11. The smallest unit of an element that retains its chemical properties is called:

a) Molecule
b) Atom
c) Ion
d) Compound
Answer: b) Atom

12. The number of protons in the nucleus of an atom is known as:

a) Mass number
b) Atomic number
c) Neutron number
d) Electron number
Answer: b) Atomic number

13. Which subatomic particle has a negative charge?

a) Proton
b) Neutron
c) Electron
d) Positron
Answer: c) Electron

14. The sum of protons and neutrons in an atom is called its:

a) Atomic number
b) Mass number
c) Isotopic number
d) Neutron number
Answer: b) Mass number

15. The concept of atomic orbitals was introduced by:

a) Niels Bohr
b) Werner Heisenberg
c) Erwin Schrödinger
d) John Dalton
Answer: c) Erwin Schrödinger

16. An isotope of an element has the same number of protons but a different number of:

a) Electrons
b) Neutrons
c) Protons
d) Atoms
Answer: b) Neutrons

17. The electronic configuration of an atom describes:

a) The number of protons
b) The arrangement of electrons in orbitals
c) The number of neutrons
d) The mass number
Answer: b) The arrangement of electrons in orbitals

18. In which type of orbitals are electrons located in the second shell of an atom?

a) s and p orbitals
b) p and d orbitals
c) s and d orbitals
d) s and f orbitals
Answer: a) s and p orbitals

19. According to the Pauli exclusion principle, each orbital can hold a maximum of:

a) One electron
b) Two electrons with opposite spins
c) Three electrons
d) Four electrons
Answer: b) Two electrons with opposite spins

20. The principle stating that electrons fill orbitals from the lowest energy to the highest is known as:

a) Hund’s rule
b) The Aufbau principle
c) The Pauli exclusion principle
d) The Heisenberg uncertainty principle
Answer: b) The Aufbau principle