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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Atomic Theory > The Atom > Sub-Atomic Particles

Sub-Atomic Particles

Within the typical atom, there are three subatomic particles: protons, neutrons, and electrons (as seen in the Helium atom below). Other particles exist as well, such as alpha and beta particles (which will be discussed later on). The Bohr Model shows the three basic subatomic particles in a simple manner. Most of an atom's mass is in the nucleus, a small dense area at the center of every atom formed by nucleons. Nucleons are protons and neutrons. All of the positivity of an atom is contained in the nucleus, because the protons have a positive charge. Neutrons are neutral, meaning they have no charge. Electrons, which have a negative charge, are located outside of the nucleus.

Introduction

The Bohr Model is out-dated but depicts the three basic subatomic particles in a comprehendible way. Electron clouds (in the figure shown below) are better representations of where electrons are found. The darker areas represent where the electrons will have a higher probability of being located and the lighter areas represent where they are less likely to be found.

Picture2.png

 

Particle Electric Charge (C) Atomic Charge Mass(g) Atomic Mass (Au) Spin
Protons +1.6022 x 10-19 +1 1.6726 x 10-24 1.0073 1/2
Neutrons 0 0 1.6740 x 10-24 1.0078 1/2
Electrons -1.6022 x 10-19 -1 9.1094 x 10-28 0.00054858 1/2
  • Au is the SI symbol for atomic mass unit.
  • As you can see the positive charge of protons cancels the negative charge of the electrons, and neutrons have no charge.
  • In regards to mass, protons and neutrons are very similar, and have a much greater mass than electrons. In calculating mass, electrons are often insignificant. 
  • Spin is the rotation of a particle. Protons, neutrons, and electrons each have a total spin of 1/2.

Protons

Protons were discovered by Earnest Rutherford in the year 1919, when he performed his gold foil experiment. He projected alpha particles (aka Helium nuclei) at gold foil and the positive alpha particles were deflected. He concluded that protons exist in a nucleus and have a positive nuclear charge. The atomic number or proton number is the number of protons present in an atom. The atomic number determines an element (e.g., the element of atomic number 6 is Carbon). 

Electrons

Electrons were discovered by Sir John Joseph Thomson in 1897. After many experiments of cathode-rays, J.J. Thomson demonstrated the ratio of mass to electric charge of cathode-rays. He confirmed that cathode-rays are fundamental particles that have a negative charge. Cathode-rays became known as electrons. Robert Millikan, through oil-drop experiments, found the value of the electronic charge.

Electrons are located in an electron cloud, which is the area surrounding the nucleus of the atom. There is usually a higher probability of finding an electron closer to to the nucleus of an atom. Electrons can abbreviated as e-. Electrons have a negative charge that is equal in magnitude to the positive charge of the protons. However, their mass is considerably less then that of a proton or neutron (such that it is usually insignificant). Unequal amounts of protons and electrons creates ions that can either be positive cations or negative anions.

Neutrons

Neutrons were discovered by James Chadwick in 1932, when he demonstrated that penetrating radiation incorporated beams of neutral particles. Neutrons are located in the nucleus with the protons. Along with protons, they make up almost all of the mass of the atom. The number of neutrons is called the neutron number and can be found by subtracting the proton number from the atomic mass number. The neutrons in an element affect which isotope the atom is of, and often times, its stability. The number of neutrons does not have to equal that of the protons.

Identification

Both of the following are appropriate ways of representing the composition of a particular atom.

Untitled.jpg

Often the proton number is not indicated since the elemental symbol conveys the same information.

Example

Here is an example of the neutral atom Carbon:  126C.  The atomic mass number of Carbon is 12 amu, the proton number is 6, and it has no charge. In neutral atoms the charge is omitted.

He Atom.png

Here is a zoomed picture of Helium in the periodic table, with the Atomic Number or Proton Number, Elemental Symbol and mass highlighted.

Every element has a specific number of protons, so the proton number is not always written (eg. second method of writing). 

  • # Neutrons = Atomic Mass Number - Proton Number
    • Atomic Mass Number is abbreviated as A.
    • Proton Number(or Atomic Number) is abbreviated Z.
  • # Protons = Proton Number or Atomic Number
  • In Neutral Atoms, # Electrons = # Protons
  • In Ions, # Electrons = # Protons - (Charge)
  • Charge is written with the number before the positive or negative sign
    • Example, 1+

Note: The Atomic Mass Number is not the same as the Atomic Mass seen on the periodic table. Click Here for More Information.

Other Basic Atomic Particles

Many of these particles (explained in detail below) are emitted through radioactive decay. Click Here for More Information. Also note that many forms of radioactive decay emit gamma rays, which are not particles.

Alpha Particles

Alpha particles can be denoted by He2+,α2+, or just α. They are helium nuclei which consist of two protons and two neutrons. The net spin on alpha particles is zero. They result from large unstable atoms through a process called alpha decay. Alpha decay is the process when an atom emits an alpha particle and loses two protons and two neutrons, therefore becoming a new element. This only occurs in elements with largely radioactive nuclei elements. The smallest noted element that emitted an alpha particle was element 52, Tellurium. Alpha particles are generally not harmful. They can be easily stopped by a single sheet of paper or by one's skin. However, they can cause considerable damage to the insides of one's body. Some uses of alpha decay are as safe power sources for radioisotope generators used in artificial heart pacemakers and space probes. 

___________________________________Alpha Decay

Decay.jpg

Beta Particles

Beta particles (β) are either free electrons or positrons with high energy and high speed, that are emitted through a process called beta decay. Positrons have the exact same mass as an electron but have a positive charge. There are two forms of this: one that emits electrons and one that emits positrons. Beta particles, which are 100 times more penetrating than alpha particles, can be stopped by household items like wood or an aluminum plate or sheet. Beta particles have the ability to penetrate living matter and can sometimes alter the structure of molecules that are struck. The alteration usually is considered as damage, and can be as severe as cancer and death. In contrast to beta particle's harmful effects, they can also be used in radiation to treat cancer. 

Beta--) or Electron Emission

Electron emission may result when excess neutrons make the nucleus of an atom unstable. As a result, one of the neutrons decays into a proton, electron, and anti-neutrino. While the proton remains in the nucleus, the electron, and anti-neutrino are emitted. The electron can be called a beta particle. The equation is shown below.

\[ _{1}^{0}\textrm{n}\rightarrow {_{1}^{1}\textrm{p}}^+ + \textrm{e}^- + \bar{\nu_{e}} \]

  • n =  Neutron
  • p+ = Proton
  • e- = Electron (Beta Particle)
  • ν= anti-neutrino

β- Decay

beta-.jpg

This process emits the subatomic particles electrons and anti-neutrinos. The electrons can be called beta particles in this case. Free neutrons also decay by this process.

 

Beta+(β+) or Positron Emission

Position emission may occur when an excess of protons makes the atom unstable. In this process, a proton is converted into a neutron, positron, and neutrino. While the neutron remains in the nucleus, the positron and the neutrino are emitted. The positron can be called a beta particle. The equation is shown below.

\[ { _{1}^{1}\textrm{p}}^+ \rightarrow _{1}^{0}\textrm{n} + \textrm{e}^+ + \nu_{e} \]

  • n =  Neutron
  • p+ = Proton
  • e+ = Positron (Beta Particle)
  • νe  = Neutrino

β+ Decay

Picture6.jpg

This process emits positrons and neutrinos. The positrons can be called beta particles in this case.

Outside Links

References

  1. Petrucci, Ralph, William Harwood, Geoffrey Herring, and Jeffry Madura.General Chemistry. 9th ed. Upper Saddle River, New Jersey: Pearson Prentince Hall, 2007.
  2. Haskin, Larry A. The Atomic Nucleus and Chemistry; D. C. Heath and Company: Lexington, MA, 1972; pp. 3-4, 43-53.
  3. Petrucci, Ralph, F. Geoffrey Herring, Jeffrey D. Madura, and Carey Bissonnette. General Chemistry. 10th ed. Upper Saddle River, New Jersey: Pearson Education, Inc., 2011.

Problems

1. Identify the number of protons, electrons, and neutrons in the following atom.

 question one.jpeg.jpg

2. Identify the subatomic particles(protons, electrons, neutrons, and positrons) present in the following: (Periodic Table will be necessary)

  • 146C
  • α
  • 35Cl-
  • β+
  • β-
  • 24Mg2+
  • 60Co
  • 3H
  • 40Ar
  • n

3. Given the following, identify the subatomic particles present. (Periodic Table will be necessary)

  • Charge +1, 3 Protons, Mass Number 6.
  • Charge -2, 7 Neutrons, Mass Number 17.
  • 26 Protons, 20 Neutrons.
  • 28 Protons, Mass Number 62.
  • 5 Electrons, Mass Number 10.
  • Charge -1, 18 Electrons, Mass Number 36.

4. Arrange the following Elements in order of increasing (a) number of protons; (b) number of neutrons; (c) mass.

27Co, when A=59;    56Fe, when Z=26;   11Na, when A=23;    80Br, when Z=35;     29Cu, when A=30;      55Mn, when Z=25 

5. Fill in the rest of the table with your knowledge of the relationships between subatomic particles.

Atomic Number Mass Number Number of Protons Number of Neutrons Number of Electrons
2     2  
  23     11
    15 16  
  85 37    
53     74  

Solutions and Explanations

1. There are 4 Protons, 5 Neutrons, and 4 electrons. This is a neutral Beryllium atom.

2. Identify the subatomic particles present in the following:

  • 146C  
    • 6 Protons, 8 Neutrons, 6 Electrons
      • There are 6 protons in accordance to the bottom proton number. There are 6 Electrons because the atom is neutral. There are 8 Neutrons because 14-6=8. 14 is the atomic mass number which is the superscript above the C.
  • α
    • 2 Protons, 2 Neutrons, 0 Electrons
      • This is an alpha particle which can also be written as 4He2+. There are two protons because the element is helium. There are no electrons because 2-(2) = 0. There are 2 neutrons because 4-2=2.
  • 35Cl-
    • 17 Protons, 18 Neutrons, 18 Electrons
      • This is a Chloride ion. According to the periodic table, there are 17 Protons because the element is Chlorine. There are 18 electrons because of the negative charge. 17-(-1) = 18. There are 18 Neutrons because 35-17=18.
  • β+
    • 0 Protons, 0 Neutrons, 0 Electrons, 1 Positron
      • This is a Beta+ particle. This can also be written as e+. "e" represents an electron, and when it has as postivie charge it is a positron. There is thus one positron
  • β-
    • 0 Protons, 0 Neutrons, 1 Electron
      • This is a Beta- particle. This can also be written as e-. This is the standard electron. There is one electron.
  • 24Mg2+
    • 12 Protons, 12 Neutrons, 10 Electrons
      • This is a Magnesium Ion. There are 12 protons from the Magnesium atom. There are 10 electrons because 12-(2) = 10. There are 12 neutrons because 24-12 = 12.
  • 60Co
    • 27 Protons, 33 Neutrons, 27 Electrons
      • The Cobalt atom has 27 protons as seen in the periodic table. There are also 27 electrons because the charge is 0. There are 33 neutrons because 60-27 = 33.
  • 3H
    • 1 Protons, 2 Neutrons, 1 Electrons
      • There is 1 Protons because of the Hydrogen element. There is 1 electron because the atom is neutral. There are 2 Neutrons because 3-1 = 2.
  • 40Ar
    • 18 Protons, 22 Neutrons, 18 Electrons
      • There are 18 Protons from the Argon element. There 18 electrons because it is neutral, and 22 Neutrons because 40 - 18 = 22.
  • n
    • 0 Protons, 1 Neutrons, 0 Electrons
      • This is a free neutron denoted by the lower case n. Therefore, it is a lone particle, and there is one neutron.

3. Given the following, identify the subatomic particles present. (Periodic Table will be necessary)

  • Charge +1, 3 Protons, Mass Number 6.
    • 3 protons, 3 Neutrons, 2 Electrons
  • Charge -2, 8 Neutrons, Mass Number 17.
    • 9 Protons, 8 Neutrons, 7 Elecetrons
  • 26 Protons, 20 Neutrons.
    • 26 Protons, 20 Neutrons, 26 Electrons
  • 28 Protons, Mass Number 62.
    • 28 Protons, 34 Neutrons, 28 Electrons
  • 5 Electrons, Mass Number 10.
    • 5 Protons, 5 Neutrons, 5 Electrons
  • Charge -1, 18 Electrons, Mass Number 36.
    • 17 Protons, 19 Neutrons, 18 Electrons

4. Arrange the following Elements in order of increasing (a) number of protons; (b) number of neutrons; (c) atomic mass.

a) Na, Mn, Fe, Co, Cu, Br

  • Z=#protons;
  • Na: z=11; Mn: Z=25, given; Fe: Z=26, given; Co: Z=27; Cu: Z=29; Br: Z=35, given

b) Na, Cu, Fe, Mn, Co, Br

  • A=#protons+#neutrons, so #n=A-#protons(Z);
  • Na: #n=23-11=12; Cu: #n=59-29=30; Fe: #n=56-26=30; Mn: #n=55-25=30; Co: #n=59-27=32; Br: #n=80-35=45

Note: Cu, Fe, Mn are all equal in their number of neutrons, which is 30.

c) Na, Mn, Fe, Co, Cu, Br

  • Na: 22.9898 amu; Mn: 54.9380 amu; Fe: 55.845 amu; Co: 58.9332 amu; Cu: 63.546 amu; Br: 79.904

Note: This is the same order as the number of protons, because as Atomic Number(Z) increases so does Atomic Mass.

5. Fill in the rest of the table with your knowledge of the relationships between subatomic particles.

Atomic Number Mass Number Number of Protons Number of Neutrons Number of Electrons
2 4 2 2 2
11 23 11 12 11
15 31 15 16 15
37 85 37 48 37
53 127 53 74 53

Note: Atomic Number=Number of Protons=Number of Electrons  and Mass Number=Number of Protons+Number of Neutrons

Contributors

  • Jiaxu (Josh) Wang (UCD)

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