2.1.4 Describe what protons, neutrons, and electrons are composed of.By considering the identity, mass number, atomic number and/or charge of an atom, you can determine the number of protons, neutrons and electrons in that atom.

In 2.1.5, describe the positions of protons, neutrons, and electrons.Design and interpret planetary models for elements up to Z18.


Frederick Soddy's study of radioactive materials (elements, which spontaneously give off particles to form new elements) gave important hints about atoms' internal structure. .Taking its name from a combination of the Greek roots isos (“equal”) and topos (“place”), he coined the termisotope.Soddy won the Nobel Prize in Chemistry in 1921 for his work on isotopes. Therefore, not all elements share the same atoms.

The following video provides an introduction to isotopes: https://www.youtube.com/watch?


Isotopes and Mass numbers

The number of neutrons can differ for atoms of the same element unlike the number of protons, which is always the same.Different atoms of the same element with varying numbers of neutrons are called isotopes.A given element has exactly one atomic number since all its isotopes contain the same number of protons.Different isotopes of the same element contain slightly different numbers of neutrons, thus they have different masses.

Here are two examples to clarify this point.

For instance, what is the atomic number (Z) and mass number (A) of an isotope of lithium with three neutrons?


A lithium isotope containing four neutrons has a atomic number (Z) and mass number (A) of 4. What are these values?


Due to the fact that the lithium atom contains 3 protons, its atomic number is always Z = 3.The mass number, however, is A= 6 for the diatomic isotope with 3neutrons, and A= 7 for the diatomic isotope with 4neutrons.Nature contains only certain isotopes.It is known that lithium has an isotope with 3neutrons and an isotope with 4neutrons, but not an isotope with 2neutrons or an isotope with 5neutrons.

The discussion of isotopes brings us back to our understanding of atomic theory.According to isotopes, not all atoms of a particular element are identical.Different elements have atoms with different neutron counts, and the mass numbers of the atoms will differ accordingly.Elements found in nature naturally occur as mixtures of isotopes with a constant ratio.A piece of lithium always contains both types of naturally occurring lithium (the type with 3neutrons and the type with 4neutrons).Furthermore, their relative abundances are the same.On average, 93% of a chunk of lithium is lithium with 4neutrons, and 7% is lithium with 3neutrons.

Isotope Notation

.The atom can be represented using the nuclear symbol which looks like this

Chemical symbol X, mass number A, and atomic number Zi identify the element. .If protons (or atomic numbers) are absent, identify the mass number instead, such as nitrogen-15.

To illustrate this point, let's consider hydrogen's three isotopes.Hydrogen atoms must have one proton exactly in the nucleus.Although the number of neutrons inside the nucleus can vary, this can lead to different mass numbers.

In one isotope of hydrogen, one proton and zero neutrons are contained in the nucleus (see the image on the left).The nuclear symbol is shown above, along with its name hydrogen-1.One of the isotopes of hydrogen has one proton and one neutron in the nucleus (in the image in the center above).In the image on the right you can see that hydrogen-2 has 1 proton and 2 neutrons in its nucleus. Another isotope of hydrogen has 1 proton and 3 neutrons in its nucleus.A nuclear symbol corresponding to hydrogen-3 can be found above.Isotopes of hydrogen, however, are referred to by their specialized names. Hydrogen-1 is sometimes called protium, hydrogen-2 is deuterium, and hydrogen-3 is tritium.

The mass of an atom

Calculating atomic mass requires knowledge of different isotopes. .Atomic mass is usually expressed as a decimal number below the chemical symbol for each element in the table.

Intensity of isotopes

A calculation of the atomic mass of an element can be performed if you know the relative abundances of its natural isotopes and the masses of their different isotopes.This is illustrated in the examples below.

Here's an example:

There are two naturally occurring isotopes of boron.In a boron sample, 20% of the atoms are boron-10, a boron isotope with 5 neutrons and a mass number of 10 amu.80 percent of the atoms are made up of boron-11, an isotope of boron with six neutrons and a mass number of 11 amu.How much mass does boron have?

How to solve:

Calculate 20% of the mass of boron-10 so you can understand how much boron-10 contributes to the average boron atom. Calculate 80% of the mass of boron-11 so you will understand how much boron-11 contributes to the average boron atom.

As a first step, divide each percentage given in the question by 100% to get their decimal forms:

To calculate the relative abundance of each isotope in decimal form, simply multiply its mass by its relative abundance (percentage):

By adding together the total masses of the isotopes, calculate the mass of the "average atom":

A boron atom, as well as the atomic mass of boron, measures 10.80 amu.

An example is:

Neon is naturally produced in three isotopes.90.48% of atoms in neon come from neon-20, which has 10 neutrons and a mass number of 19.99 amu.An additional 0.27% of the atoms are neon-21, an isotope of neon with eleven neutrons and a mass number of 20.99 amu.There are 9.25% neon-22 atoms, an isotope of neon with 12 neutrons and a mass number of 21.99 amu.How heavy is neon?

How to solve:

This problem requires us to calculate 90.48% of the mass of neon-20, which is the mass of one neon atom. We will also calculate 0.27% of the mass of neon-21 and 9.25% of the mass of neon-22, which are how much each isotope contributes to the “average neon atom.”

Using 100 as a rule of thumb, divide the percents given in the question into their decimal forms:

Divide that mass by the relative abundance (percentage) of each isotope in decimal form:

Third Step: Add up the contributions from the different isotopes to determine the total mass of the "average atom":

Hence, the average mass of an atom of neon equals 20.20 amu.

.Note that not all periodic tables show the atomic number above the element symbol and the atomic mass beneath it.In the event that you are ever confused, remember that the atomic number is always a whole number and should always be the smaller one, while the atomic mass is usually in decimal form and should always be the larger one.Atomic mass must count both protons and neutrons.

Theory of Bohr

Bohr model is useful for visualizing where protons, neutrons, and electrons are located in an element's atom, as we have discussed previously.Isotopes with different neutrons mean that the Bohr model of the atom must be slightly different - but only slightly.

A sulfur atom with mass number 33, sulfur-33.The similarities between the isotopes are their atom identities, so the atomic number (16) and number of protons (16) are also the same.Its proton number (16) equals its electron number (16) as long as it is a sulfur atom.It is the number of neutrons and the mass number that constitute the two greatest differences in the Bohr model diagrams.Mass number (33) is calculated according to the formula: mass number (16) = # of protons + # of neutrons (?).Thus, Neturon Number = 17 (see image below).