Gases that won't react

The question requires us to name a term that results from a calculation: dividing a percentage by 100. Performing this operation changes the percentage into a decimal or fraction.

Consider the initial options to see if they match the calculation's outcome:

• Atomic symbol representation is a notation that displays an atom's details such as its mass and atomic numbers, like ${}_6^{12}\text{C}$. This doesn't produce a decimal.
• Nuclear mass count refers to the total protons and neutrons in an atom's nucleus, always a whole number, not a decimal value.

Average isotopic weight is the averaged mass of an element's isotopes, factoring in their natural abundances. Although abundance data are part of its calculation, the resulting average weight is not a simple decimal derived from dividing a percentage by 100.

The fractional expression of an isotope's abundance achieved by dividing its percentage by 100 is known as the proportional isotope fraction. This represents the ratio of a specific isotope among all isotopes of that element.

$\text{Proportional Isotope Fraction}=\frac{\text{Percentage Occurrence}}{100}$

The task is to identify a term that signifies the outcome of this calculation:

$\text{Resultant Value}=\frac{\text{percentage occurrence}}{100}$

This mathematical transformation turns a percentage into a fraction or decimal. We'll use elimination to find the right term.

The atomic symbol representation (e.g., ${}_6^{12}\text{C}$) is a format for showing atomic details, such as mass and atomic numbers. It is a symbolic depiction, not a value derived through calculations, so it can be dismissed.

The nuclear mass count is the sum of protons and neutrons within an atom's nucleus, always expressed as an integer. It does not relate to percentages or their conversion into decimals, so it is not relevant here.

Average isotopic weight is the calculated mean mass of an element's isotopes, incorporating their natural occurrence rates. Though these rates are used in its calculation, the end result is more comprehensive than just dividing a percentage by 100.

After eliminating other options, we conclude that the term is proportional isotope fraction. In both math and chemistry, a fractional part of a whole is expressed as a decimal between 0 and 1, which is achieved by dividing a percentage by 100 (for example, $75\%/100=0.75$).

The Bohr model can be likened to a small-scale solar system. Here, the nucleus acts like the Sun, and the electrons are similar to planets, following specific, circular routes around the nucleus. This is a fundamental aspect of the Atomic structure section in your Matter studies.

Examine the choices that refer to internal components of an atom rather than electron paths.

• Positive particles are the protons, which reside within the nucleus.
• The atomic centers refer to the nucleus itself.

These are not paths that electrons travel on, so we can rule these out.

The term electron clouds is associated with the Quantum Mechanical model. While 'clouds' might sound like 'paths,' they actually denote a 3D area of likelihood where an electron might be, unlike the specific orbits described in the Bohr model.

From your knowledge of the Bohr model, it's clear that Niels Bohr used the term fixed energy levels (or shells) to label the designated distances from the nucleus where electrons reside. Each level is tied to a distinct energy quantum that an electron must possess to occupy that orbit.

The question prompts us to pinpoint the term used in the Bohr model to describe the 'set routes' for electrons. This is covered in the Atomic structure section of your Matter studies. Let's systematically eliminate incorrect terms to find the right one.

Let's first eliminate options describing the atom's core or particles instead of pathways.

• Positive particles are the protons, located inside the nucleus.
• Atomic centers are the nuclei, comprising protons and neutrons.

These do not define electron routes.

Next, decide between fixed energy levels and electron clouds.

• An electron cloud is a concept from the quantum mechanical model, representing a space where finding an electron is probable, rather than a definite path.
• Conversely, the Bohr model illustrates electrons in fixed, circular routes. Bohr called these routes fixed energy levels or shells.

Given the question's mention of the Bohr model and the synonym 'shells,' fixed energy levels is the correct choice. In this model, each shell is tied to a specific energy amount.

In chemistry, an element's identity is tied to its unique atomic number ($Z$), which represents how many protons are in every atom's nucleus. Altering the proton count changes the element's identity!

On the periodic table, carbon is listed as the sixth element, meaning its atomic number is 6. $\text{Atomic Number }(Z)=\text{Number of protons}=6$ Any atom with 6 protons is recognized as carbon.

Students often mix up atomic and mass numbers. Isotopes are atoms of the same element with varying neutron counts. Carbon isotopes include: - Carbon-12 has 6 protons and 6 neutrons ($6+6=12$). - Carbon-13 has 6 protons and 7 neutrons ($6+7=13$). - Carbon-14 has 6 protons and 8 neutrons ($6+8=14$). The neutron count and mass number can differ, so options C and D are incorrect.

Nuclear stability looks at whether an atom's nucleus is radioactive. Carbon-12 and Carbon-13 are stable, whereas Carbon-14 is unstable and radioactive. Not all carbon atoms are stable, so this isn't a universal trait.

The atomic number is an element's unique identifier, so all carbon atoms must possess exactly 6 protons to be deemed carbon.

To determine carbon's defining quality, consider what remains unchanged across its isotopes. Isotopes, which are covered in Matter: Atomic structure, are atoms of a single element that differ in neutron numbers. We'll use knowledge of Carbon-12 (${}^{12}\text{C}$), Carbon-13 (${}^{13}\text{C}$), and Carbon-14 (${}^{14}\text{C}$) isotopes to rule out incorrect options.

Carbon appears in isotopes like ${}^{12}\text{C}$, ${}^{13}\text{C}$, and ${}^{14}\text{C}$. These numbers indicate their mass numbers ($A$). As these can vary with isotopes, not all carbon atoms have a mass number of 12.

Neutron count ($N$) comes from subtracting atomic number ($Z=6$ for carbon) from mass number ($A$): $\begin{array}{rl}& {}^{12}\text{C}:12-6=6\text{ neutrons}\\ & {}^{13}\text{C}:13-6=7\text{ neutrons}\\ & {}^{14}\text{C}:14-6=8\text{ neutrons}\end{array}$ Different isotopes have varying neutron counts, so not all carbon atoms have 6 neutrons.

Nuclear stability examines if an atom's nucleus is stable or radioactive. - ${}^{12}\text{C}$ and ${}^{13}\text{C}$ are stable. - ${}^{14}\text{C}$ is unstable and decays. Since carbon atoms can be either stable or unstable, stability isn't a universal trait.

After eliminating other choices, we are left with the atomic number. In Atomic structure, an element's identity comes solely from its proton count. If it has 6 protons, it is carbon; any other number means it's a different element.

The question is asking for the accepted definition of the unified atomic mass unit ($u$). This definition is crucial in both physics and chemistry for expressing atomic and subatomic masses in a manageable way.

The unified atomic mass unit is internationally defined by the isotope Carbon-12 (${}^{12}\text{C}$), as per IUPAC standards.

Consider why the other choices are incorrect: Option A refers to hydrogen, which was once a standard but is no longer used for $u$. Option D mentions oxygen, which was also historically used but is now replaced by a unified standard.

Though a proton's mass is similar to $1u$, it isn't exactly $1u$. Differences exist due to binding energy and variations between protons and neutrons, making Option C incorrect.

After excluding the incorrect options, we affirm that $u$ is defined as exactly one-twelfth the mass of a Carbon-12 atom, as detailed in the Fundamental Constants section of your Data Booklet.

In chemistry, a standard unit such as the unified atomic mass unit ($u$) is essential to measure atomic masses. We will look at historical and precision aspects to eliminate wrong answers.

Hydrogen was initially used for atomic weights but isn't the current standard for the unified atomic mass unit due to better precision from other elements.

The mass of individual protons or neutrons does not define $u$, due to nuclear binding energy which affects total atomic mass. A specific isotope provides a more accurate standard.

Oxygen was used previously, with conflicting definitions by physicists and chemists. To unify the standard, Carbon-12 was chosen instead.

After eliminating other answers, the universally accepted definition remains: $u$ is 1/12th of the mass of a Carbon-12 atom.