The Mass Number Of An Atom Is The Number Of

Learning Objective

The mass number of an element is an integer number because it is the number of protons that are in the nucleus. The relative atomic mass is not an integer because elements can have more than one. The sum of the atomic number Z and the number of neutrons N gives the mass number A of an atom. Since protons and neutrons have approximately the same mass and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units, is within 1% of the whole number A. Atoms with the same atomic number but different neutron numbers, and hence different mass.

Determine the relationship between the mass number of an atom, its atomic number, its atomic mass, and its number of subatomic particles

Key Points

Neutral atoms of each element contain an equal number of protons and electrons.
The number of protons determines an element’s atomic number and is used to distinguish one element from another.
The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess.
Together, the number of protons and the number of neutrons determine an element’s mass number.
Since an element’s isotopes have slightly different mass numbers, the atomic mass is calculated by obtaining the mean of the mass numbers for its isotopes.

Terms

atomic massThe average mass of an atom, taking into account all its naturally occurring isotopes.
mass numberThe sum of the number of protons and the number of neutrons in an atom.
atomic numberThe number of protons in an atom.

Atomic Number

Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is 6 because it has 6 protons. The number of neutrons can vary to produce isotopes, which are atoms of the same element that have different numbers of neutrons. The number of electrons can also be different in atoms of the same element, thus producing ions (charged atoms). For instance, iron, Fe, can exist in its neutral state, or in the +2 and +3 ionic states.

Mass Number

An element’s mass number (A) is the sum of the number of protons and the number of neutrons. The small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers.

Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine (Cl) is 35.45 amu because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 amu (17 protons and 18 neutrons) and some with an atomic mass of 37 amu (17 protons and 20 neutrons).

Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom. For example, a lithium atom (Z=3, A=7 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 – 3 = 4).

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http://www.boundless.com/
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http://en.wiktionary.org/wiki/atomic_number
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http://www.boundless.com//biology/definition/atomic-mass–2
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“A-level Chemistry/OCR/Atoms, Bonds and Groups/Atoms and Reactions/Atoms.”

http://en.wikibooks.org/wiki/A-level_Chemistry/OCR/Atoms,_Bonds_and_Groups/Atoms_and_Reactions/Atoms
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Fundamental Subatomic Particles	Electromagnetic Radiation
Light and Other Forms of Electromagnetic Radiation

Particle	Symbol	Charge	Mass
electron	e^-	-1	0.0005486 amu
proton	p⁺	+1	1.007276 amu
neutron	n^o	0	1.008665 amu

The number of protons, neutrons, and electrons in an atom can be determined from a set of simple rules.

The number of protons in the nucleus of the atom is equal to the atomic number (Z).
The number of electrons in a neutral atom is equal to the number of protons.
The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus.
The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).

Examples: Let's determine the number of protons, neutrons, and electrons in the following isotopes.

The different isotopes of an element are identified by writing the mass number of the atom in the upper left corner of the symbol for the element. ¹²C, ¹³C, and ¹⁴C are isotopes of carbon (Z = 6) and therefore contain six protons. If the atoms are neutral, they also must contain six electrons. The only difference between these isotopes is the number of neutrons in the nucleus.

¹²C: 6 electrons, 6 protons, and 6 neutrons

¹³C: 6 electrons, 6 protons, and 7 neutrons

¹⁴C: 6 electrons, 6 protons, and 8 neutrons

Practice Problem 1:

Calculate the number of electrons in the Cl^- and Fe³⁺ ions.

The Mass Number Of An Atom Is The Number Of

Much of what is known about the structure of the electrons in an atom has been obtained by studying the interaction between matter and different forms of electromagnetic radiation. Electromagnetic radiation has some of the properties of both a particle and a wave.

Particles have a definite mass and they occupy space. Waves have no mass and yet they carry energy as they travel through space. In addition to their ability to carry energy, waves have four other characteristic properties: speed, frequency, wavelength, and amplitude. The frequency (v) is the number of waves (or cycles) per unit of time. The frequency of a wave is reported in units of cycles per second (s^-1) or hertz (Hz).

The idealized drawing of a wave in the figure below illustrates the definitions of amplitude and wavelength. The wavelength (l) is the smallest distance between repeating points on the wave. The amplitude of the wave is the distance between the highest (or lowest) point on the wave and the center of gravity of the wave.

If we measure the frequency (v) of a wave in cycles per second and the wavelength (l) in meters, the product of these two numbers has the units of meters per second. The product of the frequency (v) times the wavelength (l) of a wave is therefore the speed (s) at which the wave travels through space.

vl = s

Practice Problem 2:

What is the speed of a wave that has a wavelength of 1 meter and a frequency of 60 cycles per second?

Practice Problem 3:

Orchestras in the United States tune their instruments to an 'A' that has a frequency of 440 cycles per second, or 440 Hz. If the speed of sound is 1116 feet per second, what is the wavelength of this note?

Light is a wave with both electric and magnetic components. It is therefore a form of electromagnetic radiation.

Visible light contains the narrow band of frequencies and wavelengths in the portion of the electro-magnetic spectrum that our eyes can detect. It includes radiation with wavelengths between about 400 nm (violet) and 700 nm (red). Because it is a wave, light is bent when it enters a glass prism. When white light is focused on a prism, the light rays of different wavelengths are bent by differing amounts and the light is transformed into a spectrum of colors. Starting from the side of the spectrum where the light is bent by the smallest angle, the colors are red, orange, yellow, green, blue, and violet.

As we can see from the following diagram, the energy carried by light increases as we go from red to blue across the visible spectrum.

Mass Number Is

Because the wavelength of electromagnetic radiation can be as long as 40 m or as short as 10^-5 nm, the visible spectrum is only a small portion of the total range of electromagnetic radiation.

The electromagnetic spectrum includes radio and TV waves, microwaves, infrared, visible light, ultraviolet, x-rays, g-rays, and cosmic rays, as shown in the figure above. These different forms of radiation all travel at the speed of light (c). They differ, however, in their frequencies and wavelengths. The product of the frequency times the wavelength of electromagnetic radiation is always equal to the speed of light.

vl = c