l>Structure & Reactivity: Atoms: Protons, Neutrons, Electrons

Structure & Reactivity

AT. Historic Overview the the Atom

AT2. Old Quantum Mechanics: basic Developments

Today, we recognize that atom contain protons, neutrons and also electrons. The proton have significant mass and also a optimistic charge and also are uncovered in the nucleus of the atom. The neutrons have mass however no charge and also are likewise found in the nucleus. The electron have negative charge and also very small mass and are found outside the atom"s nucleus. The load of one atom in atomic mass systems is roughly the amount of that is protons and neutrons, since the electron don"t have actually much mass.

for example, a typical carbon atom has six protons and six neutrons, and also it has actually an atomic load of 12 amu. A carbon additionally has 6 electrons, yet they are so tiny that castle don"t add to carbon"s weight.

An aspect is defined by the number of protons in that nucleus.

You are watching: How many protons are in a carbon atom

The variety of protons in an atom is equal to the number of electrons, come balance the charge.

some carbon atoms have actually an extra ghost or two, for this reason carbon may have an atomic load of 13 or also 14 amu. However, a carbon atom can"t have actually an extra proton; an extra proton would make the a nitrogen atom. That is the six protons that make the atom behave choose carbon. Plenty of other facets may have actually slightly different atomic weights, depending on how numerous neutrons are uncovered in the nucleus. Different atoms the the same facet with different weights are dubbed isotopes. Because that example,12C,13C and14C room all isotope of carbon. They all have actually six protons, however different numbers of neutrons, as seen in a version of 12C and also 13C, below.


Neutrons are likewise in the nucleus.

A neutron has a mass comparable to a proton, however has no charge.

Compared to protons and neutrons, the massive of one electron is very small.

Problem AT2.1.

An element"s atomic number is just the number of protons in an atom of the element. Offered the following atomic numbers and also atomic weights, identify the variety of protons, neutrons and electrons in an atom the the element.

a) oxygen: atom number = 8, atomic load = 16

b) phosphorus: atomic number = 15, atomic load = 31

c) zinc: atom number = 30, atomic load = 65

d) gold: atomic number = 79, atomic load = 197

Problem AT2.2.

If a proton"s mass is 1.67 x 10-27 kg and also the fixed of one electron is 9.11 x 10-31 kg, how countless times heavier is a proton 보다 an electron?

Problem AT2.3.

If carbon in nature is around 99% 12C and 1% 13C, climate what is the average weight that a carbon atom?

Problem AT2.4.

Note that 14C is even rarer 보다 13C, because 14C is converted into 14N via radiation decay. In the event, a high-energy electron is emitted from the 14C nucleus. Define how the emission must transform the carbon right into a nitrogen, and also indicate how many protons and neutrons are found in the resulting nucleus.

Problem AT2.5.

Magnesium in nature is found in three significant isotopes. The is practically 79% 24Mg, around 11% 25Mg and also 12% 26Mg. What is the mean weight of a magnesium atom?

Problem AT2.6.

Chlorine in nature is found in two major isotopes:35Cl and also 37Cl. If the mean atomic load of chlorine is about 35.5, what percent of every isotope is found in nature?

A number of developments at the beginning of the twenty century resulted in our present understanding that the framework of atoms and also molecules a century later. At the time, some human being though protons, neutrons, and electrons were lumped with each other in the atom. This view of the atom was referred to as the "pudding model" that the atom.


Ernest Rutherford first proposed the an atom contains a really small, positively charged nucleus surrounded by empty space. The electron orbited much away native the nucleus.


Figure AT2.1. Rutherford"s version of an atom: a nucleus through electrons much away, and also lots of empty space.

Rutherford to be explaining the an outcome of an experiment in i beg your pardon alpha corpuscle (positively charged helium ions) were fired in ~ a gold foil. Most of these particles passed v the foil easily, arguing there to be a the majority of empty an are in the material. However, several of the particles bounced straight back, having actually collided with the small, very charged nuclei. The positive alpha particles were powerfully repelled by the optimistic nuclei, since like charges repel every other. He didn"t really recognize much around the place of the electrons, the negatively charged particles in the atom, but thought they orbited the nucleus favor planets approximately the sun.

Why weren"t the electrons found in the nucleus? If electrons space attracted come protons, that seems favor that"s where they need to be. Niels Bohr argued that electron are found only in specific, permitted orbits at different distances indigenous the nucleus.

That conceptual leap come specific, enabled orbits clues the development of quantum mechanics into the knowledge of the atom. Quantum mechanics is based on the idea that on a very tiny scale, countless properties just have particular values (like 1, 2, 3...) rather of any kind of value at every (like every the feasible fractions between these integers). In other words, in the world around us, we typically view things like walking increase a ramp. We deserve to heat a pot the water just a tiny bit warmer, and just a small bit warmer 보다 that, and so on. On the atom scale, however, the people is much more like walking increase a set of stairs. Probably you can heat the water come 30 oC or 40 oC, but heating to 35 oC would certainly be impossible, because heat only comes in 10 level packages. The is, in fact, just how the quantum people really works, yet on the person scale, the steps involved are so tiny that we cannot notice them.


Figure AT2.2. In the Bohr model of one atom, electrons can be found only at specific allowed distances from the nucleus.

Bohr"s design was also consistent with the earlier idea of the routine table that the elements. The idea is the electrons are uncovered in different "shells" that space each further and further native the nucleus. Every of those shells corresponds approximately to a different row in the regular table. Hydrogen and also helium have actually electrons just in the first shell, and we check out those two elements in the an initial row that the periodic table. Carbon and also oxygen"s outermost electrons are uncovered in the second shell, for this reason they present up in the 2nd row the the periodic table.


Each heat in the routine table coincides to an outer layer of electrons the are discovered further from the nucleus 보다 the outermost electrons in the row prior to it. We space going to see eventually that over there is a more variation top top this idea, however it is still pretty much the method we view the periodic table today. Hydrogen start the first shell, lithium the second, salt (the Latin, natrium) the third, potassium (Latin, kalium) the fourth, and so on.


The sports we space going come see requires that dive in the middle of the regular table. Scandium v zinc have outer electrons that are only in the third shell, not the fourth. The 3rd and the 4th shell overlap a little bit, therefore that some electrons actually start to get in the 4th shell (as in potassium and also calcium), then complete filling the 3rd shell throughout the transition metals. The reasons for that likewise have to do with quantum mechanics, but we will require to learn a little much more about energy and waves prior to we see why.

Bohr confirmed that electrons could be found in details orbits about the nucleus. He additionally showed that electrons in these various orbits have specific amounts the energy. By law this mathematically, that was offering an explanation to crucial problem. People knew the atoms deserve to absorb energy (they have the right to be cook in a flame, for example) and give the energy ago again in the kind of light. Fairly than offer off light of all colours when excited, atom only provide off very details colours. Because that example, heating lithium salts in a flame produces a red colour, however heating salt salts produces an orange colour, whereas potassium salts create a purple colour, and also so on.

this colours deserve to be separated and also studied utilizing a prism. When world did that, they found that a given atom does not create just one pure colour of light, however several different ones. When separated through a prism, the light provided off by an excited compound might be seen versus a dark surface as several different, coloured lines. This were dubbed emission lines.

It had actually been known due to the fact that the beforehand 1800"s the light had actually wave properties, and that irradiate of various colours had various wavelengths. For example, red irradiate consistes of electromagnetic waves, v a wavelength of around 700 nm, but blue light"s wavelength is around 450 nm. That way a colour deserve to actually it is in measured numerically. Since of that fact, people can look because that mathematical relationships between the emission lines observed for various atoms. Those mathematical relationships may reveal something around the atoms themselves.

Furthermore, the was recognized that different wavelengths that light synchronized to different amounts of energy. In among the very first developments in quantum mechanics, Max Planck in 1900 proposed the light travels in bundles called photons. Back they space particles, this photons do have actually wave properties. The lot of energy in a photon the light corresponds to that is wavelength.

by proposing the electrons can be found only in specific orbits, particular distances away from the nucleus, Bohr was trying to describe observations from atom spectroscopy report by another scientist named Rydberg. Rydberg had discovered a mathematical relationship between the wavelength of these emission lines. Bohr believed that, when power was added, electrons can be excited from one energy level (or orbit) to a greater one. When the electron relaxed earlier to its initial orbit, it provided off the energy it had gained in the form of light. The specific emission present occur since electrons are found at very details energy levels in an atom, for this reason a drop native one level to another constantly produces the very same amount of light energy. That specific amount of light power has a certain colour.


Figure AT2.3. The correspondence between colour, wavelength and also energy.

Bohr then provided the mathematical relationship describing electrostatic attraction and also centripetal force to show that his model of the atom was continual with Rydberg"s relationship. In fact, he can use his version to suspect the emission lines of an atom.

Problem AT2.7.

Bohr"s explanation of atom structure developed on Rydberg"s monitoring of a numerical series in spectral emission lines. Resolving a collection involves finding a sample in numbers. Discover the patterns amongst the following sequences the numbers, and predict the next number in the sequence.

a) 1, 2, 3, 4...

b) 2, 4, 6, 8...

c) 3, 5, 7, 9...

d) 1, 4, 9, 16...

e) 2, 4, 8, 16...

f) 1, 1/2, 1/4, 1/9...

Problem AT2.8.

Bohr"s idea depended partly on the usage of Coulomb"s legislation of electrostatic attraction. Coulomb"s regulation is to express mathematically together follows:

F = (k q1q2 )/ r2

in which F is the attractive force between two charged particles, q1 and also q2 space the charges on the 2 particles, r is the distance between the 2 particles and also k is a constant. A large value that F means that the charges room strongly attracted to every other.

a) expect q1 is the charge on the cell core of an atom and q2 is the charge on one electron. What wake up to the pressure of attraction between an electron and also the nucleus when the fee in the nucleus increases? Explain.

b) intend r is the street from the electron come the nucleus. What wake up to the pressure of attraction between an electron and also nucleus once the electron gets further from the nucleus?

c) using the ideas of Coulomb"s law, compare the attraction of an electron to the cell nucleus in a helium atom matches a hydrogen atom.

Problem AT2.9.

Max Planck explained the power of a photon using the adhering to relationship:

E = hν or E = hc/λ

In which E = energy; ν =frequency; λ = wavelength; c = rate of light; h = Planck"s constant

a) What wake up to the energy of light together its wavelength gets longer?

b) What happens to the energy of light as its frequency it s okay higher?

Other civilization were acquainted with these ideas and currently knew about the relationship in between light and also energy. Bohr"s design of the atom put all of these ideas together to successfully define a details atomic property:

colour = wavelength = power of irradiate = energy in between electron levels.

In various other words, an excited electron have the right to drop back to its initial orbit by offering off a photon through an power exactly the same as the distinction in energy in between the two orbits ("excited state" and also "ground state" orbits).


Figure AT2.4. An electron have the right to be assumed of together both a particle and also a wave.

However, Bohr go not explain why electrons would be uncovered at details energy level in the first place. Louis de Broglie, a historian-turned-physicist, solved this problem with the idea the wave-particle duality. De Broglie placed together the adhering to ideas:

All moving particles have actually wave properties. Electrons move around the nucleus and also they have actually wavelengths.To keep a complete standing wave along that orbit, an electron have the right to only adopt orbits of particular circumferences. Otherwise, one finish of the wave would not satisfy up v the various other end, and also it would certainly interfere through itself.Orbits with details circumferences have certain radii.Electrons are discovered at specific distances indigenous the nucleus, yet not at various other distances.

One means to show why an electron can have only certain allowed orbits is via the "particle in a box", a simple concept indigenous quantum mechanics. If a particle has wave properties, then it has actually a wavelength. The wavelength relies on particular conditions. By analogy, if you take it a guitar string and also attach it come the end of a box, the string have the right to only vibrate at certain frequencies.That"s just how guitarists can readjust the keep in mind played on a guitar string. By pushing one finish of the string against a stress on the guitar neck, the size of the string is changed, and also so is its enabled wavelength, for this reason it makes a various sound.

The wire can"t move at the two points where it is held. That method the wave has to form in such a method that it returns to the same position at both ends. Since of that, particular wavelengths won"t work, because the wave won"t be able to return to the correct position at the much end.

Furthermore, the permitted wavelengths that a etc string additionally depend top top the thickness that the string. As a result, there space two problems that regulate the tone that is played: which of the six guitar strings is plucked, and also where the wire is held against the frets.


Figure AT2.5. A particle in a box is enabled only particular wavelengths, based on the size of the box.

The same thing is true v very small particles that have actually wave properties. These particles deserve to have only details wavelengths the fit their surroundings. One electron has some property, analogous to the thickness that a etc string, that limits its feasible wavelengths. Given those limits, there room only details orbits permitted the electron. If that orbit doesn"t have the ideal circumference, the electron will not be able to form a finish wave follow me that orbit.

These principles ushered in a change in science. Quantum mechanics is a very powerful tool. It have the right to be used to that s right predict just how molecules will behave. Unfortunately, the mathematics affiliated in quantum mechanics space one or two math courses past what many introductory chemistry student are familiar with. Also so, a qualitative feel for few of the results of quantum mechanics is important sufficient that us should check out it.

Problem AT2.10.

Complete the table through the ideal information.

Element SymbolAtomic NumberMass NumberNumber the ProtonsNumber that NeutronsNumber the ElectronsCharge

This site is written and also maintained by chris P. Schaller, Ph.D., university of Saint Benedict / Saint John"s university (with contributions from other authors as noted). That is freely accessible for education use.

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