These are three questions and three complete answers.
Question 1. A) Pair: ⁴¹ ₁₉ K and ⁴² ₂₀ Ca
Protons are NOT equal (19 and 20)Neutrons are equal (22 each)Electrons are NOT equal (19 and 20)
Explanation:
The given notation is used to identify specific isotopes or atoms.
The letter is the chemical symbol. K is potassium, and Ca is calcium.
The superscript to the left of the symbol is the mass number, and the subscript to the left of the symbol is the atomic number.
Hence:
⁴¹ ₁₉ K means potassium atom with mass number 41 and atomic number 19.⁴² ₂₀ Ca means calcium atoms with mass number 42 and atomic number 20.
Atomic number is the number of protons in the nucleus of the atom. Hence:
⁴¹ ₁₉ K has 19 protons and and ⁴² ₂₀ Ca has 20 protons, and you conclude thaty this pair does not have the same number of protons.
Also, for neutral atoms (which the given pairs are) the number of electrons is equal to the number of protons. Hence:
⁴¹ ₁₉ K has 19 electrons and and ⁴² ₂₀ Ca has 20 electrons, and you conclude thaty this pair does not have the same number of electrons.
Mass number is the sum of the number of protons and the number of neutrons. Hence:
⁴¹ ₁₉ K has 41 - 19 = 22 neutrons and and ⁴² ₂₀ Ca has 42 - 20 = 22 nuetrons, and you conclude that this pair has the same number of neutrons.
Conclusion:
Protons are NOT equal (19 and 20)Neutrons are equal (22 each)Electrons are NOT equal (19 and 20)
Queston 2. B) Pair ²⁸ ₁₄ Si and ²⁹₁₄ Si
Protons are equal (14 each)Neutrons are NOT equal (14 and 15) Electrons are equal (14 each)
Explanation:
Pair ²⁸ ₁₄ Si and ²⁹₁₄ Si are different atoms of the same element (Si). So, they have the same atomic number (14 protons) but different mass number (28 and 29), due to different number of neutrons in their nuclei.
Remember, the subscript to the left of the symbol tells the atomic number, which is a constant for every atom of the same element. This pair is of Si, so both atoms have the same number of protons, which is 14 (the atomic number).
Again, since they are neutral atoms (atoms with zero net charge), the number of electrons and the number protons are equal. So, both atoms have the same number of electrons.
As per the number of neutrons, since the mass number is the sum of the protons and neutrons, given that the mass number of one atom is 28 and the mass number of the other is 29 (being the number of protons equal) you conclude that they have different number of neutrons.
²⁸ ₁₄ Si has 28 - 14 = 14 neutrons and ²⁹ ₁₄ Si has 29 - 14 = 15 neutrons.
Conclusion:
Protons are equal (14 each)Neutrons are NOT equal (14 and 15) Electrons are equal (14 each)
Question 3. C) Pair ⁵⁸ ₂₅ Mn and ⁵⁸ ₂₆ Fe
Protons are NOT equal (25 and 26)Neutrons are NOT equal (33 and 32)Electrons are NOT equal (25 and 26)
Explanation:
⁵⁸ ₂₅ Mn:
i) Left subscript, 25, means atomic number or number of protons = 25.
ii) Left superscript means mass number = protons + neutrons = 58
iii) ∴ neutrons = 58 - 25 = 33
iv) neutral atoms ⇒ number of electrons = number of protons = 25
⁵⁸ ₂₆ Fe:
i) Left subscript, 26, means atomic number or number of protons = 26.
ii) Left superscript means mass number = protons + neutrons = 58
iii) ∴ neutrons = 58 - 26 = 32
iv) neutral atoms ⇒ number of electrons = number of protons = 26
Hence, for this pair the conclusion is:
Protons are NOT equal (25 and 26)Neutrons are NOT equal (33 and 32)Electrons are NOT equal (25 and 26)
