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

### AT. Historic Overview that the Atom

AT2. Old Quantum Mechanics: basic Developments

Today, we know that atom contain protons, neutrons and electrons. The proton have far-reaching mass and a positive charge and also are discovered in the nucleus of the atom. The neutrons have actually mass however no charge and also are also found in the nucleus. The electrons have an adverse charge and also very small mass and also are uncovered outside the atom"s nucleus. The weight of an atom in atomic mass systems is around the sum of its protons and neutrons, because the electron don"t have much mass.

because that example, a usual carbon atom has six protons and also six neutrons, and it has actually an atomic weight of 12 amu. A carbon likewise has 6 electrons, yet they are so tiny that castle don"t contribute to carbon"s weight.

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

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

some carbon atoms have actually an extra neutron or two, for this reason carbon may have actually an atomic weight of 13 or even 14 amu. However, a carbon atom can"t have actually an extra proton; an extra proton would certainly make that a nitrogen atom. The is the six protons that make the atom behave choose carbon. Countless other elements may have slightly different atomic weights, depending on how many neutrons are found in the nucleus. Different atoms of the same facet with different weights are referred to as isotopes. For example,12C,13C and14C space all isotope of carbon. Lock all have six protons, however different numbers of neutrons, as seen in a design of 12C and 13C, below.

Neutrons are also in the nucleus.

A neutron has actually a mass similar 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 variety of protons in one atom of that element. Provided the following atomic numbers and atomic weights, determine the number of protons, neutrons and also electrons in an atom that the element.

a) oxygen: atomic number = 8, atomic weight = 16

b) phosphorus: atom number = 15, atomic weight = 31

c) zinc: atomic number = 30, atomic weight = 65

d) gold: atom number = 79, atomic weight = 197

Problem AT2.2.

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

Problem AT2.3.

If carbon in nature is about 99% 12C and also 1% 13C, then what is the mean weight of a carbon atom?

Problem AT2.4.

Note the 14C is also rarer than 13C, due to the fact that 14C is converted into 14N via radioactive decay. In the event, a high-energy electron is emitted from the 14C nucleus. Explain how that emission must convert the carbon into a nitrogen, and also indicate how many protons and neutrons are found in the result nucleus.

Problem AT2.5.

Magnesium in nature is uncovered in three major isotopes. It is almost 79% 24Mg, about 11% 25Mg and also 12% 26Mg. What is the typical weight of a magnesium atom?

Problem AT2.6.

Chlorine in nature is found in two major isotopes:35Cl and also 37Cl. If the median atomic load of chlorine is around 35.5, what portion of each isotope is found in nature?

A number of developments in ~ the beginning of the twenty century brought about our existing understanding of the structure of atoms and molecules a century later. At that time, some civilization though protons, neutrons, and also electrons to be lumped with each other in the atom. This check out of the atom was referred to as the "pudding model" that the atom.

Ernest Rutherford very first proposed the an atom has a really small, positively charged nucleus surrounding by empty space. The electron orbited much away indigenous the nucleus.

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

Rutherford to be explaining the an outcome of one experiment in i m sorry alpha corpuscle (positively charged helium ions) were fired at a yellow foil. Most of these particles passed v the foil easily, arguing there was a most empty room in the material. However, some of the particles bounced straight back, having collided with the small, very charged nuclei. The positive alpha particles to be powerfully repelled by the positive nuclei, because like dues repel every other. That didn"t really understand much about the ar of the electrons, the negatively charged particles in the atom, but thought they orbited the nucleus choose planets about the sun.

Why weren"t the electrons uncovered in the nucleus? If electrons room attracted come protons, it seems favor that"s whereby they must be. Niels Bohr suggested that electrons are found only in specific, enabled orbits at various distances indigenous the nucleus.

That conceptual leap come specific, allowed orbits point out the advent of quantum mechanics into the knowledge of the atom. Quantum mechanics is based on the idea the on a very small scale, many properties only have certain values (like 1, 2, 3...) rather of any kind of value at all (like every the possible fractions between these integers). In various other words, in the world roughly us, we usually view things choose walking up a ramp. We deserve to heat a pot the water just a little bit warmer, and also just a tiny bit warmer than that, and so on. On the atom scale, however, the people is an ext like walking increase a collection of stairs. Probably you could heat the water come 30 oC or 40 oC, however heating come 35 oC would certainly be impossible, because heat just comes in 10 degree packages. The is, in fact, how the quantum human being really works, yet on the human being scale, the steps involved are so tiny that we cannot an alert them.

Figure AT2.2. In the Bohr model of an atom, electrons can be discovered only at details allowed ranges from the nucleus.

Bohr"s model was likewise consistent with the previously idea that the periodic table the the elements. The idea is the electrons are discovered in different "shells" that room each further and further native the nucleus. Each of those shells corresponds around to a different row in the regular table. Hydrogen and also helium have actually electrons just in the first shell, and also we check out those two elements in the very first row the the periodic table. Carbon and also oxygen"s outermost electrons are discovered in the 2nd shell, therefore they display up in the second row the the routine table.

Each heat in the routine table corresponds to an outer layer of electrons the are uncovered further native the nucleus 보다 the outermost electrons in the row before it. We are going come see eventually that over there is a additional variation top top this idea, yet it is still pretty lot the means we view the regular table today. Hydrogen starts the first shell, lithium the second, sodium (the Latin, natrium) the third, potassium (Latin, kalium) the fourth, and so on.

The sport we space going to see involves that dive in the middle of the routine table. Scandium with zinc have actually outer electrons that are only in the third shell, no the fourth. The third and the 4th shell overlap a small bit, therefore that some electrons actually start to enter the 4th shell (as in potassium and calcium), then end up filling the third shell throughout the shift metals. The factors for that likewise have to execute with quantum mechanics, however we will require to learn a little more about energy and waves before we check out why.

Bohr proved that electrons might be discovered in specific orbits roughly the nucleus. He also showed the electrons in these different orbits have certain amounts the energy. By act this mathematically, the was supplying an explanation to an essential problem. Civilization knew the atoms have the right to absorb energy (they have the right to be cook in a flame, for example) and also give the energy earlier again in the form of light. Quite than offer off light of all colours once excited, atom only give off very specific colours. For example, heating lithium salt in a fire produces a red colour, yet heating salt salts produces an orange colour, vice versa, potassium salts create a purple colour, and also so on.

these colours can be separated and studied using a prism. When people did that, they discovered that a offered atom does not create just one pure color of light, yet several various ones. When separated by a prism, the light given off by an excited compound can be seen versus a dark surface ar as numerous different, coloured lines. This were referred to as emission lines.

It had actually been known since the early 1800"s the light had actually wave properties, and also that irradiate of different colours had different wavelengths. For example, red light consistes the electromagnetic waves, with a wavelength of around 700 nm, yet blue light"s wavelength is around 450 nm. That way a colour can actually it is in measured numerically. Because of that fact, human being can look because that mathematical relationships in between the emission lines observed for different atoms. Those mathematics relationships might reveal something around the atom themselves.

Furthermore, the was recognized that different wavelengths of light synchronized to different quantities of energy. In among the very first developments in quantum mechanics, Max Planck in 1900 proposed the light travel in bundles referred to as photons. Return they space particles, these photons do have actually wave properties. The lot of power in a photon of light corresponds to that is wavelength.

through proposing the electrons can be found only in particular orbits, certain distances far from the nucleus, Bohr to be trying to define observations from atomic spectroscopy report by another scientist named Rydberg. Rydberg had discovered a math relationship between the wavelength of these emission lines. Bohr thought that, when power was added, electrons might be excited native one power level (or orbit) to a higher one. Once the electron relaxed ago to its initial orbit, it offered off the energy it had obtained in the form of light. The details emission currently occur because electrons are uncovered at very particular energy levels in an atom, for this reason a drop from one level to another always produces the exact same amount of irradiate energy. That certain amount the light energy has a specific colour.

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

Bohr then used the mathematical relationships describing electrostatic attraction and also centripetal force to display that his model of the atom was continuous with Rydberg"s relationship. In fact, he can use his version to suspect the emission lines of one atom.

Problem AT2.7.

Bohr"s explanation of atomic structure constructed on Rydberg"s monitoring of a numerical collection in spectral emissions lines. Addressing a collection involves finding a pattern in numbers. Discover the patterns amongst the complying with sequences the numbers, and predict the following 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 partially on the use of Coulomb"s law of electrostatic attraction. Coulomb"s law is express mathematically as follows:

F = (k q1q2 )/ r2

in i m sorry F is the attractive force between two charged particles, q1 and also q2 are the fees on the 2 particles, r is the distance between the two particles and also k is a constant. A large value the F way that the charges are strongly attractive to every other.

a) suppose q1 is the charge on the nucleus of an atom and q2 is the charge on an electron. What happens to the pressure of attraction between an electron and also the nucleus as soon as the charge in the cell nucleus increases? Explain.

b) expect r is the street from the electron come the nucleus. What wake up to the pressure of attraction in between an electron and nucleus when the electron gets more from the nucleus?

c) making use of the concepts of Coulomb"s law, compare the attraction of one electron come the cell core in a helium atom matches a hydrogen atom.

Problem AT2.9.

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

E = hν or E = hc/λ

In i beg your pardon E = energy; ν =frequency; λ = wavelength; c = speed of light; h = Planck"s constant

a) What wake up to the energy of light together its wavelength it s okay longer?

b) What happens to the power of light together its frequency it s okay higher?

Other people were acquainted with these principles and currently knew about the relationship between light and also energy. Bohr"s version of the atom put all of these ideas together come successfully describe a specific atomic property:

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

In other words, one excited electron can drop earlier to its original orbit by providing off a photon with an power exactly the exact same as the difference in energy in between the two orbits ("excited state" and "ground state" orbits).

Figure AT2.4. One electron can be believed of together both a particle and also a wave.

However, Bohr walk not describe why electrons would be discovered at specific energy levels in the an initial place. Louis de Broglie, a historian-turned-physicist, solved this difficulty with the idea the wave-particle duality. De Broglie put together the complying with ideas:

All moving particles have actually wave properties. Electron move approximately the nucleus and also they have wavelengths.To keep a complete standing wave along the orbit, an electron have the right to only embrace orbits of particular circumferences. Otherwise, one finish of the wave would not accomplish up v the various other end, and also it would interfere v itself.Orbits with certain circumferences have specific radii.Electrons are uncovered at certain distances from the nucleus, however not at other distances.

One method to illustrate why an electron could have only certain allowed orbits is via the "particle in a box", a simple concept native quantum mechanics. If a particle has wave properties, then it has a wavelength. Its wavelength counts on certain conditions. By analogy, if you take it a etc string and attach it come the ends of a box, the string can only vibrate at specific frequencies.That"s exactly how guitarists can adjust the note played top top a etc string. By pushing one finish of the string against a stress on the etc neck, the length of the cable is changed, and also so is its permitted wavelength, for this reason it renders a various sound.

The wire can"t relocate at the 2 points whereby it is held. That method the wave has actually to form in together a means that it return to the same position at both ends. Since of that, certain wavelengths won"t work, since the wave won"t be able to return to that correct position at the far end.

Furthermore, the allowed wavelengths that a etc string likewise depend on the thickness of the string. Together a result, there are two problems that control the tone that is played: i beg your pardon of the six guitar strings is plucked, and also where the wire is held against the frets.

Figure AT2.5. A particle in a crate is permitted only specific wavelengths, based on the dimensions of the box.

The very same thing is true with very little particles that have actually wave properties. These particles deserve to have only certain wavelengths that fit your surroundings. One electron has some property, analogous to the thickness that a guitar string, that limits its possible wavelengths. Given those limits, there are only certain orbits permitted the electron. If its orbit doesn"t have the ideal circumference, the electron will not it is in able to kind a finish wave follow me that orbit.

These principles ushered in a transformation in science. Quantum mechanics is a very powerful tool. It can be used to correctly predict how molecules will certainly behave. Unfortunately, the mathematics associated in quantum mechanics are one or two math courses beyond what many introductory chemistry students are acquainted with. Also so, a qualitative feeling for several of the consequences of quantum mechanics is important sufficient that we should discover it.

Problem AT2.10.

Complete the table through the suitable information.

 Element Symbol Atomic Number Mass Number Number that Protons Number of Neutrons Number the Electrons Charge H 1 1 H 1 0 H 1 2 H 2 1 H 3 1 4 9 2 +2 6 12 6 6 12 13 12 0 43 55 43 20 40 +2 Si 14 28 0 19 28 +4 Fe 26 30 23 35 44 -1 K 22 17 21 15 15 0 13 27 +3 S 16 16 0 Pd 106 46 +1 24 28 21 50 68 50 Hg 80 120 79 79 118 78

This website is written and also maintained by chris P. Schaller, Ph.D., college of Saint Benedict / Saint John"s college (with contributions from various other authors as noted). The is freely available for education use.

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