What are the answers to the following chemistry exam questions covering isotopes, atomic models, and electron capacities?

Here are the answers to your chemistry exam questions:

I. Write True if the statement is correct Or False if it is Incorrect (5 pts)

1. Isotopes of an element have Similar chemical properties.
   • True
2. Mendeleev arranged the elements based on increasing order of atomic number.
   • False (He arranged them by increasing atomic mass.)
3. Isobars are different atoms that have the same mass number.
   • True
4. Atoms of the same element have the same number of neutrons.
   • False (Isotopes of the same element have different numbers of neutrons.)
5. The mass of an electron is almost equal to the mass of a proton.
   • False (The mass of an electron is much smaller than that of a proton.)

II. Match column 'A' with column 'B' (5 Pts)

6. Dalton's Atomic Model $\rightarrow$ C. Billiard Model
7. J.J. Thomson Atomic Model $\rightarrow$ D. Plum-pudding Model
8. Rutherford's Atomic Model $\rightarrow$ B. Planetary Model
9. Bohr's Atomic Model $\rightarrow$ A. Orbit
10. Democritus Atomic Model $\rightarrow$ E. Solid sphere

III. Choose The Correct Answer from the following alternatives (10 pts)

11. The Energy level in the third principal quantum number can accommodate $\boxed{\text{C. 18}}$ electrons.

    • Explanation: For a principal quantum number $n$, the maximum number of electrons is given by $2n^2$. For $n=3$, the maximum is $2(3^2) = 2 \times 9 = 18$.

12. The mass of an atom is concentrated in $\boxed{\text{B. Nucleus}}$.

    • Explanation: The nucleus contains protons and neutrons, which account for almost all of an atom's mass.

13. Which fundamental particle does not present in the hydrogen atom? $\boxed{\text{C. neutron}}$.

    • Explanation: The most common isotope of hydrogen ($^1_1$H, protium) has one proton and one electron but no neutrons.

14. What is the average atomic mass of an element which Isotopic composition is 90% of $^{20}$X and 10% of $^{22}$X? $\boxed{\text{D. 20.2}}$.

    • Calculation: $$\text{Average atomic mass} = (0.90 \times 20) + (0.10 \times 22) = 18 + 2.2 = 20.2$$

15. Nucleus was discovered by $\boxed{\text{B. Rutherford}}$.

    • Explanation: Ernest Rutherford discovered the atomic nucleus through his gold foil experiment.

16. $1s^2 2s^2 2p^6 3s^2 3p^4$ is an electronic configuration of $\boxed{\text{D. }_{18}\text{Ar}}$.

    • Explanation: The given electron configuration has $2+2+6+2+4 = 16$ electrons, which corresponds to Sulfur (S). However, Sulfur is not an option. Assuming there is a typo in the question and the configuration should have been $1s^2 2s^2 2p^6 3s^2 3p^6$ (18 electrons), then it would correspond to Argon (Ar).

17. How many Protons, electrons and neutrons are found in an atom of $_{35}^{80}$Br respectively? $\boxed{\text{C. 35, 35, 45}}$.

    • Explanation:
      • Atomic number (Z) = 35, so there are 35 protons.
      • For a neutral atom, the number of electrons equals the number of protons, so there are 35 electrons.
      • Mass number (A) = 80.
      • Number of neutrons = A - Z = $80 - 35 = 45$.

18. An element has the valence shell electron configuration of $2s^2 2p^3$ to which group and period does it belong? $\boxed{\text{B. VA and 2}}$.

    • Explanation:
      • The highest principal quantum number is 2, so it belongs to Period 2.
      • The total number of valence electrons is $2+3=5$. Elements with 5 valence electrons are in Group VA (or Group 15).

19. The earliest of the attempts to classify elements was that of $\boxed{\text{D. Dobereiner's triads}}$.

    • Explanation: Dobereiner's triads (1829) were among the first attempts to group elements with similar properties.

20. How many elements