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Atoms are the building blocks of all matter. Everything around us, from the air we breathe to the food we eat, is made up of atoms. Understanding the structure of the atom helps us learn how substances combine, react, and form new materials. In this worksheet, we will explore the different parts of an atom, important theories, and some basic calculations related to atomic structure.
What is an Atom?
An atom is the smallest unit of matter that cannot be divided further by chemical means. Each atom consists of three main particles: protons, neutrons, and electrons. These particles determine the properties and behavior of an atom.
- Protons are positively charged particles found in the nucleus (center of the atom).
- Neutrons have no charge and are also found in the nucleus.
- Electrons are negatively charged particles that move around the nucleus in energy levels or shells.
The number of protons in an atom is called its atomic number, and the total number of protons and neutrons together is called the mass number.
Important Atomic Theories
Scientists have studied atoms for many years, and different models have been proposed to explain their structure. Some important theories include:
- Dalton’s Atomic Theory (1808) – John Dalton stated that all matter is made of tiny, indivisible atoms, and each element has identical atoms.
- Thomson’s Model (1897) – J.J. Thomson discovered electrons and proposed the “plum pudding model,” where negatively charged electrons were spread in a positive sphere.
- Rutherford’s Model (1911) – Ernest Rutherford found that the nucleus is dense and positively charged, with electrons moving around it.
- Bohr’s Model (1913) – Niels Bohr suggested that electrons move in fixed orbits (shells) around the nucleus.
- Modern Quantum Model – This model explains that electrons move in specific energy levels but their exact location cannot be determined.
Why is the Study of Atoms Important?
Understanding atoms helps us in many fields of science, such as chemistry, physics, and biology. It explains why substances react, how elements form compounds, and even how electricity works.
This worksheet will help students practice questions on atomic structure, including calculating atomic and mass numbers, identifying subatomic particles, and understanding atomic models. By solving these exercises, students will build a strong foundation in chemistry and develop problem-solving skills.
Worksheet for Class 9 Chemistry Structure of the Atom
Ques 1. Who discovered the neutron?
a) Niels Bohr
b) J. Chadwick
c) J.J. Thomson
d) E. Goldstein
Ques 2. What does Rutherford’s gold foil experiment tell us about atoms?
a) Most of an atom is empty space.
b) Electrons move in fixed circular orbits.
c) The center of an atom is small and dense.
d) Most of an atom’s mass is in the nucleus.
Ques 3. The atomic mass of an element is twice its atomic number. If the L-shell has six electrons:
a) What is its valency?
b) Identify the element.
Ques 4. Which of these statements about a proton is incorrect?
a) It has a positive charge.
b) It has the same mass as a neutron.
c) It is found in the nucleus of an atom.
d) It is part of cathode rays.
Ques 5. An atom has a mass number of 39 and contains 20 neutrons. What is its atomic number?
Ques 6. What are the electronic configurations of the following ions?
i) H–
ii) S²⁻
iii) N⁻¹
iv) N²⁺
Ques 7. What did Niels Bohr propose to improve Rutherford’s atomic model?
Ques 8. Which atomic property was correctly explained by Thomson’s model of the atom?
a) Atom stability
b) Hydrogen atom spectrum
c) Overall neutrality of an atom
d) Placement of electrons, protons, and neutrons in an atom
Ques 9. How many nucleons are in an Americium (Am) atom with atomic number 95 and mass number 243? Also, how many electrons and neutrons does it have?
Ques 10. Fill in the blanks:
a) The weighted average of all natural isotopes’ masses is called an atom’s _________.
b) The number of _______ in the nucleus equals the number of _______ in a neutral atom.
c) The _______ contains most of an atom’s mass.
d) The highest number of protons and neutrons in a hydrogen isotope is ________.
e) Inside an atom, only certain special orbits called _______ are allowed for electrons.
Ques 11. Which pair of elements has the same number of electrons in the outermost shell?
a) Na, Mg
b) Zn, Fe
c) Pn, Sb
d) K, Rb
Ques 12. The arrangement of electrons in different atomic shells was suggested by:
a) John Dalton
b) Bohr and Bury
c) Henry Moseley
d) Rutherford
Ques 13. The correct statement about electron configuration is:
a) It shows which orbitals are occupied rather than how many there are.
b) It represents only the outer electrons of an atom.
c) It shows exactly which orbitals electrons occupy.
d) The shape of orbitals is determined by electron configuration.
Ques 14. Isobars are atoms that:
a) Have the same mass number but different atomic numbers.
b) Have the same number of protons but different electrons.
c) Have the same number of protons and electrons.
d) Have the same atomic number but different mass numbers.
Ques 15. Why did Rutherford use gold foil for his alpha particle experiment?
Ques 16. Boron exists naturally as two isotopes with atomic masses of 10.01 and 11.01. If boron’s average atomic mass is 10.81, what is the percentage of each isotope?
Ques 17. Write the electron arrangement for argon and phosphorus. Also, if K and M shells contain 2 and 8 electrons, how many are in the L shell?
Ques 18. Which subatomic particle determines the identity of an element?
a) Electron
b) Neutron
c) Proton
d) Electron + Proton
Ques 19. The symbols ¹⁷₃₅Cl and ³⁵Cl are correct, while ³⁵₁₇Cl and ³⁵Cl are not. Why?
