Atoms and Isotopes
An atom contains 17 protons, 18 neutrons, and 17 electrons. Identify the element, determine the mass number, and write the isotope notation.
Element: Chlorine (Cl)
Mass number: 17 + 18 = 35
Isotope notation: ³⁵₁₇Cl
Determine whether CO₂ is ionic or molecular. Explain your reasoning.
Molecular/covalent.
Carbon and oxygen are both nonmetals, so they form a molecular compound by sharing electrons.
Give the name and charge of SO₄²⁻.
Sulfate, 2−
Name Mg₃(PO₄)₂
Magnesium phosphate
Name N₂O₃.
N₂ = dinitrogen
O₃ = trioxide
Answer: Dinitrogen trioxide
Determine the number of protons, neutrons, and electrons in ³¹₁₅P³⁻.
Protons = 15
Neutrons = 31 − 15 = 16
Electrons = 15 + 3 = 18
Write the formula for phosphorus pentachloride.
Phosphorus = P
Penta = 5
Chlorine = Cl
Formula: PCl₅
Write the formula for calcium nitrate. Show your work.
Calcium = Ca²⁺
Nitrate = NO₃⁻
One Ca²⁺ requires two NO₃⁻ ions
Formula: Ca(NO₃)₂
Name SnCl₄.
Tin(IV) chloride
Cl is −1:
4 Cl = −4
Therefore Sn = +4.
Write the formula for dinitrogen pentoxide.
Di = 2 nitrogen atoms
Penta = 5 oxygen atoms
Formula: N₂O₅
An atom has 12 protons, 12 neutrons, and 10 electrons. Identify the element and determine its charge.
12 protons → Magnesium (Mg)
Charge = 12 − 10 = +2
Ion: Mg²⁺
Explain the difference between an ionic compound and a molecular compound.
Ionic compounds: Metal + nonmetal; electrons are transferred and ions form.
Molecular compounds: Nonmetal + nonmetal; electrons are shared between atoms.
Give the name of Al₂(SO₄)₃.
Aluminum sulfate
Name Fe₂O₃. Show how you determine the Roman numeral.
Oxygen = −2
3 O = −6
Therefore, the 2 Fe atoms must total +6.
Each Fe = +3
Answer: Iron(III) oxide
A student names FeCl₃ as "iron chloride." What information is missing from the name? Give the complete name.
The Roman numeral indicating the charge of iron is missing.
FeCl₃:
3 Cl⁻ = −3
Therefore Fe = +3.
Answer: Iron(III) chloride
An element has an atomic number of 35 and a mass number of 80. Determine the number of protons, neutrons, and electrons in the neutral atom.
Protons = atomic number = 35
Neutrons = 80 − 35 = 45
Electrons = protons in a neutral atom = 35
Explain what happens when Na reacts with Cl to form NaCl. Include what happens to the electrons and the charges of the ions.
Na loses one electron:
Na → Na⁺ + e⁻
Cl gains one electron:
Cl + e⁻ → Cl⁻
The Na⁺ and Cl⁻ ions attract because they have opposite charges.
Answer: Na transfers one electron to Cl, forming Na⁺ and Cl⁻.
Write the formula for iron(III) sulfate. Show your work.
Iron(III) = Fe³⁺
Sulfate = SO₄²⁻
2 Fe³⁺ = +6
3 SO₄²⁻ = −6
Formula: Fe₂(SO₄)₃
Name Cu(NO₃)₂. Show how you determine the charge of copper.
Nitrate = NO₃⁻
2 nitrate ions = −2
Therefore Cu must be +2.
Answer: Copper(II) nitrate
A compound contains Al³⁺ and SO₄²⁻. Determine the correct formula and explain how the charges balance.
Al³⁺ = +3
SO₄²⁻ = −2
LCM = 6:
2 Al³⁺ = +6
3 SO₄²⁻ = −6
Formula: Al₂(SO₄)₃
Which atom has the larger atomic radius: Na or Cl? Explain why.
Na has the larger atomic radius.
Atomic radius generally decreases from left to right across a period because the effective nuclear charge increases.
Write the correct formula for aluminum oxide. Show your work using the charges of the ions.
Aluminum = Al³⁺
Oxygen = O²⁻
2 Al³⁺ = +6
3 O²⁻ = −6
Formula: Al₂O₃
Explain why parentheses are needed in a formula such as Ca(NO₃)₂.
Parentheses show that the subscript applies to the entire polyatomic ion.
Ca(NO₃)₂ means there are two nitrate ions.
Write the formula for lead(II) nitrate.
Lead(II) = Pb²⁺
Nitrate = NO₃⁻
Need two nitrate ions.
Formula: Pb(NO₃)₂
A compound contains Fe³⁺ and PO₄³⁻. Write the formula, name the compound, and explain why a Roman numeral is necessary.
SURPRISE! This is the double point question!
So this is worth 1000 points if you can get it right
Fe³⁺ = +3
PO₄³⁻ = −3
The charges cancel 1:1.
Formula: FePO₄
Because iron can have multiple charges, the Roman numeral identifies the charge.
Answer: Iron(III) phosphate