Honors Chemistry: Unit 3 - Atomic Structure, Composition, Radioactive Decay, and Half-Life

Comprehensive practice flashcards covering Honors Chemistry Unit 3: the historical development of atomic models, subatomic composition, nuclear decay mechanisms, and quantitative half-life calculations.

What concept did Democritus propose regarding matter, and from what Greek word did he derive the term 'atom'?

Democritus proposed that all matter is composed of small, indivisible particles called atoms, deriving the term from the Greek word 'atomos' meaning 'indivisible'.

What was Aristotle's view of matter, and which four elements did he believe composed all matter?

Aristotle challenged Democritus by asserting that matter was continuous and indivisible, proposing that all matter consisted of Earth, Water, Air, and Fire (an ideology accepted for roughly $2000$ years).

Which two of John Dalton's 5 Postulates of Atomic Theory were later proven to be incorrect?

Postulate 2 (that atoms of a given element are identical in size, mass, and other properties, disproven by isotopes) and Postulate 3 (that atoms cannot be subdivided, created, or destroyed, disproven by subatomic particles and nuclear reactions).

What was the name and premise of John Dalton's 1803 atomic model?

The 'Solid Sphere Model', which theorized that an atom was a solid sphere possessing a neutral charge.

What subatomic particle did J.J. Thomson discover in 1897, and what experimental apparatus did he use?

He discovered the negatively charged electron (originally named 'corpuscles') using the Cathode Ray Experiment.

What two names are commonly given to J.J. Thomson's 1904 atomic model?

The 'Plum Pudding Model' and the 'Chocolate Chip Cookie Model'.

Which experiment is shown in this diagram, and what primary discovery did it establish about atomic structure?

Ernest Rutherford's Gold Foil Experiment (1911), which demonstrated that atoms are mostly empty space with a dense, positively charged center called the nucleus.

What was Ernest Rutherford's atomic model called, and how did it configure electrical charges?

The 'Nuclear Model', which concentrated positive charge in a central nucleus while negatively charged electrons occupied the surrounding empty space.

How did Niels Bohr modify Rutherford's atomic model in 1913?

Bohr proposed that electrons travel around the nucleus in specific circular orbits of fixed sizes and quantized energy levels, establishing the 'Planetary Model' (also called 'Solar System Model' or 'Heliocentric Model').

What did Erwin Schrödinger propose regarding electron behavior in 1926?

Schrödinger theorized that electrons behave as waves rather than localized particles, exhibiting wave-particle duality as illustrated by the Double Slit Experiment.

What subatomic particle was discovered using this Polonium Geiger Counter experimental setup with Beryllium metal and Paraffin wax?

The neutron, discovered by James Chadwick in 1932 through scattering data.

What atomic model did Werner Heisenberg and Erwin Schrödinger develop to describe electron position uncertainty?

The 'Cloud Model' (or 'Quantum Model'), which represents electrons within probability clouds termed orbitals rather than fixed paths.

What are the charge, mass number, location, and symbol of a proton?

A proton has a positive charge, a Mass Number of $1$, is located in the nucleus, is denoted by the symbol $p^+$, and defines the element via the Atomic Number ($Z$).

What are the charge, mass number, location, and symbol of an electron?

An electron has a negative charge, a Mass Number of $0$ (virtually massless), is located in Energy Levels, and is denoted by the symbol $e^-$.

What are the charge, mass number, location, and symbol of a neutron?

A neutron has a neutral charge, a Mass Number of $1$, is located in the nucleus, and is denoted by the symbol $n^0$.

What is the approximate density of an atomic nucleus, and what forces hold it together?

The nucleus has a density of approximately $2.3 \times 10^7\,\text{kg/m}^3$ (over $10\times$ denser than the Sun's core) and is bound together by nuclear forces (proton-neutron, proton-proton, and neutron-neutron interactions).

What is the formula used to calculate the Mass Number of an atom?

$$\text{Mass Number} = \text{\# of Protons} + \text{\# of Neutrons}$$

What is an isotope?

Atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different masses.

Based on this atomic diagram containing 8 protons, 9 neutrons, and 8 electrons, what is the isotope's identity and notation?

Oxygen-17 (or $^{17}_{8}\text{O}$), because $8$ protons define Oxygen ($Z = 8$) and $8\text{ protons} + 9\text{ neutrons} = 17$ for the Mass Number.

How is the number of Energy Levels of an atom determined from the Periodic Table?

By looking at the Period Number of the element.

What are valence electrons and where must they reside?

Valence electrons are electrons located strictly in the outermost Energy Level that govern chemical bonding and reactions.

How are valence electrons generally determined for s-block and p-block elements?

For the s-block (teal region), the Group Number equals the number of valence electrons (except Helium, which has $2$); for the p-block (green region), the one's digit of the Group Number equals the number of valence electrons.

For the isotope Aluminum-27, what are the values for Mass Number, $p^+$, $e^-$, $n^0$, valence electrons ($ve^-$), and Energy Levels ($EL\text{'s}$)?

$\text{Mass \#} = 27$, $p^+ = 13$, $e^- = 13$, $n^0 = 14$, $ve^- = 3$, and $EL\text{'s} = 3$.

What is radioactive decay?

The spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by the emission of particles, electromagnetic radiation, or both.

Under what condition regarding internal forces does radioactive decay occur in a nucleus?

Decay occurs when the electrostatic repulsive force between positive protons overcomes the attractive nuclear force binding the nucleons.

According to the Band of Stability shown in this graph, how does the stable neutron-to-proton ($n/p$) ratio change from low to high atomic numbers?

Elements with low proton numbers are stable at an $n/p$ ratio of approximately $1:1$, whereas elements with high proton numbers require an $n/p$ ratio of approximately $1.5:1$ to maintain nuclear stability.

What is an alpha particle, and what changes occur in the nucleus during alpha decay?

An alpha particle is a Helium-4 nucleus ($^4_2\text{He}$) composed of two protons and two neutrons; alpha decay results in a daughter nucleus with two fewer protons and two fewer neutrons.

What daughter nuclide is produced when Polonium-218 undergoes alpha decay: $^{218}_{84}\text{Po} \rightarrow \,^4_2\text{He} + \,^A_Z\text{X}$?

Lead-214 ($^{214}_{82}\text{Pb}$), since $A = 218 - 4 = 214$ and $Z = 84 - 2 = 82$.

What three subatomic particles are formed when a free neutron breaks down, as illustrated here?

A proton, an electron, and a neutrino.

What occurs during beta decay (beta minus), and what types of isotopes typically undergo it?

A neutron converts into a proton and emits an electron ($^0_{-1}\beta$) to produce an isobar; it typically occurs in isotopes with a high neutron-to-proton ratio.

What is an isobar in nuclear chemistry?

An isotope of one element that shares the same Mass Number as an isotope of a different element.

What nuclear equation represents the beta decay of Aluminum-28?

$$^{28}_{13}\text{Al} \rightarrow \,^{28}_{14}\text{Si} + \,^0_{-1}\beta$$

What is positron emission (beta plus), and what is a positron?

A decay process where a proton converts into a neutron and emits a positron ($^0_{+1}\beta$); a positron is the antimatter counterpart of an electron, having the same size and mass but a positive electrical charge.

What occurs during electron capture in an unstable nucleus?

The nucleus absorbs an inner-orbital electron, combining it with a proton to convert it into a neutron and creating an isobar.

What is gamma decay?

A process where a nucleus in an excited energy state releases high-frequency electromagnetic radiation as gamma rays ($\gamma$) to return to ground state without changing mass or atomic number.

What is the distinction between spontaneous fission and nuclear fusion?

Spontaneous fission is the splitting of a heavy nucleus into two smaller nuclei without a bombarding particle, while nuclear fusion is the combining of two light nuclei to form a heavier nucleus.

What is a radioactive decay series?

A sequential series of radioactive decays that an unstable nucleus undergoes until a stable, non-radioactive isotope is formed.

What is the definition of half-life?

The amount of time required for half of the atoms of a radioactive nuclide to decay.

What is the half-life decay formula, and what does each variable signify?

$A = A_0 \times \left(\frac{1}{2}\right)^{t/h}$, where $A$ is the remaining amount, $A_0$ is the initial amount, $t$ is the elapsed time, $h$ is the half-life, and $t/h$ is the total number of half-lives elapsed.

If a $1.20\,\text{g}$ sample of Argon-35 ($h = 1.83\,\text{s}$) decays for $5.49\,\text{s}$, what is the mass of the remaining isotope?

$0.15\,\text{g}$, calculated by recognizing $5.49 / 1.83 = 3$ half-lives: $A = 1.20 \times \left(\frac{1}{2}\right)^3 = 1.20 \times \frac{1}{8} = 0.15\,\text{g}$.

What is carbon dating, and up to what approximate age is it reliable?

A process that determines the age of organic material (substances containing Carbon-Hydrogen bonds) up to about $50{,}000\,\text{years}$ by measuring the ratio of Carbon-12 to Carbon-14.

What is the half-life of Carbon-14, and what percentage of Carbon-14 remains in an organism after two half-lives ($11{,}460\,\text{years}$)?

Carbon-14 has a half-life of $5{,}730\,\text{years}$; after two half-lives ($11{,}460\,\text{years}$), $25\%$ of the initial Carbon-14 remains.

A homosapien specimen contains $9\%$ ($0.09$) of the expected living Carbon-14 level ($h = 5730\,\text{years}$). Using the Power Rule of logarithms, what is its approximate age?

Approximately $19{,}900\,\text{years}$ old (calculated as $t = \frac{5730 \log(0.09)}{\log(1/2)} \approx 19{,}905\,\text{years}$).