Learning Objectives

• Negatively Charged Subatomic Particles That Have Almost No Mass
• Negatively Charged Subatomic Particles

• Define atom and subatomic particle.
• Describe the locations, charges, and relative masses of protons and electrons.
• Determine the number of protons and electrons in an element.

The only stable negatively charged subatomic particles is electron. All other hadronic (Pi-) and leptonic (muon-), negative charged particles are unstable. Well, antiprotons are negatively charged and are stable under condition they do not get in touch with matter hadronic particles. A subatomic particle with no charge. A negatively charged subatomic particle. The smallest particle of an element that retains the properties of that.

An atom is defined as the smallest unit of an element that still has the properties of that element. For example, a piece of aluminum foil is shiny, silver in color, lightweight, and conductors heat and electricity. If that piece of foil is torn in half, each of those halves would exhibit the same characteristics of the initial piece. Each half could be divided over and over and over again, and each new fragment would still retain the same properties, until the atomic level is reached.

The idea of an atom was first introduced by the Greek philosopher Democritus in 450 B.C. However, his theories were largely-forgotten until the early 1800s, when John Dalton used the concept of an atom to explain why elements seemed to combine in whole-number ratios. As mentioned in the first section of this chapter, these ratios are indicated by the subscripts of chemical formulas, and the derivation of these ratios will be discussed in greater detail in the next chapter. As Dalton's theories became increasingly popular, additional scientists attempted to prove the existence of these small particles and were ultimately successful. In the course of these studies, atoms were determined to be electrically-neutral, which means that they carry no overall charge, and were thought to be indivisible.

Subatomic Particles

While the former discovery was later proven to be correct, the latter was untrue. Contrary to what Dalton and his contemporaries believed, atoms can, in fact, be broken apart into smaller units called subatomic particles. Ultimately, three main types of subatomic particles have been discovered.

Electrons

Electrons, which were first discovered in 1897, are negatively-charged subatomic particles and are, therefore, symbolized using the notation 'e⁻.' In particular, every electron carries a −1 charge. Electrons have incredibly small masses, but occupy the majority of an atom's volume. Initially, scientists believed that electrons were tiny particles that were randomly-dispersed across a considerable volume, just as raindrops are little bits of water that are scattered throughout rain clouds. However, this concept of an 'electron cloud' was later proven to be inaccurate. This theory will be revisited and corrected in a later section of this chapter.
Electrons are highly important, because a specific subset of electrons, called valence electrons, are solely-responsible for determining how elements interact, or bond, with one another. The concept of bonding is the focus of Chapter 3 in this text.

Protons

Protons, which were discovered in 1919, are subatomic particles that each bear a +1 charge and are, correspondingly, symbolized using the notation 'p⁺.' Protons are 2,000 times more massive than electrons. This ratio can be approximated by comparing the mass of a bowling ball to the mass of a penny. However, despite their relatively large mass, protons occupy a very small percentage of an atom's volume. The densely-packed space at the center of an atom in which protons are found is called the nucleus. The adage 'opposites attract' can be used to explain why electrons remain anchored within atoms. Since the central space within an atom is positively-charged, all negatively-charged electrons within that atom will be attracted to, and, therefore, bound within an atom by, the nucleus.

Recall that the atomic number of an element is defined as the number of protons contained within an atom of that element. Therefore, since atomic numbers are unique values, the identity of an element is solely-dependent on the number of protons present in an atom of that element. For example, every atom of carbon, C, that exists in the known universe is defined to contain 6 protons, because its atomic number is 6, and no other element can contain exactly 6 protons. Furthermore, it was stated above that atoms as a whole are electrically-neutral, but contain both electrons and protons, which are charged particles. In order for all of this information to remain valid, the number of positively-charged protons and negatively-charged electrons in an atom must be equal, so that their combined charges effectively 'cancel out' to a net zero, or neutral, charge. Therefore, the atomic number of an element must indicate not only the number of protons found in an atom of that element, but also the number of electrons that are contained in an atom of that element.

Example (PageIndex{1})

Use a periodic table to determine the number of protons and the number of electrons contained in an atom of each of the following elements.

1. Silicon
2. Cd
3. Bromine

Solutions

The number of protons in an atom is defined by the element's atomic number, which is found above the elemental symbol within a box on the periodic table. Furthermore, since an atom must have an overall neutral charge, the number of protons and electrons found within an atom of an element must be equal.

1. Since silicon, Si, has an atomic number of 14, every silicon atom contains 14 protons and 14 electrons.
2. Since Cd, cadmium, has an atomic number of 48, every cadmium atom contains 48 protons and 48 electrons.
3. Since bromine, Br, has an atomic number of 35, every bromine atom contains 35 protons and 35 electrons.

Neutrons

The final type of subatomic particle, the neutron, will be discussed in the next section.

A typical atom consists of 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 are discussed below). 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, composed of nucleons. Nucleons include protons and neutrons. All the positive charge of an atom is contained in the nucleus, and originates from the protons. Neutrons are neutrally-charged. Electrons, which are negatively-charged, are located outside of the nucleus.

Introduction

The Bohr model is outdated, but it depicts the three basic subatomic particles in a comprehensible way. Electron clouds are more accurate representations of where electrons are found. Darker areas represent where the electrons are more likely to be found, and lighter areas represent where they are less likely to be found.

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.
• The positive charge of protons cancels the negative charge of the electrons. Neutrons have no charge.
• With regard to mass, protons and neutrons are very similar, and have a much greater mass than electrons. Compared with neutrons and protons, the mass of an electron is usually negligible.
• Spin is associated with the rotation of a particle. Protons, neutrons, and electrons each have a total spin of 1/2.

Protons

Protons were discovered by Ernest Rutherford in the year 1919, when he performed his gold foil experiment. He projected alpha particles (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 involving 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 are negatively-charged; these 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 than that of a proton or neutron (and as such is usually considered insignificant). Unequal amounts of protons and electrons create ions: 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 determine the isotope of an atom, and often its stability. The number of neutrons is not necessarily equal to the number of protons.

Identification

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

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

Example
Consider a neutral atom of 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. Above is the atomic symbol for helium from the periodic table, with the atomic number, elemental symbol, and mass indicated.

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

• # 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 an alpha particle is zero. They result from large, unstable atoms through a process called alpha decay. Alpha decay is the process by which an atom emits an alpha particle, thereby becoming a new element. This only occurs in elements with large, radioactive nuclei. The smallest noted element that emits alpha particles is 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. Alpha decay is used as a safe power source for radioisotope generators used in artificial heart pacemakers and space probes.

Figure: Alpha Decay involves the emission of an alpha particle from the nucleus

Beta Particles

Beta particles (β) are either free electrons or positrons with high energy and high speed; they are emitted in a process called beta decay. Positrons have the exact same mass as an electron, but are positively-charged. There are two forms of beta decay: the emission of electrons, and the emission of 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 they strike. The alteration usually is considered damage, and can cause cancer and death. In contrast to beta particle's harmful effects, they can also be used in radiation to treat cancer.

Beta^- (β^-) 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, an electron, and an anti-neutrino. The proton remains in the nucleus, and the electron and anti-neutrino are emitted. The electron is called a beta particle. The equation for this process is given below:

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

• n = Neutron
• p⁺ = Proton
• e^- = Electron (beta particle)
• ν_e = Anti-neutrino

β^- Decay

Beta⁺(β⁺)or Positron Emission

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

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

• n = Neutron
• p⁺ = Proton
• e⁺ = Positron (beta particle)
• ν_e= Neutrino

β⁺ Decay

Outside Links

• Basic Sub-Atomic Particles: http://www.youtube.com/watch?v=lP57g...eature=related
• Alpha Particles: http://en.wikipedia.org/wiki/Alpha_decay
• Beta Particles: http://en.wikipedia.org/wiki/Beta_particle
• What are Sub-Atomic Particles?: http://www.youtube.com/watch?v=uXcOqjCQzh8
• Atomic Number and Mass Number: http://www.youtube.com/watch?v=lDo78hPTlgk

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.

2. Identify the subatomic particles (protons, electrons, neutrons, and positrons) present in the following:

• (ce{^{14}_6C})
• (alpha)
• (ce{^{35}Cl^-})
• (beta^+)
• (beta^-)
• (ce{^{24}Mg^{2+}})
• (ce{^{60}Co})
• (ce{^3H})
• (ce{^{40}Ar})
• (^1_0n)

3. Given the following, identify the subatomic particles present. (The periodic table is required to solve these problems)

• 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:

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 with the proton number in the subscript. There are 6 electrons because the atom is neutral. There are 8 neutrons because 14-6=8. 14 is the atomic mass number in the superscript.
• 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 due to 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. It can also be written as e⁺. 'e' represents an electron, but when it has as positive charge it is a positron.
• β^-
  • 0 protons, 0 neutrons, 1 electron
    • This is a beta^- particle, and can also be written as e^-. This is a standard 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 proton because the element is hydrogen. 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.

3. Given the following, identify the subatomic particles present. (The periodic table is required to solve these problems)

• 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 electrons
• 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 lements 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

Negatively Charged Subatomic Particles That Have Almost No Mass

• 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:

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

Negatively Charged Subatomic Particles

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