Chemical elements

1. Which atomic model gave an answer to each of the following? (4 points)

a ) Discovered the electron.

b ) Explained that chemical reactions occur in fixed mass proportions.

c ) Proposed that it is impossible to know the exact position and momentum of a subatomic particle simultaneously (uncertainty).

d ) Explained atomic energy levels through the light emitted or absorbed by atoms (atomic spectra).

e ) Proposed that atoms have a tiny, dense, positively charged center.

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2. Given: Symbol=Fe, A=50, charge=0. Find the missing values: (1 point)

a ) Atomic number Z:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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3. Given: Z=23, n=28, charge=0. Find the missing values: (1 point)

a ) Symbol:

b ) Mass number A:

c ) Number of protons p:

d ) Number of electrons e:

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4. Given: Symbol=P, A=31, charge=0. Find the missing values: (1 point)

a ) Atomic number Z:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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5. Given: Z=16, A=32, charge=-2. Find the missing values: (1 point)

a ) Symbol:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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6. Given: A=39, e=18, charge=+1. Find the missing values: (1 point)

a ) Symbol:

b ) Atomic number Z:

c ) Number of protons p:

d ) Number of neutrons n:

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7. Given: Z=18, n=22, charge=0. Find the missing values: (1 point)

a ) Symbol:

b ) Mass number A:

c ) Number of protons p:

d ) Number of electrons e:

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8. Given: Symbol=Cr, n=28, charge=+3. Find the missing values: (1 point)

a ) Atomic number Z:

b ) Mass number A:

c ) Number of protons p:

d ) Number of electrons e:

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9. Given: Z=13, A=27, e=10. Find the missing values: (1 point)

a ) Symbol:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Charge (enter as integer):

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10. Given: Z=82, n=125, charge=+2. Find the missing values: (1 point)

a ) Symbol:

b ) Mass number A:

c ) Number of protons p:

d ) Number of electrons e:

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11. Given: Z=32, A=64, charge=-2. Find the missing values: (1 point)

a ) Symbol:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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12. Given: Symbol=Br, A=79, charge=-1. Find the missing values: (1 point)

a ) Atomic number Z:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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13. Given: Z=27, A=59, charge=0. Find the missing values: (1 point)

a ) Symbol:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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14. Given: Symbol=B, n=6, charge=+3. Find the missing values: (1 point)

a ) Atomic number Z:

b ) Mass number A:

c ) Number of protons p:

d ) Number of electrons e:

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15. Given: Z=54, A=110, charge=0. Find the missing values: (1 point)

a ) Symbol:

b ) Number of protons p:

c ) Number of neutrons n:

d ) Number of electrons e:

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16. Given: Z=47, n=60, charge=+1. Find the missing values: (1 point)

a ) Symbol:

b ) Mass number A:

c ) Number of protons p:

d ) Number of electrons e:

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17. Given: A=32, n=16, e=18. Find the missing values: (1 point)

a ) Symbol:

b ) Atomic number Z:

c ) Number of protons p:

d ) Charge (enter as integer):

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18. Select the category that fits better: Chemically inert under normal conditions. (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Noble gases (Group 18) have complete valence shells and are virtually unreactive.

19. Select the category that fits better: Ductile (can be drawn into wires). (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Ductility is a characteristic of metals; delocalized electrons allow layers to slide without breaking bonds.

20. Select the category that fits better: Generally good conductors of heat and electricity. (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Metals have free (delocalized) electrons that carry both charge and thermal energy.

21. Select the category that fits better: Tends to lose electrons and form positive ions (cations). (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Metals have low ionization energies and readily lose electrons to form cations.

22. Select the category that fits better: Malleable (can be hammered into thin foils). (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Malleability is a key metallic property due to the non-directional nature of metallic bonding.

23. Select the category that fits better: Exhibits properties intermediate between metals and nonmetals. (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Metalloids (e.g., Si, Ge, As) lie along the 'staircase' and share characteristics of both metals and nonmetals.

24. Select the category that fits better: Can easily be found as solid, liquid, or gas at room temperature. (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Nonmetals exist in all three states at room temperature: e.g., C (solid), Br₂ (liquid), O₂ (gas).

25. Select the category that fits better: Does not conduct electricity as a pure solid but can conduct electricity when dissolved in water. (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: This describes ionic compounds formed by nonmetals (e.g., NaCl). In solution, ions are free to move and conduct current.

26. Select the category that fits better: Tends to gain electrons and form negative ions (anions). (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Nonmetals have high electronegativity and tend to gain electrons (e.g., Cl → Cl⁻).

27. Select the category that fits better: Shiny appearance. (1 point)

Noble gas Metal Nonmetal Metalloid

Feedback: Metallic luster results from free electrons reflecting light.

28. Indicate the type of chemical bond in: NaCl: (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: NaCl is formed by electrostatic attraction between Na⁺ and Cl⁻ ions — a classic ionic compound.

29. Indicate the type of chemical bond in: Cu/Ni alloy (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: Alloys are metal mixtures held together by metallic bonding (delocalized electrons in a lattice).

30. Indicate the type of chemical bond in: P₄ (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: White phosphorus consists of discrete P₄ molecules with covalent P–P bonds.

31. Indicate the type of chemical bond in: H₂O₂ (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: Hydrogen peroxide is a discrete covalent molecule (O–O and O–H covalent bonds).

32. Indicate the type of chemical bond in: MgO (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: MgO is formed by Mg²⁺ and O²⁻ ions — ionic bonding.

33. Indicate the type of chemical bond in: SO₂ (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: SO₂ is a discrete covalent molecule (nonmetal + nonmetal).

34. Indicate the type of chemical bond in: Al (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: Aluminium is a metal held together by metallic bonding.

35. Indicate the type of chemical bond in: CH₄ (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: Methane (CH₄) is a discrete covalent molecule.

36. Indicate the type of chemical bond in: LiBr (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: Lithium bromide is an ionic compound formed by Li⁺ and Br⁻ ions.

37. Indicate the type of chemical bond in: C (graphite) (1 point)

Ionic Metallic Covalent molecule Covalent network

Feedback: Graphite is a covalent network solid where each carbon is covalently bonded to three others in extended layers.

38. Indicate the most probable substance that: Can dissolve well in water — Ag, KBr, CO₂ (1 point)

Ag KBr CO₂

Feedback: KBr is an ionic salt that dissociates readily in water. Ag is nearly insoluble; CO₂ dissolves slightly but reacts to form carbonic acid.

39. Indicate the most probable substance that: Likely used in semiconductor devices — Ge, Al, Cl₂ (1 point)

Ge Al Cl₂

Feedback: Germanium (Ge) is a metalloid used in early semiconductors. Al is a conductor and Cl₂ is a nonmetal gas.

40. Indicate the most probable substance that: Is the least reactive element — Zn, K, Au (1 point)

Zn K Au

Feedback: Gold (Au) is one of the least reactive metals; it does not tarnish or corrode under normal conditions.

41. Indicate the most probable substance that: Has the highest melting point — NH₃, CaO, CH₄ (1 point)

NH₃ CaO CH₄

Feedback: CaO (calcium oxide) is an ionic solid with very strong electrostatic forces, giving it a melting point of ~2613 °C. NH₃ and CH₄ are molecular substances with weak intermolecular forces.

42. Indicate the most probable substance that: Has a low melting point — CH₄, Al₂O₃, NaCl (1 point)

CH₄ Al₂O₃ NaCl

Feedback: Methane (CH₄) is a nonpolar molecule held together only by weak van der Waals forces, giving it a melting point of −182 °C.

43. Indicate the most probable substance that: Exists as discrete molecules — Cl₂, Fe, CaO (1 point)

Cl₂ Fe CaO

Feedback: Cl₂ exists as diatomic molecules. Fe is a metallic solid and CaO is an ionic network solid.

44. Indicate the most probable substance that: Very reactive metal — Na, Cu, Pt (1 point)

Na Cu Pt

Feedback: Sodium (Na) reacts vigorously with water and air. Cu and Pt are much less reactive (noble metals).

45. Indicate the most probable substance that: Poor electrical conductor in all states — CCl₄, NaCl, Al (1 point)

CCl₄ NaCl Al

Feedback: CCl₄ is a covalent molecule with no free ions or electrons in any state. NaCl conducts when molten or dissolved; Al is an excellent metal conductor.

46. Indicate the most probable substance that: High boiling point due to strong ionic forces — MgCl₂, NH₃, O₂ (1 point)

MgCl₂ NH₃ O₂

Feedback: MgCl₂ is an ionic compound with strong Mg²⁺–Cl⁻ electrostatic forces, giving it a boiling point of ~1412 °C. NH₃ and O₂ are molecular with much weaker forces.

47. Classify each of the following images as: pure element, pure compound, homogeneous mixture, or heterogeneous mixture. (2 points)

i) Pure element, ii) Pure compound, iii) Homogeneous mixture, iv) Heterogeneous mixture i) Pure compound, ii) Pure element, iii) Heterogeneous mixture, iv) Homogeneous mixture i) Homogeneous mixture, ii) Heterogeneous mixture, iii) Pure element, iv) Pure compound i) Heterogeneous mixture, ii) Homogeneous mixture, iii) Pure compound, iv) Pure element

Feedback: Image i shows single atom molecules, image ii a compound made of 1 element plus 3 atoms of another element, image iii shows 2 different compounds in different parts, and image iv shows a uniform mixture of 2 compounds.

48. Indicate whether the following is a chemical or physical change: A piece of paper tearing. (1 point)

Chemical Physical

Feedback: Tearing changes shape but does not produce a new substance.

49. Indicate whether the following is a chemical or physical change: Milk turning sour. (1 point)

Chemical Physical

Feedback: Souring is bacterial fermentation producing new substances (lactic acid).

50. Indicate whether the following is a chemical or physical change: Salt dissolving in water. (1 point)

Chemical Physical

Feedback: Ions separate but no new substance forms; the salt can be recovered by evaporation.

51. Indicate whether the following is a chemical or physical change: A car engine burning fuel. (1 point)

Chemical Physical

Feedback: Combustion produces CO₂ and H₂O — new substances are formed.

52. Indicate whether the following is a chemical or physical change: Ice forming in a freezer. (1 point)

Chemical Physical

Feedback: Freezing is a change of state; H₂O remains H₂O.

53. Indicate whether the following is a chemical or physical change: A nail rusting. (1 point)

Chemical Physical

Feedback: Rusting forms iron oxide (Fe₂O₃) — a new substance.

54. Indicate whether the following is a chemical or physical change: Crushing a soda can. (1 point)

Chemical Physical

Feedback: The can changes shape but no new chemical is formed.

55. Indicate whether the following is a chemical or physical change: Baking a cake. (1 point)

Chemical Physical

Feedback: Baking triggers chemical reactions (protein denaturation, CO₂ from baking powder) producing new substances.

56. Indicate whether the following is a chemical or physical change: Water evaporating from a lake. (1 point)

Chemical Physical

Feedback: Evaporation is a change of state; water molecules remain H₂O.

57. Indicate whether the following is a chemical or physical change: A log burning in a fire. (1 point)

Chemical Physical

Feedback: Combustion of wood produces CO₂, H₂O, and ash — new substances.

58. Indicate whether the following is a chemical or physical change: Breaking a glass bottle. (1 point)

Chemical Physical

Feedback: Breaking changes shape but not chemical composition.

59. Indicate whether the following is a chemical or physical change: Cooking pasta. (1 point)

Chemical Physical

Feedback: Cooking causes starch gelatinization and protein denaturation — chemical changes.

60. Indicate whether the following is a chemical or physical change: Freezing juice into ice pops. (1 point)

Chemical Physical

Feedback: Freezing is a change of state; no new substances form.

61. Indicate whether the following is a chemical or physical change: Metal corroding. (1 point)

Chemical Physical

Feedback: Corrosion produces metal oxides or hydroxides — new substances.

62. Indicate whether the following is a chemical or physical change: Cutting hair. (1 point)

Chemical Physical

Feedback: Cutting changes shape but not chemical composition.

63. Indicate whether the following is a chemical or physical change: Digestion of food. (1 point)

Chemical Physical

Feedback: Enzymes break macromolecules into simpler ones — new substances are formed.

64. Indicate whether the following is a chemical or physical change: Dissolving sugar in tea. (1 point)

Chemical Physical

Feedback: Sugar molecules disperse but remain chemically unchanged.

65. Indicate whether the following is a chemical or physical change: Firewood turning into ash. (1 point)

Chemical Physical

Feedback: Combustion produces ash, CO₂, and H₂O — new substances.

66. Indicate whether the following is a chemical or physical change: Melting butter in a pan. (1 point)

Chemical Physical

Feedback: Melting is a change of state; chemical composition is unchanged.

67. Indicate whether the following is a chemical or physical change: A battery producing electricity. (1 point)

Chemical Physical

Feedback: A battery converts chemical energy to electrical energy via redox (chemical) reactions.

68. Magnesium has three isotopes: Mg-24 (abundance 80%), Mg-25 (abundance 10%), and Mg-26 (abundance 10%). Calculate the molar mass of magnesium in g/mol. (2 points)

Feedback: Molar mass = (24 × 0.80) + (25 × 0.10) + (26 × 0.10) = 19.20 + 2.50 + 2.60 = 24.30 g/mol.

69. Beryllium is used in nuclear reactors to slow down (moderate) or reflect neutrons back into the core, increasing efficiency.<br>Balance the following nuclear reaction: $^{9}_{4}Be + ^{4}_{2}He → ^{A}_{Z}X + ^{1}_{0}n$. (3 points)

a ) What is the value of A?

b ) What is the value of Z?

c ) What is the symbol of the product X?

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70. Polonium was first isolated by Marie Curie in 1898 and named after her Polish homeland.<br>Balance the following nuclear reaction: $^{210}_{84}Po → ^{206}_{82}Pb + ^{A}_{Z}X$. (3 points)

a ) What is the value of A?

b ) What is the value of Z?

c ) What is the symbol of the product X?

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