MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 4e Atomic Structure Isotopes

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This deck focuses on 4e Atomic Structure Isotopes, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What quantity does atomic number $Z$ represent for an element?

Answer: Number of protons in the nucleus. The atomic number $Z$ defines the element's identity by specifying the number of protons, which determines its chemical properties.

Flashcard 2: Identify the number of electrons in a neutral atom with atomic number $Z$.

Answer: Electrons $= Z$. In neutral atoms, electron count equals the atomic number $Z$ to balance the positive charge from protons.

Flashcard 3: Calculate average atomic mass for $^{35}\text{Cl}$ at $75\%$ and $^{37}\text{Cl}$ at $25\%$.

Answer: $35.5\ \text{amu}$. Weighted average: $0.75 \times 35 + 0.25 \times 37 = 26.25 + 9.25 = 35.5$ amu.

Flashcard 4: What is the charge (in units of $e$) and approximate location of a neutron in an atom?

Answer: Charge $0$; located in the nucleus. Neutrons have no net charge and contribute to the nuclear mass while stabilizing the nucleus against proton repulsion.

Flashcard 5: What is the charge (in units of $e$) and approximate location of an electron in an atom?

Answer: Charge $-1e$; located outside the nucleus. Electrons possess a negative charge equal to the elementary charge and occupy orbitals in the electron cloud surrounding the nucleus.

Flashcard 6: What quantity does mass number $A$ represent for a nuclide?

Answer: Total nucleons: $A = p + n$. The mass number $A$ equals the sum of protons and neutrons, representing the total number of nucleons in the nucleus.

Flashcard 7: Identify the formula for neutrons $n$ in terms of mass number $A$ and atomic number $Z$.

Answer: $n = A - Z$. Subtracting atomic number $Z$ from mass number $A$ yields the neutron count, as $A$ includes both protons and neutrons.

Flashcard 8: What does the notation $^{A}_{Z}X$ specify about an atom or ion?

Answer: Element $X$, with mass $A$ and atomic number $Z$. Nuclide notation $^{A}_{Z}X$ indicates the element symbol $X$ with its specific mass number $A$ and atomic number $Z$.

Flashcard 9: What is the definition of isotopes of the same element?

Answer: Same $Z$ (protons), different $A$ (neutrons). Isotopes share the same atomic number $Z$ but differ in mass number $A$ due to varying neutron counts.

Flashcard 10: What is the definition of an ion in terms of electrons and protons?

Answer: Species with $e^- \neq p^+$ (net charge not zero). An ion forms when the number of electrons differs from protons, resulting in a nonzero net charge.

Flashcard 11: State the relationship between net ionic charge, protons $p$, and electrons $e$.

Answer: Net charge $= p - e$ (in units of $e$). Net charge arises from the difference between positively charged protons and negatively charged electrons.

Flashcard 12: Find the number of neutrons in $^{37}_{17}\text{Cl}$.

Answer: $20$ neutrons. Neutron number is calculated as mass number minus atomic number: $37 - 17 = 20$.

Flashcard 13: Find the number of protons, neutrons, and electrons in neutral $^{23}_{11}\text{Na}$.

Answer: $p=11$, $n=12$, $e=11$. For neutral sodium, protons equal $Z=11$, neutrons are $23-11=12$, and electrons match protons.

Flashcard 14: Find the number of electrons in $^{56}_{26}\text{Fe}^{3+}$.

Answer: $23$ electrons. The +3 charge indicates loss of 3 electrons from neutral iron's 26, leaving 23 electrons.

Flashcard 15: Find the net charge (in units of $e$) for a species with $p=16$ and $e=18$.

Answer: Net charge $=-2$. Net charge is protons minus electrons: $16 - 18 = -2$, indicating an anion.

Flashcard 16: Which option best describes a cation: loss of electrons or gain of electrons?

Answer: Loss of electrons. Cations are positively charged ions formed by losing electrons, reducing electron count below protons.

Flashcard 17: Which option best describes an anion: loss of electrons or gain of electrons?

Answer: Gain of electrons. Anions are negatively charged ions formed by gaining electrons, increasing electron count above protons.

Flashcard 18: What is the approximate relative mass of a proton compared with an electron?

Answer: Proton is about $1836\times$ the electron mass. Protons are much heavier than electrons, with a mass ratio of approximately 1836:1.

Flashcard 19: What is the approximate relative mass of a neutron compared with a proton?

Answer: Neutron mass is approximately equal to proton mass. Neutrons and protons have nearly identical masses, both contributing significantly to atomic mass.

Flashcard 20: Which subatomic particle primarily determines an element's chemical identity?

Answer: Protons (atomic number $Z$). The number of protons determines the element's position in the periodic table and its chemical behavior.

Flashcard 21: Which subatomic particle primarily changes between isotopes of the same element?

Answer: Neutrons. Isotopes vary in neutron number while maintaining the same proton count, affecting mass but not chemical identity.

Flashcard 22: State the formula for the average atomic mass from isotopic masses and fractional abundances.

Answer: Average mass $= \sum_i (m_i \times f_i)$. Average atomic mass is the weighted sum of each isotope's mass $m_i$ multiplied by its fractional abundance $f_i$.

Flashcard 23: What is the definition of atomic mass unit in terms of $^{12}\text{C}$?

Answer: $1\ \text{amu} = \frac{1}{12}$ the mass of one $^{12}\text{C}$ atom. The amu is defined relative to carbon-12, where 1 amu equals one-twelfth its atomic mass.

Flashcard 24: What is the charge (in units of $e$) and approximate location of a proton in an atom?

Answer: Charge $+1e$; located in the nucleus. Protons carry a positive charge equal to the elementary charge and form part of the dense central nucleus along with neutrons.