Atomic Structure, Periodicity, and Matter:The Structure of the Atom

Introduction

All matter consists of elements, of which there are more than 100. Each element has its own unique properties, and the atoms of each element are the smallest parts that retain the properties of that element. Since most atoms consist of the same three basic particles: protons, neutrons, and electrons, then the question becomes “what makes the atoms of one element different from another?” The answer lies in the number and arrangement of these three basic subatomic particles. For this reason, it is important to understand the fundamental structure of an atom and the characteristics of its protons, neutrons and electrons.

Early experiments studying the relationship between matter and electricity during the late 1800’s led scientists to the discovery of the first subatomic particle, the electron. Electrons are negatively charged particles, found in energy levels surrounding the center or nucleus of the atom, and are in constant motion.

Further investigations identified the nucleus as being composed of two types of particles, protons and neutrons. The proton is a positively charged particle, and the neutron is a particle with no electric charge.

The Nucleus: Protons

In 1920, Ernest Rutherford refined the concept of the nucleus and concluded that the nucleus contained positively charged particles called protons. The proton is a subatomic particle carrying a charge equal but opposite to the negative charge of the electron. That is, the proton has a charge of +1, and the electron has a charge of -1. Therefore, an atom is electrically neutral as long as it has an equal number of protons and electrons.

However, when the number of protons does not equal the number of electrons, the atom becomes charged and is known as an ion. Ions will be covered in more detail in this lesson; but it is important to note, at this time, that it is the movement of the electrons, and not the protons, that creates the positive or negative charge which changes an atom into an ion.

Although the number of electrons and neutrons can vary in individual atoms of an element, the number of protons cannot. Each and every atom or ion of an element must have exactly the same number of protons. All carbon atoms, for example, must have 6 protons. If a proton were added to a carbon atom, then the atom would become an atom of nitrogen, having only the properties of nitrogen. Similarly, if a proton were removed from a carbon atom, it would become an atom of boron with the properties of boron.

Since the number of protons in an atom of an element is unique to that element, this number is significant and is known as the element’s atomic number. In the section of this lesson dealing with the periodic table, you will see that the elements are arranged by increasing atomic number. Additionally, the proton has approximately the same mass as a neutron, and the combined mass of these two nuclear particles is known as the mass number.

Although, the proton is often discussed as a single indivisible subatomic particle, it is important to recognize recent theories that no longer consider the proton as an elementary particle. The proton is referred to as a nucleon which is made up of three quarks. As in most theories, the concept of the proton is in a constant condition of being refined.

The Nucleus: Neutrons

In 1932, James Chadwick was credited with the discovery of a second subatomic particle contained in the nucleus of the atom. Chadwick described this particle as having no electrical charge and the approximate mass of a proton. This particle is the neutron. Unlike the protons, the number of neutrons can and do vary in the atoms of an element. Atoms of the same element with differing numbers of neutrons are called isotopes. Because neutrons are electrically neutral, a variation in the number of neutrons in the nucleus has no effect on the charge of the atom. However, the number of neutrons will affect the mass of an atom and the atomic mass of an element, and may affect the stability of the nucleus.

Since the number of neutrons plus the number of protons in an atom is equal to the mass number of that atom, the neutrons are an important factor in the determination of the mass of any particular atom. In addition, the naturally occurring isotopes of an element determine the atomic mass of that element. The atomic mass is the average of the mass numbers of the naturally occurring isotopes of an element. For example, naturally occurring copper consists of 69.17% copper atoms with a mass number of 63 amu and 30.83% copper atoms with a mass number of 65 amu. By multiplying the atomic mass of each isotope by its relative abundance and adding the two products, the average atomic mass of naturally occurring copper is calculated to be 63.55 amu.

The neutrons also play an important role in the stability of the nucleus. There is a significant correlation between the neutron to proton ratio (n/p) and the stability of the nucleus. Atoms with low atomic numbers (less than 20) are most stable when the neutron to proton ratio is 1:1. However, as the atomic number increases, more and more neutrons are needed to produce a strong nuclear force sufficient to hold the nucleus together. Thus, the neutron to proton ratio gradually increases in stable atoms to a maximum ratio of approximately 1.5 neutrons to 1 proton. This neutron to proton ratio range of 1.5 to 1 is known as the band of stability. Atoms with an n/p ratio within this band have stable nuclei, and atoms with a ratio outside the band of stability contain nuclei that are radioactive.

The Mass Spectrometer

A mass spectrometer is a device used to separate atoms of slightly different masses, thereby proving that many samples of naturally occurring elements are actually composed of a mixture of isotopes.

In a mass spectrometer, a sample is placed in a vacuum chamber, where it is vaporized. There, the vaporized sample of the element is bombarded with high energy electrons coming from a heated filament. When a particle in the sample is hit by one of these electrons, it becomes ionized, usually to a positive ion (cation) due to the loss of an electron. The ions are then focused into a narrow beam of particles and accelerated by an electric field. Because of their charge, the particles can be deflected by magnetic fields. The amount of deflection depends on the speed, charge, and mass of the particles.

The ions are deflected in an arc whose radius is inversely proportional to the mass of the ion. Lighter ions are deflected more than heavier ions. By varying the strength of the magnetic field, ions of different masses can be focused on a detector fixed at the end of a curved tube. If a sample contains particles of several different masses, each different mass corresponds to a different deflection. The resulting information is stored in and analyzed by a computer. This information will usually be presented as a bar graph or line graph. Once the mass number and percent of naturally occurring isotopes of an element is known, it is a relatively simple mathematical process to determine the atomic mass of an element as we shall see in the next section.

How to Calculate an Atomic Mass

In order to calculate the atomic mass of an element, two values must be known:
The exact mass for each naturally occurring stable isotope of the element and the percent
abundance of each isotope. These values can be found in chemistry and physics reference books. To find the atomic
mass of an element, multiply the exact mass of each isotope times the percent abundance (expressed as a decimal) of
the isotope. Then add the results for each isotope together, and the sum will be the atomic mass for the
element.

Electrons

The subatomic particle of most importance to the chemist is the electron, because of its involvement in chemical
reactions. Electrons are subatomic, negatively charged particles that surround the nucleus of an atom. The electrons
each have a single fundamental unit of negative electric charge, or –1. It is much lighter than either the proton or
neutron. One electron’s mass is approximately 1/1840^th of the mass of either a proton or neutron.

Scientists no longer think of electrons as following the fixed orbits as described by Bohr’s planetary model of the
atom. Instead, electrons form regions of negative charge around the nucleus which are referred to as energy levels. There are seven principal energy levels or regions around the nucleus where electrons are likely to be moving. Each principle energy level is divided into sublevels. Each sublevel is composed of a number of orbitals (not to be confused with the Bohr model’s planetary orbit) which can hold one or two electrons.

Obviously, the radii of the energy levels increase as they get further from the nucleus. The outer energy levels,
being larger, can contain more sublevels with a greater number of electrons. The maximum number of electrons for the
first four principle energy levels, as their distance from the nucleus increases, is summarized in the chart
below.

Electron Orbitals

In 1926, Erwin Schrödinger took the electron arrangement one giant step further based on the quantum theory. He developed mathematical equations describing the probable location and energy of an electron in a hydrogen atom. From his calculations, the quantum mechanical model of the atom was developed. Previous models, although very useful in providing a visual image of the basic atomic structure, were based on the observations of large objects in motion. The quantum mechanical model is not based on visual analogies, but rather on mathematical calculations. The quantum mechanical model agrees with the Bohr model that there are seven principal energy levels, but does not define the exact path of an electron around the nucleus. Instead, it describes the probability of finding an electron in a certain position which may be depicted as a blurry cloud of negative charge. In addition, within each principal energy level, the quantum mechanical model gives us one or more electron cloud shapes. This is because the mathematics of the quantum theory divides the principal energy levels into energy sublevels, labeled s, p, d, and f. Each orbital in each sublevel has its own unique cloud shape. All s orbitals are spherical and all p orbitals are dumbbell shaped. However, not all d or f orbitals have the same shape.

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