IntroductionOther Basic Atomic ParticlesBeta Particles

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.

You are watching: Which subatomic particles contribute to an atom’s mass number but not its atomic number?

## 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The Bohr model is outdated, but it depicts the three basic subatomic particles in a comprehensible way.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.

ParticleElectric Charge (C)Atomic ChargeMass (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.

## 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.

Consider a neutral atom of carbon: $$\ce{^{12}_{6}C}$$. 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

See more: Which Of These Is The Only Moveable Bone In The Skull, Bones Of The Skull Flashcards

### 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.

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) νe = Neutrino

β+ Decay