Atomic Structure

Concept:
Atomic Number (Z) = Number of protons (P).
Mass Number (A) = Number of protons (P) + Number of neutrons (N).
For a neutral atom, Number of electrons (E) = Number of protons (P).
For an ion, the number of electrons changes:
Positive ion (cation, like Y ): Formed by losing electrons. E = P - (charge).
Negative ion (anion): Formed by gaining electrons. E = P + |charge|. Step 1: Identify given information for element Y
Atomic number of Y (Z) = 19.
Mass number of Y (A) = 39.
We are interested in the ion Y . Step 2: Determine the number of protons (P) The atomic number is equal to the number of protons. P = Z = 19. (The element with Z=19 is Potassium, K). Step 3: Determine the number of neutrons (N) Mass Number (A) = Protons (P) + Neutrons (N). So, Neutrons (N) = Mass Number (A) - Protons (P). N = . Number of neutrons = 20. Step 4: Determine the number of electrons (E) in the ion Y First, consider a neutral atom of Y. In a neutral atom, Number of electrons = Number of protons = 19. The ion is Y , which means it has a charge of +1. A positive charge indicates that the atom has lost one electron. Number of electrons in Y = (Number of electrons in neutral Y) - (Number of electrons lost) Number of electrons in Y = . Step 5: Summarize the results for Y
Electrons (E) = 18
Protons (P) = 19
Neutrons (N) = 20 This matches option (2). (Option (1) E=19, P=19, N=20 would be correct for a neutral atom Y, not the ion Y ).

Concept: This question compares the masses of subatomic particles. Protons, neutrons, and electrons are the fundamental constituents of atoms. Mesons are a type of subatomic particle composed of a quark and an antiquark. Step 1: Approximate masses of common subatomic particles
Electron ( ): Mass . This is often considered as approximately of the mass of a proton.
Proton ( ): Mass . This is approximately 1 atomic mass unit (amu).
Neutron ( ): Mass . The neutron is slightly more massive than the proton, but their masses are very similar (also approx. 1 amu).
Meson: Mesons are a broad category of subatomic particles. Their masses vary depending on the type of meson, but they are generally intermediate between the mass of an electron and the mass of a proton/neutron. For example:
Pion ( -meson): Mass (approx. 264 times the electron mass, or 1/7 of proton mass).
Kaon (K-meson): Heavier than pions. Even the lightest mesons (pions) are significantly more massive than electrons. Step 2: Comparing the masses
Mass of Electron:
Mass of Proton:
Mass of Neutron:
Mass of typical Meson (e.g., pion): Comparing the powers of 10, is much smaller than or . The order of masses is approximately: Electron Neutron. The handwritten note "e n with n slightly heavier) supports that electron is the lightest of these three. Step 3: Identifying the lightest particle The electron has by far the smallest mass among the options listed (and among the fundamental constituents of an atom).

Concept: The period number in the periodic table corresponds to the highest principal energy level (electron shell) occupied by electrons in an atom of that element. The atomic number (Z) equals the number of electrons in a neutral atom. Step 1: Determine the number of electrons The atomic number of the element is 20. For a neutral atom, the number of electrons = atomic number = 20. Step 2: Write the electronic configuration (distribution of electrons in shells) Electrons fill shells in the order K, L, M, N... Maximum electrons per shell:
K-shell (n=1): Can hold up to 2 electrons.
L-shell (n=2): Can hold up to 8 electrons.
M-shell (n=3): Can hold up to 18 electrons (but often fills to 8 before the N-shell starts, as per Bohr-Bury rules for simpler configurations).
N-shell (n=4): Can hold up to 32 electrons. Let's fill the 20 electrons:
K-shell: 2 electrons (Remaining electrons: )
L-shell: 8 electrons (Remaining electrons: )
M-shell: According to the Bohr-Bury scheme, the M-shell will take up to 8 electrons before the N-shell begins to fill if the M-shell is the outermost or penultimate shell being filled for these lighter elements. So, 8 electrons go into the M-shell. (Remaining electrons: )
N-shell: The remaining 2 electrons will go into the N-shell. So, the electronic configuration is 2 (K), 8 (L), 8 (M), 2 (N). The handwritten note "2+8+8+2" on the image confirms this configuration. Step 3: Identify the period number The period number is determined by the highest principal energy level (shell) that contains electrons. In this configuration (2, 8, 8, 2):
K-shell is the 1st shell.
L-shell is the 2nd shell.
M-shell is the 3rd shell.
N-shell is the 4th shell. Since electrons are present in the N-shell (the 4th shell), the element belongs to the 4th period. (The element with atomic number 20 is Calcium, Ca).

Concept: To find the total number of atoms, we first need to calculate the number of moles of each substance, then the number of molecules, and finally the total number of atoms by considering the number of atoms per molecule. Number of moles ( ) = Mass ( ) / Molar mass ( ). Number of molecules = Number of moles Avogadro's number ( ). Total number of atoms = Number of molecules Number of atoms per molecule. Since the mass (18g) and Avogadro's number are constant for all options, the number of atoms will be proportional to (Number of atoms per molecule) / (Molar mass). A simpler approach for comparison is to calculate moles, then total moles of atoms. Step 1: Calculate Molar Masses (M)
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: (Atomic masses: H=1, C=12, O=16 g/mol approximately) Step 2: Calculate number of moles ( ) for 18g of each substance
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:
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: Step 3: Calculate total number of moles of atoms (Total moles of atoms = moles of substance number of atoms in one molecule)
: 1 molecule has atoms. Total moles of atoms = .
: 1 molecule has atoms. Total moles of atoms = .
: 1 molecule has atoms. Total moles of atoms = .
: 1 molecule has atoms. Total moles of atoms = . Step 4: Compare the total number of moles of atoms Comparing the values:
: 3 moles of atoms
: 1.125 moles of atoms
: moles of atoms
: 5.625 moles of atoms has the maximum number of moles of atoms, and thus the maximum number of atoms.

Concept: The standard notation for representing an element (or isotope) is , where:
is the chemical symbol of the element.
is the Atomic Number (number of protons).
is the Mass Number (number of protons + number of neutrons). Step 1: Given information
Number of electrons in element X = 6.
Number of neutrons in element X = 8. Step 2: Determine the Atomic Number ( ) For a neutral atom (which an "element" generally implies unless specified as an ion), the number of electrons is equal to the number of protons. Number of protons = Number of electrons = 6. The Atomic Number ( ) is defined as the number of protons. So, . (An element with atomic number 6 is Carbon, C, but the symbol X is used here). Step 3: Determine the Mass Number ( ) The Mass Number ( ) is the sum of the number of protons and the number of neutrons. Number of protons = 6 Number of neutrons = 8 So, . Step 4: Write the correct representation Using the notation : Substitute and . The representation is . Step 5: Compare with the options
(1) : Mass number is incorrect.
(2) : This matches our derived representation.
(3) : Atomic number and mass number positions/values are incorrect.
(4) : Atomic number is incorrect. Therefore, the correct representation is .

Concept:
Mass Number ( ) = Number of protons ( ) + Number of neutrons ( ).
Atomic Number ( ) = Number of protons ( ). For a neutral atom, also equals the number of electrons.
For an ion, the number of electrons changes. A positive charge means electrons are lost. A negative charge means electrons are gained. Step 1: Given information
Charge of the ion = +3 (3 positive charges).
Mass number of the atom ( ) = 27.
Number of neutrons ( ) = 14. We need to find the number of electrons in the ion. Step 2: Find the number of protons ( ) We know . So, . . The number of protons in the atom (and also in the ion, as protons don't change when an ion forms) is 13. This means the atomic number ( ) of the element is 13. (This element is Aluminium, Al). Step 3: Determine the number of electrons in the neutral atom For a neutral atom, the number of electrons is equal to the number of protons. Number of electrons in a neutral atom = 13. Step 4: Determine the number of electrons in the ion The ion has 3 positive charges (+3). A positive charge indicates that the atom has lost electrons. A +3 charge means the atom has lost 3 electrons. Number of electrons in the ion = Number of electrons in neutral atom - Number of electrons lost Number of electrons in the ion = . Therefore, the number of electrons in the ion is 10. This matches option (2).

Concept: Isotopes are variants of a particular chemical element. Understanding their definition is key. Step 1: Defining Isotopes Isotopes of an element are atoms that have:
The same number of protons (which means they have the same atomic number, ). Since they have the same number of protons, they are atoms of the same element.
A different number of neutrons.
Because they have a different number of neutrons (but the same number of protons), they will have a different mass number ( ). Step 2: Analyzing the properties based on the definition
Chemical Properties: Chemical properties are primarily determined by the number of electrons, specifically the valence electrons. Since isotopes of an element have the same number of protons, their neutral atoms have the same number of electrons and the same electron configuration. Therefore, isotopes of an element generally have very similar (nearly identical) chemical properties.
Physical Properties: Some physical properties, particularly those dependent on mass (like density, diffusion rate, melting/boiling points to a very slight extent), can differ slightly between isotopes due to the difference in mass number. So, they do not have exactly the same physical properties, though the differences might be small.
Atomic Number: By definition, isotopes of the same element have the same atomic number (same number of protons).
Number of Neutrons: By definition, isotopes of the same element have a different number of neutrons. Step 3: Evaluating the options
(1) The same physical properties: Incorrect. Physical properties dependent on mass can differ slightly.
(2) different chemical properties: Incorrect. They have nearly identical chemical properties because they have the same electron configuration.
(3) different number of neutrons: Correct. This is the defining characteristic of isotopes of an element.
(4) different atomic number: Incorrect. They have the same atomic number (same number of protons). Therefore, isotopes of an element have a different number of neutrons.

Step 1: Understand Rutherford's Alpha-Particle Scattering Experiment.
In 1909, Ernest Rutherford conducted an experiment where alpha particles were directed at a thin gold foil. The results of this experiment led to the discovery of the atomic nucleus. Key observations from the experiment include: 1. Most alpha particles passed straight through the foil without deflection. 2. A small fraction of alpha particles were deflected at large angles, indicating that they encountered a concentrated region of positive charge within the atom. These observations contradicted the prevailing "plum pudding" model of the atom proposed by J.J. Thomson, which suggested that the positive charge was uniformly distributed throughout the atom. Instead, Rutherford concluded that the positive charge (and most of the mass) must be concentrated in a small, dense region at the center of the atom, now known as the atomic nucleus. Step 2: Analyze Each Option.
Option (1): atomic nucleus — Correct, as Rutherford's experiment directly led to the discovery of the atomic nucleus.
Option (2): electron — Incorrect, as electrons were discovered earlier by J.J. Thomson using cathode ray experiments.
Option (3): number of proton — Incorrect, as the number of protons was not directly determined by this experiment.
Option (4): number of neutron — Incorrect, as neutrons were discovered later by James Chadwick in 1932.
Step 3: Final Answer. $ $

Step 1: Understand the definition of isotopes.
Isotopes are atoms of the same element that have the same number of protons (and thus the same atomic number) but different numbers of neutrons. This difference in neutrons results in different mass numbers. For an element, the atomic number (Z) is fixed. Step 2: Recall the atomic number of Chlorine (Cl).
Chlorine (Cl) has an atomic number of 17. This means all isotopes of chlorine must have 17 protons. Step 3: Analyze the common isotopes of Chlorine.
The most common isotopes of chlorine are Chlorine-35 ( ) and Chlorine-37 ( ).
has 17 protons and neutrons.
has 17 protons and neutrons. Both have the same atomic number (17) but different mass numbers (35 and 37), which fits the definition of isotopes. Step 4: Evaluate the given options.

(1) Chlorine. and :} While is an isotope, and are the most prevalent and typically referred to common stable isotopes. However, fundamentally, and would also be isotopes of chlorine as long as they both have 17 protons. But when asked for "correct isotopes," it usually refers to the most recognized or stable ones.
(2) Chlorine. and :} is a radioactive isotope of chlorine. This pair shows two isotopes.
(3) Chlorine. and :} This pair represents the two most abundant and stable natural isotopes of chlorine. They both have the same atomic number (17) but different mass numbers (35 and 37). This is a correct pair of isotopes.
(4) Chlorine. and :} This represents the same isotope, not a pair of different isotopes. Step 5: Select the best fit.
Among the given options, the pair and is the most commonly recognized and correct example of naturally occurring isotopes of Chlorine. (3) Chlorine. and }