Chemistry — Semester A Practice

Question 1 of 10

TEKS 5A-5C Medium

Which of the following neutral atoms has the SMALLEST atomic radius?

A Carbon (C)

B Neon (Ne)

C Lithium (Li)

D Fluorine (F)

Explanation

📌 Atomic radius decreases left-to-right across a period because nuclear charge increases while electrons fill the same shell — the stronger pull compresses the atom. Among Li (3 protons), C (6), and F (9), fluorine has the strongest nuclear pull and the smallest radius. (Note: Ne is often shown larger because covalent radius isn't well-defined for noble gases.)

Question 2 of 10

TEKS 6A-6E Medium

Carbon-12 and carbon-14 are both naturally occurring forms of the element carbon. Which statement correctly describes their relationship?

A They are isotopes because they have the same number of protons but different numbers of neutrons

B They are allotropes because they are different structural forms of carbon

C They are ions because one has gained two extra particles

D They are different elements because they have different mass numbers

Explanation

📌 Isotopes are atoms of the SAME element (same proton count, so both are carbon) with DIFFERENT neutron counts. Carbon-12 has 6 neutrons, carbon-14 has 8. Ions differ in electrons. Allotropes (graphite, diamond) are different molecular structures, not different atoms.

Question 3 of 10

TEKS 5A-5C Easy

Elements in Group 1 of the periodic table (Li, Na, K, …) are known as:

A Noble gases

B Alkaline earth metals

C Halogens

D Alkali metals

Explanation

📌 Group 1 (excluding hydrogen) = alkali metals. They are highly reactive soft metals that form +1 cations and react vigorously with water to produce hydrogen gas and a strong base. Group 2 = alkaline earth metals (Be, Mg, Ca, …). Group 17 = halogens. Group 18 = noble gases.

Question 4 of 10

TEKS 1A-3F Medium Calc Word

A student measures the mass of a metal sample as 12.450 g and its volume as 4.50 mL. What is the density of the metal, reported to the correct number of significant figures?

A 2.77 g/mL

B 2.7666 g/mL

C 2 g/mL

D 2.8 g/mL

Explanation

📌 Density = mass ÷ volume = 12.450 g ÷ 4.50 mL = 2.766666… g/mL. Significant figures rule for division: the answer carries the same sig figs as the LEAST precise measurement. 4.50 mL has 3 sig figs (mass has 5), so the answer rounds to 3 sig figs: 2.77 g/mL.

Question 5 of 10

TEKS 4A-4D Easy Word

The reaction 2 H₂(g) + O₂(g) → 2 H₂O(l) is best classified as which type of reaction?

A Synthesis (combination)

B Single replacement

C Double replacement

D Decomposition

Explanation

📌 Synthesis: two or more substances combine to form ONE product (A + B → AB). Here, H₂ and O₂ combine to form H₂O. Decomposition is the opposite (AB → A + B). Replacement reactions involve elements swapping positions in compounds.

Question 6 of 10

TEKS 7A-7E Medium

What is the correct formula for the ionic compound formed between calcium and chlorine?

A CaCl₂

B Ca₂Cl

C CaCl

D Ca₂Cl₃

Explanation

📌 Calcium forms Ca²⁺ (Group 2). Chlorine forms Cl⁻ (Group 17). To balance the +2 charge, you need TWO Cl⁻ ions per one Ca²⁺ → formula CaCl₂. The subscripts cross-cross from the charges: Ca²⁺ + Cl⁻ → Ca₁Cl₂ = CaCl₂. Net charge of the formula unit is 0.

Question 7 of 10

TEKS 7A-7E Easy

Magnesium reacts with oxygen to form magnesium oxide (MgO). Based on the electron transfer in this reaction, MgO is best classified as which type of compound?

A Ionic

B Metallic

C Polar covalent

D Covalent

Explanation

📌 Magnesium (a metal, group 2) transfers its two valence electrons to oxygen (a nonmetal, group 16), producing Mg²⁺ and O²⁻ ions. The opposite charges attract → ionic bond. Rule of thumb: metal + nonmetal with large electronegativity difference → ionic. Two nonmetals → covalent.

Question 8 of 10

TEKS 4A-4D Medium Word

Aluminum reacts with oxygen gas to produce aluminum oxide: Al + O₂ → Al₂O₃. When this equation is balanced using the smallest whole-number coefficients, what is the coefficient of O₂?

A 3

B 4

C 1

D 2

Explanation

📌 Balanced: 4 Al + 3 O₂ → 2 Al₂O₃. Start by balancing aluminum: 2 Al₂O₃ contains 4 Al, so you need 4 Al on the left. Then balance oxygen: 2 Al₂O₃ contains 6 O atoms; since O₂ provides 2 O per molecule, you need 3 O₂. Always balance metals/nonmetals first and leave free elements (like O₂) for last.

Question 9 of 10

TEKS 1A-3F Easy

A student is investigating how the temperature of water affects how fast sugar dissolves. To make this a properly controlled experiment, what should the student do?

A Use the same mass of sugar and the same volume of water in each trial, varying only the water temperature

B Vary both the water temperature and the stirring speed to gather more data

C Use different amounts of sugar at each temperature to see which dissolves fastest

D Run each trial only once to save time and average the result later

Explanation

📌 In a controlled experiment, the independent variable (water temperature) is the ONLY thing that changes between trials. All other conditions — sugar mass, water volume, stirring, container — must stay constant. Otherwise, you can't attribute the observed effect to temperature.

Question 10 of 10

TEKS 6A-6E Easy

Which subatomic particle has a negative charge and the smallest mass?

A Nucleus

B Electron

C Neutron

D Proton

Explanation

📌 Electron: −1 charge, mass ≈ 1/1836 of a proton. Protons are +1 charge with mass ≈ 1 amu. Neutrons have no charge, mass ≈ 1 amu. The nucleus is not a particle — it's the dense central region containing protons and neutrons.

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