2.1.4 Identify the relative masses and relative charges of protons, neutrons, and electrons.Identify and calculate the number of protons, neutrons, and electrons contained in an atom, given its identity, mass number, atomic number, and charge.

Position the protons, neutrons, and electrons.Defining planetary models up to Z=18.


As a result of the study of radioactive materials (elements that spontaneously give off particles that form new elements) by Frederick Soddy, we learned a great deal about atoms' internal structure.A study by him demonstrated that some substances with different radioactive properties and different atomic weights actually belonged to the same element.Using root words isos ("equal") and topos ("place"), he coined the term isotope.He said that isotopes have identical outsides but different interiors. Soddy received the Nobel Prize in Chemistry in 1921 with this claim.Therefore, all elements do not have the same atoms.

An introduction to isotopes can be found on YouTube: https://www.youtube.com/watch?


Isotopes and Mass numbers

.A variety of neutron-numbered atoms that belong to the same element are called isotopes.All isotopes of an element have the same atomic number because they all contain the same number of protons.The problem is that different isotopes of an element contain different amounts of neutrons, so their masses are different.

To clarify this point, consider the following two examples.

An isotope of lithium containing three neutrons has the following atomic number (Z) and mass number (A).


An isotope of lithium containing four neutrons has an atomic number of Z and a mass number of A. What are they?


Lithium has three protons, therefore its atomic number is always 3.However, A= 6 is the mass number of an isotope with 3neutrons, and A= 7 is the mass number of an isotope with 4neutrons.Nature only contains certain isotopes.For instance, lithium exists as a 3neutron isotope and as a 4neutron isotope, but not as twoneutrons or as a 5neutron isotope.

It is this discussion of isotopes that brings us back to our understanding of atomic theory. .In some elements, different numbers of neutrons can be found in the atoms, so the mass number of the atom will vary.Elements in nature are uniform mixtures composed of their naturally occurring isotopes at a constant ratio.Therefore, a piece of lithium always contains both types of naturally occurring lithium (the type with 3neutrons and the type with 4neutrons).It always comprises both in the same relative amounts (or "relative abundances").93% of a chunk of lithium will always contain 4neutrons, while the remaining 7% will always contain 3neutrons.

Isotope Notation

As the nucleus for an element can differ over time, we must specify or be able to calculate the atomic number (Z) and the mass number (A) for that atom.Atoms are shown using atomic symbols, which take the form of

.A nitrogen nucleus with 7 protons and 8 neutrons might be written asSince all nitrogen atomsmust have 7 protons in their nucleus, sometimes the 7 can be omitted in writing the symbol for nitrogen. This same practice can be used to write the name of nitrogen.You can omit the protons (or the atomic number) and indicate the mass number after the name, for example, nitrogen-15.

I would like to use hydrogen's three isotopes as an example.In each hydrogen atom, there must be exactly one proton.As a result, the number of neutrons in the nucleus can differ, resulting in different mass numbers (see image below).

An isotope of hydrogen has a proton and no neutrons in its nucleus (as in the image on the left).The nuclear symbol for hydrogen-1 can be seen above.In the image on the right, another isotope of hydrogen has a proton and a neutron in its nucleus.In the above image, the nuclear symbol shows hydrogen-2, a third isotope of hydrogen that has a proton and two neutrons as its nuclei (in the image above on the right).The hydrogen-3 symbol is written above.(Unlike many other isotopes, hydrogen isotopes are also known by their specialized names. Hydrogen-1 is sometimes called protium, hydrogen-2 deuterium, and hydrogen-3 tritium.)

Mass of Atom

When calculating atomic mass, knowing about the different isotopes is essential.In the periodic table, the atomic mass(sometimes called atomic weight) of an element is the number of atoms in a naturally occurring sample of the element. The atomic mass is reported in atomic mass units (amu).Each element's atomic mass is usually written as an integer below its chemical symbol in the table.

An abundance of isotopes

An element's atomic mass can be calculated by knowing the relative abundances and masses of its natural isotopes.Following are some examples of how this can be done.

Voici an example:

Natural isotopes of boron exist.An atom of boron with a mass number of 10 amu is composed of 20% boron-10, an isotope of boron with 5 neutrons.Another 80% of the atoms are boron-11 atoms, which are boron isotopes with 6 neutrons and mass number 11 amu.Amount of atomic mass of boron?

How to solve:

Taking the percentages given in the question and dividing by 100%, convert them into decimal forms.

To find the relative abundance of each isotope, multiply each mass by the percentage in decimal form.

Atomic mass of boron is 10.80 amu.

Voici an example:

Natural neon comes in three isotopes.A sample of neon contains 90.48% neon-20 atoms, which is an isotope of neon with 10 neutrons and a mass number of 19.99 amu.Another 0.27% of the atoms are neon-21, an isotope with 11 neutrons and a mass number of 20.99 amu.The final 9.25% of the atoms consist of neon-22, an isotope with 12 neutrons and a mass number of 21.99 amu.What is the atomic weight of neon?

This solution works:

Divide each percentage in the question by 100% to get its decimal equivalent:

Step Two: Multiply the mass of each isotope by its relative abundance (percentage):

In step three, the average mass of an atom is calculated by adding the masses of each of the different isotopes:

Hence, neon's atomic mass is 20.20 amu.

In our example, boron's (symbol C) atomic mass is 10.81 and neon's (symbol Ne) is 20.18, both of which are very close to what we calculated.Not all periodic tables display the element's atomic number above its symbol and its atomic mass below it.If you are ever confused about atomic mass, you should keep in mind that it is in decimal form and always the larger of the two numbers, while the atomic number is always a whole number, and it should always be the smaller of the two.A mass of an atom must include both the number of protons and the average number of neutrons.

Boltzmann Model

For visualizing the placement of protons, neutrons, and electrons in an atom of an element, the Bohr model is useful.Isotopes have different neutrons, so the Bohr model of an atom must change - but only slightly.Think for example of an atom of sulfur with a mass number of 32, sulfur-32 (see below),

It consists of an atom of sulfur with a mass number of 33, sulfur-33.Atomic identity would be the common characteristic between the isotopes, so their atomic number (16) and number of protons (16) would remain the same.Due to the fact that it is an atom of sulfur, proton (16) equals electron (16).Among the two most noticeable differences in the Bohr model diagram are the number of neutrons and mass number.Therefore, mass number (33) equals # of protons + # of neturons. The formula is: Mass number = # of protons (16) + # of neturons (?).NTU = 17 (bipolar electrons).