2.1.4State the family member masses and relative fees of protons, neutrons and also electrons. Calculate the number of protons, neutrons and also electrons in atoms from the identity, massive number, atom number and/or charge.

You are watching: An atom with 3 protons and 4 neutrons

2.1.5 State the place of protons, neutrons and electrons. Draw and also interpret planetary models for elements up to Z=18.


The study of radioactive products (elementsthat spontaneously provide off corpuscle to form new elements) through Frederick Soddy gave essential clues around the inner structure that atoms. His work showed that some substanceswith different radioactive properties and also different atomic weights were in truth the very same element. That coined the termisotopefrom the Greek rootsisos (íσος “equal”) and topos (τóπος “place”). He explained isotopes as, “identical outsides but different insides.” Soddy won the Nobel prize in Chemistry in 1921 for his work.Therefore, not all atomsof a given element are identical.

Watch the following video clip for an introduction to isotopes: https://www.youtube.com/watch?v=GsJPxR6IfZI



Isotopes and Mass numbers

Unlike the number of protons, i m sorry is always the exact same for every atoms that the same element, the variety of neutrons have the right to be different. Atoms of the same element with different numbers of neutron are known asisotopes. Due to the fact that the isotopes of any kind of given aspect all save on computer the same number of protons, they have actually the exact same atomic number. However, due to the fact that the isotope of a given aspect contain different numbers of neutrons, various isotopes have different mass numbers.

Review the adhering to two instances to aid clarify this point.

Example:What is the atomic number (Z) and also the massive number (A) of one isotope that lithium comprise 3 neutrons? A lithium atom includes 3 proton in that nucleus.

Given-atomic number (Z)= # of proton = 3# of neutrons = 3Solve-mass number (A)= # of protons + # the neutrons= 3 + 3 = 6

Example:What is the atom number (Z) and the fixed number (A) of one isotope that lithium containing 4neutrons? A lithium atom consists of 3protons in that is nucleus.

Given-atomic numberZ= # of protons = 3# of neutron = 4Solve-mass number (A)= # of proton + # of neutrons= 3 + 4 = 7

Notice that since the lithium atom constantly has 3protons, the atom number for lithium is alwaysZ= 3. The mass number, however, isA= 6for the isotope with 3neutrons, andA= 7for the isotope through 4neutrons. In nature, only particular isotopes exist. Because that instance, lithium exists as an isotope through 3neutrons and as one isotope through 4neutrons, however it doesn’t exists as an isotope v 2neutrons or as an isotope with 5neutrons.

This whole conversation of isotopes brings us back to our expertise of atom theory. Based ~ above isotopes, atoms of a given element cannot all be identical. Atoms of a given facet can have different numbers that neutrons, and therefore, different mass numbers. It turns out that elements found in nature exist as uniform mixtures v a consistent ratio of your naturally occurring isotopes. In other words, a piece of lithium constantly contains both species of naturally emerging lithium (the type with 3neutrons and also the type with 4neutrons). Moreover, it always contains the two in the same relative amounts (or “relative abundances”). In a chunk that lithium, 93% will constantly be lithium through 4neutrons, when the remaining 7% will constantly be lithium through 3neutrons.

Isotope Notation

Because the nucleus for an element can differ because that the same element, we need to specify or have the ability to calculate the atom number (Z) and the massive number (A) for that atom. The atom deserve to be shown with anuclear symbol which takes the form

where X is the chemical symbol for the element, Ais themass number, and also Zis the atom number. For example, a nitrogen nucleus containing 7 protons and 8 neutrons would beSince all nitrogenatomsmust have 7 protons in your nucleus, periodically the 7 is omitted and the prize is written simply asThis exact same practice have the right to be applied when creating the name. Omit the number of protons (or atom number) and identify the mass number ~ the name and hyphen, for example, nitrogen-15.

For another example, let"s think about hydrogen"s three isotopes. Every hydrogen atoms need to have specifically 1 proton in the nucleus. However, the variety of neutrons in the nucleus deserve to vary, top to various mass numbers (see the picture below).

One isotope of hydrogen has 1 proton and 0 neutrons in the nuclues (in the image on the left above). The atom symbol is written above and has actually the surname hydrogen-1. One more isotope the hydrogenhas 1 proton and 1 neutron in the nuclues (in the photo in the center above). The atom symbol is written over and has the name hydrogen-2.A third isotope the hydrogenhas 1 proton and also 2 neutrons in the nuclues (in the photo on the appropriate above). The atom symbol is written over and has actually the surname hydrogen-3. (Unlike numerous other isotopes, isotopes of hydrogen are also referred come with committed names. The hydrogen-1 may additionally be referred to as protium, hydrogen-2 as deuterium, and hydrogen-3 as tritium.)

Atomic Mass

Knowing about the various isotopes is necessary when it pertains to calculating atom mass. Theatomic mass(sometimes referred to as atom weight) the an facet is the weighted mean mass the the atoms in a naturally arising sample of the element.Atomic fixed is generally reported in atomic fixed units(amu).Most regular tables give the atom mass of every element. The atomic mass is frequently a decimal number normally written listed below the chemical symbol that each element in the table.

Isotope Abundance

It is feasible to calculation the atom mass of one element, provided you know the family member abundances the element’s naturally arising isotopes and the masses of those different isotopes. The examples listed below show just how this calculation is done.


Boron has two naturally developing isotopes. In a sample of boron, 20% that the atoms room boron-10, i beg your pardon is an isotope the boron through 5 neutrons and a mass variety of 10 amu. The various other 80% that the atoms are boron-11, i m sorry is one isotope of boron with 6 neutrons and a mass variety of 11 amu. What is the atom mass the boron?


To do this problem, we will calculate 20% that the massive of boron-10, i beg your pardon is just how much the boron-10 isotope contributes come the “average boron atom.” us will additionally calculate 80% that the fixed of boron-11, i beg your pardon is exactly how much the boron-11 isotope contributes to the “average boron atom.”

Step One:Convert the percentages given in the question right into their decimal forms by dividing each percent by 100%:

Decimal kind of 20% = 0.20Decimal kind of 80% = 0.80

Step Two:Multiply the fixed of every isotope by its family member abundance (percentage) in decimal form:

20% that the massive of boron-10 = 0.20 x 10 amu = 2.00 amu80% of the massive of boron-11 = 0.80 x 11 amu = 8.80 amu

Step Three:Find the full mass the the “average atom” by including together the contributions from the various isotopes:

Total mass of typical atom = 2.00 amu + 8.80 amu = 10.80 amu

The massive of an average boron atom, and thus boron’s atom mass, is 10.80 amu.


Neon has actually three naturally emerging isotopes. In a sample that neon, 90.48% the the atoms are neon-20, i m sorry is an isotope of neon with 10 neutrons and also a mass number of 19.99 amu. An additional 0.27% of the atom areneon-21, which is an isotope the neon with 11 neutrons and also a mass variety of 20.99 amu. The final 9.25% of the atom areneon-22, i m sorry is one isotope of neon with 12 neutrons and also a mass number of 21.99 amu. What is the atom mass that neon?


To carry out this problem, we will calculate 90.48% the the massive of neon-20, i beg your pardon is just how muchneon-20 contributes come the “average neon atom.” us will also calculate 0.27% of the mass ofneon-21 and also 9.25% the the massive ofneon-22, i beg your pardon are how much theneon-21 and also theneon-22 isotopes add to the “average neon atom” respectively.

Step One:Convert the percentages provided in the question right into their decimal develops by dividing each percentage by 100%:

Decimal kind of 90.48% = 0.9048Decimal form of 0.27% = 0.0027Decimal kind of 9.25% = 0.0925

Step Two:Multiply the massive of each isotope by its relative abundance (percentage) in decimal form:

90.48% the the fixed ofneon-20 =0.9048 x 20 amu = 18.10 amu0.27% the the fixed of neon-21 = 0.0027 x 21 amu = 0.057 amu9.25% of the massive ofneon-22 = 0.0925 x 22 amu = 2.04 amu

Step Three:Find the full mass the the “average atom” by including together the contribute from the various isotopes:

Total fixed of mean atom = 18.10 amu + 0.057 amu + 2.04 amu

The fixed of an typical neon atom, and thus neon’s atom mass, is 20.20 amu.

See more: If You Are Driving An Suv Or Large Vehicle, Enter Most Entrance Ramps__________.

Notice the atomic mass native the periodic table because that boron (symbol B) is 10.81 and the atomic mass that neon (symbol Ne) is 20.18, both which are very close to what us calculated in our examples. Take time to an alert that not all periodic tables have the atomic number above the element’s symbol and the atomic mass listed below it. If girlfriend are ever confused, remember that the atomic number is constantly a totality number and also should constantly be the smaller sized of the two, when the atomic mass for most numbers is in decimal type and should constantly be the bigger of the two. (The atom mass must encompass both the number of protons and also the average number of neutrons.)

Bohr Model

As discussed previously,the Bohr design is beneficial for visualizing the placement of the protons, neutrons, and also electrons in the atom of one element. Through the sport of neutrons in isotopes, the Bohr model of the atom must readjust - however only slightly. For example, take into consideration an atom of sulfur v a mass variety of 32, sulfur-32 (see picture below),

and one atom the sulfur with a mass variety of 33, sulfur-33. The similarities in between the isotopes would be the atom identity, therefore, the atom number (16) and also # of protons (16) remain the same. Due to the fact that it is one atom that sulfur, the proton # (16) still amounts to the electron # (16). The two significant differences in the Bohr version diagrams would be the massive number and # of neutrons. The calculate of fixed number = # of protons + # the neturons, therefore,mass number (33) = # of proton (16) + # of neturons (?). Neturon #, then = 17 (see photo below).