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Quizzes > Science

Atoms and Molecules Quiz: Elements and Compounds Practice

Quick, free molecule quiz to check your chemistry basics. Instant results.

Editorial: Review CompletedCreated By: Gabriel MoppsUpdated Aug 27, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration showing atoms and molecules, for a chemistry quiz on teal background with element symbols and bonds

This atoms and molecules quiz helps you check your understanding of elements, compounds, bonding, and atomic structure. If you want a boost first, try the structure of atom quiz, then explore electron configuration quiz, or review how substances are categorized in the elements and compounds quiz. Come back anytime and track your progress as you answer.

What is the smallest unit of an element that retains its chemical properties?
Molecule
Atom
Electron
Proton
An atom is the smallest unit of a chemical element that retains its unique properties because it consists of a nucleus and electrons arranged in shells. Molecules are formed by two or more atoms bonded together, while protons and electrons are subatomic particles. The atom as a whole defines the identity of each element.
Which particle carries a negative electrical charge?
Electron
Proton
Nucleus
Neutron
Electrons carry a negative charge and orbit the atomic nucleus in various shells. Protons are positively charged, and neutrons are neutral. The nucleus is the central core of the atom containing protons and neutrons.
What is the atomic number of carbon?
14
8
6
12
The atomic number equals the number of protons in an atom's nucleus. Carbon has six protons, giving it the atomic number 6. The other numbers refer to mass numbers or other elements.
Which of the following elements is a noble gas?
Sodium
Chlorine
Neon
Oxygen
Neon is in Group 18 of the periodic table, known as the noble gases, characterized by full valence electron shells. Chlorine and oxygen are halogens and chalcogens respectively, and sodium is an alkali metal. Noble gases are inert under standard conditions.
What is the chemical formula for water?
H2O
HO2
H2
OH
Water consists of two hydrogen atoms covalently bonded to one oxygen atom, giving the formula H2O. HO2 is the hydroperoxyl radical, OH is a hydroxyl radical, and H2 is molecular hydrogen. The molecular structure defines the chemical formula.
How many protons are present in a neutral helium atom?
1
3
2
4
Helium has an atomic number of 2, which equals the number of protons in its nucleus. A neutral helium atom also has two electrons. Four would be the mass number if two neutrons are included as well.
A molecule consisting of two atoms of the same element is called what?
Ionic
Monatomic
Polyatomic
Diatomic
Diatomic molecules consist of two atoms of the same element bonded together. Examples include O2 and N2. Monatomic refers to single atoms, polyatomic to molecules with more than two atoms, and ionic describes bonding type.
Which of the following is primarily a covalent compound?
CO2
NaCl
MgO
Fe
CO2 features covalent bonds where electrons are shared between carbon and oxygen. NaCl and MgO are ionic compounds, and Fe is a metallic element. Covalent compounds form between nonmetal atoms sharing electrons.
What is the mass number of an isotope with 17 protons and 18 neutrons?
34
18
17
35
The mass number equals the sum of protons and neutrons in an atom's nucleus. With 17 protons and 18 neutrons, the mass number is 35. The other values reflect only one type of subatomic particle or a miscalculation.
In the periodic table, elements within the same group have what in common?
Ionization energy
Number of valence electrons
Atomic mass
Number of neutrons
Elements in the same group share the same number of valence electrons, which gives them similar chemical reactivity. Atomic mass and ionization energies vary down a group due to increasing atomic size. Neutrons do not determine group placement.
Which type of chemical bond is formed by the sharing of electron pairs between atoms?
Covalent bond
Hydrogen bond
Ionic bond
Metallic bond
Covalent bonds arise when atoms share pairs of electrons to achieve full valence shells. Ionic bonds result from electrostatic attraction between charged ions, metallic bonds involve pooling electrons in metals, and hydrogen bonds are weak attractions between polar molecules.
Which statement best describes a chemical compound?
A mixture of elements without bonding
A single atom in isolation
A homogenous physical blend of atoms
A substance made of two or more elements chemically combined
A compound consists of two or more different elements that are chemically bonded in fixed proportions. A mixture lacks chemical bonding and can be separated by physical means. Homogeneous blends can be mixtures, not compounds, and single atoms are elements, not compounds.
What is the ionic charge of a sodium ion in most compounds?
0
-1
+2
+1
Sodium loses one electron to achieve a full valence shell, resulting in a +1 charge. A -1 charge would require gaining an electron, and +2 or neutral charges do not reflect sodium's usual chemistry.
Which element has an atomic mass closest to 40 amu?
Argon
Potassium
Chlorine
Calcium
Argon has an atomic mass of approximately 39.95 amu, which is closest to 40 amu. Calcium is about 40.08, but argon is slightly closer. Potassium and chlorine deviate more from 40.
What term describes atoms of the same element that have different numbers of neutrons?
Ions
Isomers
Allotropes
Isotopes
Isotopes are variants of a given element that have the same number of protons but different numbers of neutrons. Ions differ in electron count, allotropes are different structural forms, and isomers refer to molecules with the same formula but different structures.
How many electrons can occupy the second energy level (shell) of an atom?
2
32
18
8
The second shell comprises one 2s orbital (2 electrons) and three 2p orbitals (6 electrons), for a total of 8. The first shell holds 2, and higher shells can hold more.
What is the full electron configuration of an oxygen atom in its ground state?
1s2 2s2 3s2
1s2 2s2 2p4
1s2 2p6
1s2 2s2 2p6
Oxygen has 8 electrons: two fill the 1s orbital, two fill the 2s orbital, and four occupy the 2p orbitals. The configuration 1s2 2s2 2p6 describes neon, not oxygen.
Which molecule exhibits a trigonal planar geometry?
CH4
H2O
NH3
BF3
BF3 has three bonding pairs and no lone pairs on boron, giving it a trigonal planar shape. NH3 is trigonal pyramidal, CH4 is tetrahedral, and H2O is bent.
What is the molar mass of carbon dioxide (CO2)?
28.02 g/mol
18.02 g/mol
32.00 g/mol
44.01 g/mol
CO2 has one carbon (12.01 g/mol) and two oxygens (2 × 16.00 g/mol), totaling 44.01 g/mol. 28.02 g/mol is N2, 18.02 g/mol is H2O, and 32.00 g/mol is O2.
In a water molecule, what is the approximate bond angle between the hydrogen atoms?
120°
109.5°
90°
104.5°
The two lone pairs on oxygen push the hydrogen atoms closer, reducing the H - O - H angle to about 104.5°. A perfect tetrahedral angle is 109.5°, but water's bent shape is narrower.
Which element has the highest electronegativity on the Pauling scale?
Chlorine
Nitrogen
Fluorine
Oxygen
Fluorine has the highest electronegativity (3.98) of all elements, making it the strongest electron attractor. Oxygen and chlorine follow, but both are lower. Electronegativity trends increase across a period and decrease down a group.
What is the principal quantum number of the outermost electron in a sodium atom?
1
3
2
4
Sodium (atomic number 11) has its outermost electron in the 3s orbital, so the principal quantum number is 3. Lower numbers refer to inner shells, and 4 would be next higher.
How many sigma bonds are present in an ethene (C2H4) molecule?
6
7
4
5
Ethene has a C=C double bond comprising one sigma and one pi bond, plus four C - H sigma bonds, totaling five sigma bonds. The remaining bond in the double bond is a pi bond.
What is the term for the minimum energy required to remove an electron from a gaseous atom or ion?
Activation energy
Electron affinity
Ionization energy
Electronegativity
Ionization energy is defined as the energy needed to remove an electron from a gaseous atom or ion. Electron affinity is the energy change when adding an electron, electronegativity measures attraction in a bond, and activation energy applies to reactions.
According to molecular orbital theory, what is the bond order of the oxygen molecule (O2)?
2
1
1.5
3
Molecular orbital theory predicts O2 has 10 bonding electrons and 6 antibonding electrons, giving a bond order of (10 - 6)/2 = 2. A bond order of 1.5 applies to O2+..., not neutral O2.
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Study Outcomes

  1. Understand Atomic Structure -

    Use your knowledge of protons, neutrons, and electrons to identify each subatomic particle and its role within an atom.

  2. Differentiate Elements from Compounds -

    Recognize the key characteristics that distinguish pure elements from compound substances based on their atomic and molecular composition.

  3. Analyze Chemical Formulas -

    Interpret chemical notation to determine the types and numbers of atoms in given molecules and compounds.

  4. Apply Periodic Table Knowledge -

    Use periodic table trends to predict element properties such as atomic number, mass, and valence electrons.

  5. Evaluate Molecule Formation -

    Examine how atoms bond and interact to form stable molecules through ionic and covalent bonding principles.

  6. Reinforce Chemistry Concepts -

    Strengthen your grasp of fundamental chemistry topics by tackling targeted quiz questions on atoms, elements, and compounds.

Cheat Sheet

  1. Atomic Structure Essentials -

    Review how atoms consist of a dense nucleus of protons and neutrons surrounded by an electron cloud; the atomic number (protons) defines the element, while mass number sums protons and neutrons (NIST). For example, carbon (6 protons, 6 neutrons) has a mass number of 12. Try the mnemonic "PEN: Protons, Electrons, Number" to recall the core components.

  2. Periodic Table Patterns -

    Examine periodic trends like atomic radius, ionization energy, and electronegativity, which increase or decrease predictably across periods and groups (IUPAC). Recognizing that fluorine has the highest electronegativity helps in chemical reasoning. A handy phrase "Fr is funky" reminds you that francium has the largest atomic radius.

  3. Bonding Basics: Ionic vs Covalent -

    Ionic bonds form when electrons transfer from one atom to another (e.g., NaCl), while covalent bonds share electrons between atoms (e.g., H2O) to satisfy the octet rule (American Chemical Society). Understanding bond types is crucial for an atoms and molecules quiz focused on compounds. Visualize ionic lattices versus shared-pair models to solidify the difference.

  4. Molecular Shapes and VSEPR Theory -

    Use Valence Shell Electron Pair Repulsion (VSEPR) theory to predict molecule geometries by minimizing electron pair repulsions around the central atom (UC Berkeley). For instance, methane (CH4) is tetrahedral with 109.5° bond angles, while carbon dioxide (CO2) is linear at 180°. Sketching Lewis structures first can help you master shapes for your next chemistry quiz.

  5. Stoichiometry and Balancing Equations -

    Master the mole concept and balance chemical equations to calculate reactant and product masses using Avogadro's number (6.022×1023) (Purdue University). For example, balance 2H2 + O2 → 2H2O to determine moles of water produced. Practice converting grams to moles and back to grams to boost your quiz confidence.

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