How To Find Atomic Mass Of Isotopes

Boilerplate Atomic Mass

The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance.

Learning Objectives

Calculate the boilerplate atomic mass of an chemical element given its isotopes and their natural affluence

Key Takeaways

Key Points

An element can have differing numbers of neutrons in its nucleus, simply information technology always has the aforementioned number of protons. The versions of an element with dissimilar neutrons accept dissimilar masses and are called isotopes.
The average atomic mass for an element is calculated past summing the masses of the element'southward isotopes, each multiplied by its natural affluence on Globe.
When doing any mass calculations involving elements or compounds, ever use average diminutive mass, which can be institute on the periodic tabular array.

Key Terms

mass number: The total number of protons and neutrons in an diminutive nucleus.
natural affluence: The affluence of a particular isotope naturally found on the planet.
average atomic mass: The mass calculated by summing the masses of an element'southward isotopes, each multiplied by its natural abundance on World.

The atomic number of an element defines the element's identity and signifies the number of protons in the nucleus of ane atom. For example, the element hydrogen (the lightest element) will e'er have i proton in its nucleus. The element helium will e'er have two protons in its nucleus.

Isotopes

Atoms of the same element can, all the same, have differing numbers of neutrons in their nucleus. For example, stable helium atoms be that contain either i or two neutrons, merely both atoms have ii protons. These different types of helium atoms have different masses (3 or four atomic mass units ), and they are called isotopes. For any given isotope, the sum of the numbers of protons and neutrons in the nucleus is chosen the mass number. This is because each proton and each neutron weigh one atomic mass unit (amu). Past adding together the number of protons and neutrons and multiplying by ane amu, you tin can summate the mass of the atom. All elements be as a collection of isotopes. The word 'isotope' comes from the Greek 'isos' (meaning 'same') and 'topes' (significant 'identify') considering the elements can occupy the same place on the periodic tabular array while being different in subatomic construction.

Lithium Atom: Stylized lithium-7 atom: 3 protons (red), 4 neutrons (black), and 3 electrons (blue). (Lithium also has some other, rarer isotope with merely two neutrons.)

Calculating Average Atomic Mass

The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied past its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).

Average atomic mass = f₁M_ane + f_iiM₂ +… + f_nM_nwhere f is the fraction representing the natural abundance of the isotope and K is the mass number (weight) of the isotope.

The average diminutive mass of an element can be found on the periodic tabular array, typically nether the elemental symbol. When data are bachelor regarding the natural abundance of various isotopes of an element, it is simple to calculate the boilerplate atomic mass.

For helium, there is approximately ane isotope of Helium-3 for every 1000000 isotopes of Helium-4; therefore, the average diminutive mass is very close to iv amu (four.002602 amu).
Chlorine consists of ii major isotopes, 1 with 18 neutrons (75.77 per centum of natural chlorine atoms) and one with 20 neutrons (24.23 pct of natural chlorine atoms). The atomic number of chlorine is 17 (it has 17 protons in its nucleus).

To calculate the average mass, first convert the percentages into fractions (separate them past 100). Then, calculate the mass numbers. The chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35 amu. To calculate the average diminutive mass, multiply the fraction by the mass number for each isotope, then add them together.

Average atomic mass of chlorine = (0.7577 [latex]\cdot[/latex] 35 amu) + (0.2423 [latex]\cdot[/latex] 37 amu) = 35.48 amu

Another example is to summate the atomic mass of boron (B), which has two isotopes: B-10 with xix.9% natural abundance, and B-11 with 80.i% affluence. Therefore,

Average atomic mass of boron = (0.199 [latex]\cdot[/latex] 10 amu) + (0.801 [latex]\cdot[/latex] 11 amu) = 10.fourscore amu

Whenever we practise mass calculations involving elements or compounds (combinations of elements), we ever use boilerplate atomic masses.

Mass Spectrometry to Measure Mass

Mass spectrometry is a powerful characterization method that identifies elements, isotopes, and compounds based on mass-to-charge ratios.

Learning Objectives

Define the chief application of a mass spectrometer

Primal Takeaways

Cardinal Points

Mass spectrometers work on samples in a gaseous state.
The gaseous samples are ionized by an ion source, which adds or removes charged particles ( electrons or ions). Examples of ion sources include inductively coupled plasma and electron affect.
Mass analyzers separate ionized samples according to their mass-to-charge ratio. Time-of-flight and quadrupole are examples of mass analyzers.
A particle'south mass tin be calculated very accurately based on parameters such equally how long it takes to travel a certain distance or its angle of travel.
Mass spectrometers are so authentic that they can determine the types of elements in a compounds or mensurate the differences between the mass of dissimilar isotopes of the same cantlet.

Key Terms

ionization: Whatsoever procedure that leads to the dissociation of a neutral atom or molecule into charged particles (ions).
plasma: A country of affair consisting of partially ionized gas, usually at loftier temperatures.
mass-to-charge ratio: The best way to dissever ions in a mass spectrometer. This number is calculated past dividing the ions weight by its accuse.

Mass spectrometry (MS) is a powerful technique that can place a broad variety of chemical compounds. It is used to determine a particle's mass, the elemental composition of a sample, and the chemical structures of larger molecules.

Mass spectrometers split up compounds based on a holding known as the mass-to-charge ratio: the mass of the atom divided by its charge. Kickoff, the sample is ionized. Ionization is the procedure of converting an atom or molecule into an ion by adding or removing charged particles such equally electrons or ions. Once the sample is ionized, information technology is passed through some form of electric or magnetic field. A particle'south mass tin can be calculated based on parameters such as how long it takes to travel a certain distance or its angle of travel.

Schematic of Mass Spectrometer: A sample is loaded onto the mass spectrometer, where it undergoes vaporization and ionization. The components of the sample are ionized past ane of a multifariousness of methods, such equally the ionizing filament. The ions are separated in an analyze by magnetic fields. They are separated according to their mass-to-charge ratios. The ions are detected, usually by a quantitative method such equally a Faraday collector. The ion point is candy into a mass spectrum.

The Make-Up of Mass Spectrometry (MS) Instruments

MS instruments consist of two main components:

An ion source, which can convert sample molecules into ions
A mass analyzer, which sorts the ions past mass by applying electromagnetic fields

There are a wide variety of techniques for ionizing and detecting compounds.

Ionizing Compounds

Inductively coupled plasma (ICP) flame: Motion picture of an ICP flame viewed through green welder's drinking glass.

The ion source is the part of the mass spectrometer that ionizes the compound. Depending on the information desired from mass spectrometry analysis, different ionization techniques may exist used. For example, the most common ion source for analyzing elements is inductively coupled plasma (ICP). In ICP, a ten,000-caste C "flame" of plasma gas is used to disintegrate sample molecules and strip the outer electrons from those atoms.

The plasma is usually generated from argon gas. Plasma gas is electrically neutral overall, only a substantial number of its atoms are ionized past the loftier temperature.

Electron impact (EI) is another method for generating ions. In EI, the sample is heated until it becomes a gas. It is then passed through a axle of electrons. This high-energy axle strips electrons from the sample molecules, leaving behind a positively charged radical species.

Mass Analyzers

Mass analyzers split up the ions co-ordinate to their mass-to-charge ratios. There are many types of mass analyzers. Each has its strengths and weaknesses, including:

how accurately they can measure similar mass-to-charge ratios
the range of masses and sample concentrations they tin measure.

For instance, a time-of-flight (TOF) analyzer uses an electric field to advance the ions through the same potential and then measures the time they accept to reach the detector. Since the particles all have the same accuse, their velocities depend merely on their masses, and lighter ions volition reach the detector first.

Fourth dimension-of-Flight mass analyzer: Schematic of a fourth dimension-of-flight (TOF) mass analyzer.

Another blazon of detector is a quadrupole. Here, ions are passed through four parallel rods, which apply a varying electrical voltage. As the field changes, ions respond by following complex paths. Depending on the practical voltage, simply ions of a certain mass-to-charge ratio volition pass through the analyzer. All other ions will be lost by standoff with the rods.

Using a Mass Spectrometer to Mensurate Mass

Here is how a mass spectrometer would analyze a sample of sodium chloride (table common salt).

In the ion source, the sample is vaporized (turned into gas) and ionized into sodium (Na⁺) and chloride (Cl^–) ions.
Sodium atoms and ions have just i isotope and a mass of well-nigh 23 amu.
Chloride atoms and ions come in ii isotopes, with masses of approximately 35 amu (at a natural affluence of well-nigh 75 percentage) and approximately 37 amu (at a natural abundance of near 25 percent).
The mass analyzer office of the spectrometer contains electric and magnetic fields, which exert forces on ions traveling through these fields. The angle at which the ion moves through the fields depends on its mass-to-accuse ratio: lighter ions change direction more than heavier ions.
The streams of sorted ions pass from the analyzer to the detector, which records the relative abundance of each ion type. This information is used to determine the chemical composition of the original sample (i.e. that both sodium and chlorine are present in the sample) as well as its isotopic composition (the ratio of chlorine-35 to chlorine-37).