One atom Hà 1 proton, 0 neutron, 1 electron and another H+ à 1 proton, 0 neutron, 0 electron. This last one is an ion because it has lost an electron).
  4. What is an ion? Define an anion and cation? Provide examples.
    An ion is an atom that has gained or lost electrons and therefore it is a charged atom. An anion is a negative ion. (Ex.Cl-). It has gained one additional electron. (Therefore it has 1 more electron than protons and is negatively charged) A cation is a positive ion. (Ex. Na+). It has lost one electron. (Therefore it has 1 more proton than electrons and is positively charged).
  5. How does a neutral atom become an ion?
    It gains or loses electrons.
  6. What is an isotope? Provide examples.
    Is an atom that has the same number of protons as another atom but differs in the amount neutrons. Examples carbon with a mass of 12 amuand carbon with a mass of 13 amu. You represent this as C-12 or C-13. Another way to represent this is: .
  7. a. What is the mass of a chlorine atom which has 17 protons and 20 neutrons?
    b. What is the mass of a chlorine atom which has 17 protons and 18 neutrons?
    c. What symbol could you use to differentiate between both isotopes?
    • a. 37 amu.
    • b. 35 amu.
    • c. Cl-37 and Cl-35 You can also use nuclear symbols: .
  8. How many protons, neutrons and electrons does K-41 have?
    • Protons 19
    • Neutron 22
    • Electrons 19
  9. State the number of protons, neutrons and electrons in the ion ?
    • Protons 7
    • Neutrons 8
    • Electrons 10
  10. State the electron arrangement for atoms of aluminium, nitrogen and fluorine.
    • Al 2.8.3
    • N 2.5
    • F 2.7
  11. State the electron arrangements of the following
    species:
    • S2– 2.8.8
    • P3– 2.8.8
  12. An element has an electron configuration of 2.8.5. State what period and what group its found in. How many valence electrons does it have? What is the element?
    Period (Row) 3 (because electrons in three main energy levels) and group (column 5 )(because it has 5 valence electrons. It is P.
  13. Define valence electrons and explain what relationship do valence electrons have with a group.
    • Valence electrons- electrons in theoutmost shell.
    • Number of valence electrons in the outmostshell is equal to group (column).
  14. Which elements have stable ELECTRON configurations? Why do they have stable electron configurations?
    Noble gases because they have full outershells
  15. Why do elements combine chemically with other
    elements?
    To achieve full outer shells They can do this by gaining or losing electrons (Or next unit you will also see they can do it by sharing electrons).
  16. Why don´t noble gases want to react (chemically
    combine) with other elements?
    They already have full outershells. They are already stable electronically. Thay are happy atoms.
  17. Why do elements in group 2 tend to form 2+ charges
    when they become ions?
    They all have 2 valence electrons. They will tend to lose 2 valenceelectrons. When they do lose thesetwo electrons, they will have two more protons than electrons.
  18. Why do elements in group 7 tend to form 1- charges
    when they become ions?
    They all have 7 valence electrons. They will tend to gain 1 valenceelectrons. When they do gain 1 electron, they will have one more electron than protons and thus negative charge.
  19. What charge do elements in group 5 tend to have
    when they become happy atoms?
    3-
  20. What is electron shielding?
    Electrons in internal shells that interfere between the attraction of the positive nucleus and outside electrons.
  21. How many shielding electrons does C have? How many shielding electrons does N have? How many shielding electrons does O have? Why do they all the elements in a the same period have the same amount of shielding electrons?
    All of them have two valence electrons. All of these elements are in the same period and therefore they all have the same number of energy levels and the same number of complete internal energy levels.
  22. Which electrons are lost or gained when elements chemically combine?
    Only valence electrons.
  23. What is electronegativity?
    The tendency an atom has to attract an electron from another atom.
  24. Which is the most electronegative element in the
    periodic table?
    Fluorine.
  25. Which is the least electronegative element in the periodic table?
    Fr and Cs.
  26. Place the following elements in increasing order of electronegativity Mg, Be , Sr. What is the electronegativity trend as you go down a group Explain why.
    • Sr, Mg, Be. (the further away you get from F , the less electronegative) The electronegativity decreases going
    • down a group. Eventhough there is a greater number of protons going down a group, there are also an increasing number of energy elevels that interfere between the attraction of the positive nucleus and outside electron.
  27. Place the following elements in increasing order
    of electronegativity Si, Na Mg. What is the electronegativity trend as you go across a period?Explain why.
    Na, Mg , Si (the closer the element is to F the more electronegative) Within a same period there is the same number of shielding electrons and therefore the same interference however the elements to the right have a greater number of protons in their nucleus which attract the outside electron more.
  28. What group belong to the halogens?
    Group 7.
  29. What group belong to the alkali metals?
    Group 1.
  30. What group belong to the alkaline earth metals?
    Group 2.
  31. What group are the noble gases?
    Group 8.
  32. Provide two examples of transitions metals.
    Fe, Cr (several answers).
  33. Which are the metals, nonmetals and metalloids in the following sketch of the periodic table?
  34. Define the octet rule.
    Principle that states that atoms tend to form compounds so that each atom can have eight electrons in its outermost energy level.
  35. State which compounds are covalent or ionic and what occurs with valence electrons.
    a. Li2O- ionic because it is a metal and non metal. The “Li” are losing electrons while the “O” is gaining electrons. In other words there is a transfer ofelectrons. Li is a positive ion while O is a negative ionand they attract to each other.

    b.CS2- covalent because it is a non metal and nonmetal. They are sharing theirvalence electrons.
  36. State the steps to write an ionic formula given two elements.
    a. Write symbols and charges.

    b. Criss cross charges to form subscripts.

    c. Write final formula with lowest ratio and no 1.
  37. Form ionic formulas with the following elements:
    a. Sodium(Na) and Oxygen (O)

    b. Sodium (Na) and Chlorine (Cl)

    c. Magnesium (Mg) and Sulfur (S)

    d. Aluminum (Al) and Nitrogen (N)
    • a. Na2O
    • b. NaCl
    • c. MgS
    • d. AlN
  38. A single covalent bond consists of ____ bonding electrons. A double covalent bond consists of ___ bonding electrons. A triple covalent bond consists of ______ bonding electrons.
    A single covalent bond consists of 2 bonding electrons. A double covalent bond consists of 4 bonding electrons. A triple covalent bond consists of 4 bonding electrons.
  39. State the steps to drawing a Lewis Structure for a covalent structure.
    • 1.Make a skeleton with the elements. (Place the least electronegative element in center except for hydrogen – wich shall always be in the edges).
    • 2. Add up all valence electrons.
    • 3. Place single covalent bonding pairs between atoms. Add the restof electrons as unshared pairs of electrons.
    • 4. If octet rule is not reached for all atoms try double ortriple bonds.
  40. Write the Lewis structure for the following molecules:
    a. NH3
    b. O2
    c. H2O
    d. F2
    e. N2
    f. CO2
    g. CH2O
    h. HF
    i. CH4
    .

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