for each element listed below, indicate if the element is a metal, a nonmetal, or a semimetal.
63. cobalt
64. oxygen
65. carbon
66. boron
67. iodine
68. zinc
69. arsenic
70. mercury
71. antimony
72. aluminum
three elements are represented by the letters a, b, and c. the three elements have the same number of valence electrons. element c has electrons in three energy levels and element a is the lightest of the three. element b has a total of twenty electrons.
73. what is element a?
74. what is element b?
75. what is element c?
answer each question below about the element chlorine:
76. how many protons does chlorine have?
77. in which group does chlorine belong?
78. what is the name of this group?
79. how many valence electrons does chlorine have?
80. how does the atomic radius of chlorine compare to fluorine?
81. what type of ion is chlorine most likely to form?
82. how does the electronegativity of chlorine compare to sulfur?
83. how does the ionization energy of chlorine compare to bromine?

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# Brief Explanations:
- **63. Cobalt**: Metals are elements that typically have high electrical and thermal conductivity, are malleable and ductile, and tend to lose electrons in chemical reactions. Cobalt ($Co$) is a transition metal.
- **64. Oxygen**: Non - metals are elements that usually have low electrical and thermal conductivity, are brittle (in solid state), and tend to gain electrons in chemical reactions. Oxygen ($O$) is a non - metal as it is a gas at room temperature, has low conductivity, and gains electrons (e.g., in the formation of $O^{2 -}$ ions).
- **65. Carbon**: Carbon ($C$) is a non - metal. It can form a wide variety of compounds (organic and inorganic). It has properties like covalent bonding (sharing of electrons) which is typical of non - metals.
- **66. Boron**: Boron ($B$) is a metalloid (semimetal). It has some properties of metals (e.g., can form alloys to some extent) and some of non - metals (e.g., forms covalent compounds like $BF_3$).
- **67. Iodine**: Iodine ($I$) is a non - metal. It is a halogen (Group 17 element). Halogens are non - metals as they are diatomic in their elemental form ($I_2$), have low conductivity, and gain electrons (e.g., $I^-$ formation).
- **68. Zinc**: Zinc ($Zn$) is a metal. It is a transition metal with typical metallic properties such as good electrical conductivity (used in batteries for its ability to conduct electricity in electrochemical cells), malleability (can be shaped into sheets), and it loses electrons in reactions (e.g., $Zn
ightarrow Zn^{2 +}+2e^-$).
- **69. Arsenic**: Arsenic ($As$) is a metalloid. It has properties intermediate between metals and non - metals. For example, it has some metallic luster but also forms covalent compounds like $AsH_3$.
- **70. Mercury**: Mercury ($Hg$) is a metal. It is the only metal that is liquid at room temperature. It has high electrical conductivity (used in thermometers and electrical switches in some cases) and shows metallic bonding.
- **71. Antimony**: Antimony ($Sb$) is a metalloid. It has some metallic properties (e.g., can be used in alloys) and some non - metallic properties (e.g., forms covalent compounds like $SbCl_3$).
- **72. Aluminum**: Aluminum ($Al$) is a metal. It is a lightweight metal with high electrical and thermal conductivity, is malleable (used in aluminum foil), and loses electrons in reactions (e.g., $4Al + 3O_2
ightarrow 2Al_2O_3$, where $Al$ goes from $0$ to $+ 3$ oxidation state).

For the elements $A$, $B$, and $C$:
- Let's assume they are in the same group (since they have the same number of valence electrons).
- **73. Element $A$**:
    - If element $C$ has electrons in three energy levels (third - period element), and $A$ is the lightest. Let's assume they are in Group 2 (alkaline earth metals). The lightest alkaline earth metal is beryllium ($Be$).
- **74. Element $B$**:
    - If $B$ has 20 electrons, then the number of protons (in a neutral atom) is also 20. The element with atomic number 20 is calcium ($Ca$).
- **75. Element $C$**:
    - Since $C$ has three energy levels and is in the same group as $A$ and $B$ (assuming Group 2). The third - period Group 2 element is magnesium ($Mg$).

For chlorine ($Cl$):
- **76. Protons in chlorine**:
    - The atomic number of an element is equal to the number of protons. For chlorine ($Cl$), the atomic number is 17. So, the number of protons is 17.
- **77. Group of chlorine**:
    - Chlorine is in Group 17 of the periodic table.
- **78. Name of the group**:
    - Group 17 elements are called halogens.
- **79. Valence electrons in chlorine**:
    - Chlorine has an electron configuration of $1s^{2}2s^{2}2p^{6}3s^{2}3p^{5}$. The valence shell is the third shell ($n = 3$), and the number of valence electrons is 7.
- **80. Atomic radius comparison (Cl vs F)**:
    - As we move down a group (from $F$ to $Cl$), the atomic radius increases. This is because there are more energy levels (electron shells) in $Cl$ ($n = 3$) compared to $F$ ($n = 2$). So, the atomic radius of $Cl$ is larger than that of $F$.
- **81. Type of ion chlorine forms**:
    - Chlorine has 7 valence electrons and tends to gain 1 electron to achieve a noble - gas configuration ($Cl^-$). So, it forms an anion (negatively - charged ion).
- **82. Electronegativity comparison (Cl vs S)**:
    - Electronegativity generally increases across a period. Chlorine ($Cl$) is to the right of sulfur ($S$) in the third period. So, chlorine is more electronegative than sulfur.
- **83. Ionization energy comparison (Cl vs Br)**:
    - Ionization energy decreases down a group. Chlorine ($Cl$) is above bromine ($Br$) in Group 17. So, the ionization energy of $Cl$ is higher than that of $Br$ because the outermost electrons in $Cl$ are closer to the nucleus (less shielding effect from fewer inner shells) and are more tightly bound.

# Answer:
- 63. Metal
- 64. Non - metal
- 65. Non - metal
- 66. Semimetal
- 67. Non - metal
- 68. Metal
- 69. Semimetal
- 70. Metal
- 71. Semimetal
- 72. Metal
- 73. Beryllium ($Be$)
- 74. Calcium ($Ca$)
- 75. Magnesium ($Mg$)
- 76. 17
- 77. Group 17
- 78. Halogens
- 79. 7
- 80. The atomic radius of $Cl$ is larger than that of $F$
- 81. Anion ($Cl^-$)
- 82. Chlorine is more electronegative than sulfur
- 83. The ionization energy of $Cl$ is higher than that of $Br$

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