Online Test — Atoms and Molecules
25 Questions • 15 min • Chapter MCQ
15:00
Question 1 of 25
The smallest particle of an element that can take part in a chemical reaction is the:
Molecule
Atom
Mixture
Compound
Explanation: The atom is the smallest particle of an element.
Question 2 of 25
The first scientific atomic theory was given by:
Newton
Dalton
Ohm
Faraday
Explanation: Dalton gave the first complete scientific atomic theory.
Question 3 of 25
According to Dalton, atoms combine in:
Random amounts
Simple whole-number ratios
Decimals
Halves only
Explanation: Dalton said atoms combine in simple whole-number ratios.
Question 4 of 25
A limitation of Dalton's theory is that atoms are actually:
Indivisible
Made of smaller particles
Created easily
All identical
Explanation: Atoms are made of protons, neutrons, and electrons, so they are divisible.
Question 5 of 25
Atoms of the same element with different masses are called:
Molecules
Isotopes
Ions
Mixtures
Explanation: Isotopes are atoms of the same element with different masses.
Question 6 of 25
Which subatomic particle carries a positive charge?
Electron
Proton
Neutron
Photon
Explanation: The proton carries a positive charge.
Question 7 of 25
The particle with no charge, found in the nucleus, is the:
Proton
Electron
Neutron
Ion
Explanation: The neutron has no charge and is found in the nucleus.
Question 8 of 25
The atomic number (Z) of an atom is the number of:
Neutrons
Protons
Electrons + neutrons
Shells
Explanation: The atomic number is the number of protons in the nucleus.
Question 9 of 25
The mass number (A) is equal to:
Protons only
Protons + neutrons
Electrons only
Neutrons − protons
Explanation: The mass number is the total number of protons and neutrons (A = Z + N).
Question 10 of 25
For $^{23}_{11}Na$, the number of neutrons is:
11
12
23
34
Explanation: Neutrons = A − Z = 23 − 11 = 12.
Question 11 of 25
Isotopes are atoms of the same element with the same atomic number but different:
Number of electrons
Mass numbers
Charges
Chemical properties
Explanation: Isotopes have the same atomic number but different mass numbers.
Question 12 of 25
The three isotopes of hydrogen differ in their number of:
Protons
Electrons
Neutrons
Shells
Explanation: Hydrogen isotopes have the same protons but different neutrons.
Question 13 of 25
Isotopes of an element have the same ______ properties.
Physical
Chemical
Magnetic
Electrical
Explanation: Isotopes have the same chemical properties (same electrons).
Question 14 of 25
Carbon-14 is an isotope used for:
Cooking
Carbon dating
Lighting
Welding
Explanation: Carbon-14 is used in carbon dating of ancient objects.
Question 15 of 25
Atoms of different elements with the same mass number are called:
Isotopes
Isobars
Isotones
Ions
Explanation: Isobars have the same mass number but different atomic numbers.
Question 16 of 25
The shells in which electrons are arranged are named, from the innermost:
A, B, C
K, L, M
X, Y, Z
1, 2, 3 only
Explanation: The shells are named K, L, M, N from the innermost outward.
Question 17 of 25
The maximum number of electrons in the K shell is:
2
8
18
32
Explanation: The K shell can hold a maximum of 2 electrons.
Question 18 of 25
The electrons in the outermost shell are called:
Core electrons
Valence electrons
Neutrons
Protons
Explanation: The outermost-shell electrons are the valence electrons.
Question 19 of 25
The electronic configuration of sodium (Z = 11) is:
2, 8, 1
2, 8, 8
8, 2, 1
2, 9
Explanation: Sodium's 11 electrons fill as K = 2, L = 8, M = 1 → 2, 8, 1.
Question 20 of 25
The combining capacity of an atom is called its:
Valency
Mass number
Atomic number
Density
Explanation: Valency is the combining capacity of an atom.
Question 21 of 25
The smallest particle of a substance that can exist independently is a:
Proton
Atom
Molecule
Neutron
Explanation: A molecule is the smallest particle of a substance that can exist on its own.
Question 22 of 25
The molecular formula H₂O shows that water has 2 hydrogen atoms and:
2 oxygen atoms
1 oxygen atom
3 oxygen atoms
no oxygen
Explanation: H₂O means 2 hydrogen atoms and 1 oxygen atom.
Question 23 of 25
The molecular mass of a substance is the sum of the ______ of its atoms.
Charges
Atomic masses
Volumes
Valencies
Explanation: Molecular mass is the sum of the atomic masses of all atoms in the molecule.
Question 24 of 25
Avogadro's number is:
6.022 × 10²³
3.14
9.8
100
Explanation: Avogadro's number is 6.022 × 10²³, the number of particles in one mole.
Question 25 of 25
One mole of a substance has a mass equal to its molecular mass in:
Kilograms
Grams
Litres
Newtons
Explanation: One mole has a mass equal to its molecular mass expressed in grams.