Atomic Number and Mass Number. Learning Objective 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 neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet.
October 16, Licenses and Attributions. CC licensed content, Shared previously. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:. Both the atomic number and mass are written to the left of the chemical symbol. The "A" value is written as a superscript while the "Z" value is written as a subscript.
For an example of this notation, look to the chromium atom shown below:. Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. Symbol-mass format for the above atom would be written as Cr In this notation, the atomic number is not included. You will need to refer to a periodic table for proton values. Calculate each of the three subatomic particles and give specific group or period names for each atom. Allison Soult , Ph.
Department of Chemistry, University of Kentucky. Learning Outcomes Define atomic and mass numbers. Determine the number of protons, neutrons, and electrons in an atom. Identify the charge and relative mass of subatomic particles. Label the location of subatomic particles in the atom. Determine the mass of an atom based on its subatomic particles.
Atomic Number The atomic number represented by the letter Z of an element is the number of protons in the nucleus of each atom of that element. 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. 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. Click here to check your answer to Practice Problem 1. 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.
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