HiSET Science: Atomic and Subatomic Structure

Basic Organization of Matter
An element is the most basic type of matter. It has unique properties and cannot be broken down into other elements.

The smallest unit of an element is the atom.
A chemical combination of two or more types of elements is called a compound. Compounds often have properties that are very different from those of their constituent elements.
The smallest independent unit of an element or compound is known as a molecule.
Most elements are found somewhere in nature in single-atom form, but a few elements only exist naturally in pairs. These are called diatomic elements, of which some of the most common are hydrogen, nitrogen, and oxygen.
Elements and compounds are represented by chemical symbols, one or two letters, most often the first in the element name. More than one atom of the same element in a compound is represented with a subscript number designating how many atoms of that element are present.
Water, for instance, contains two hydrogens and one oxygen. Thus, the chemical formula is H2O. Methane contains one carbon and four hydrogens, so its formula is CH4.

Protons, Neutrons, and Electrons
The three major subatomic particles are the proton, neutron, and electron.
The proton, which is located in the nucleus, has a relative charge of +1.
The neutron, which is located in the nucleus, has a relative charge of 0.
The electron, which is located outside the nucleus, has a relative charge of –1.
The proton and neutron, which are essentially the same mass, are much more massive than the electron and make up the mass of the atom.
The electron's mass is insignificant compared to the mass of the proton and neutron.

Orbits and Orbitals
An orbit is a definite path, but an orbital is a region in space. The Bohr model described electrons as orbiting or following a definite path in space around the nucleus of an atom. But, according to Heisenberg's uncertainty principle, it is impossible to determine the location and the momentum of an electron simultaneously. Therefore, it is impossible to draw a definite path or orbit of an electron. An orbital as described by the quantum-mechanical model or the electron-cloud model is a region in space that is drawn in such a way as to indicate the probability of finding an electron at a specific location.

The distance an orbital is located from the nucleus corresponds to the principal quantum number.
The orbital shape corresponds to the subshell or azimuthal quantum number. The orbital orientation corresponds to the magnetic quantum number.

Quantum Numbers
The principal quantum number (n) describes an electron's shell or energy level and actually describes the size of the orbital.

Electrons farther from the nucleus are at higher energy levels. The subshell or azimuthal quantum number (l) describes the electron's sublevel or subshell (s, p, d, or f) and specifies the shape of the orbital.

Typical shapes include spherical, dumbbell, and clover leaf. The magnetic quantum number (ml) describes the orientation of the orbital in space. The spin or magnetic moment quantum number (ms) describes the direction of the spin of the electron in the orbital.

Atomic Number and Mass Number
The atomic number of an element is the number of protons in the nucleus of an atom of that element. This is the number that identifies the type of an atom.
For example, all oxygen atoms have eight protons, and all carbon atoms have six protons. Each element is identified by its specific atomic number.
The mass number is the number of protons and neutrons in the nucleus of an atom. Although the atomic number is the same for all atoms of a specific element, the mass number can vary due to the varying numbers of neutrons in various isotopes of the atom.

Isotopes
Isotopes are atoms of the same element that vary in their number of neutrons. Isotopes of the same element have the same number of protons and thus the same atomic number. But, because isotopes vary in the number of neutrons, they can be identified by their mass numbers. For example, two naturally occurring carbon isotopes are carbon-12 and carbon-13, which have mass numbers 12 and 13, respectively. The symbols

represents the element symbol, M represents the mass number, and A represents the atomic number.

Average Atomic Mass

The average atomic mass is the weighted average of the masses of all the naturally occurring isotopes of an atom in comparison to the carbon-12 isotope. The unit for average atomic massis the atomic mass unit (u). Atomic masses of isotopes are measured using a mass spectrometer by bombarding a gaseous sample of the isotope and measuring its relative deflections. Atomic masses can be calculated if the percent abundances and the atomic masses of the naturally occurring isotopes are known.

Cathode Ray Tube (CRT)

Electrons were discovered by Joseph John Thomson through scientific work with cathode ray tubes (CRTs). Cathode rays had been studied for many years, but it was<br> Thomson who showed that cathode rays were negatively charged particles.

Although Thomson could not determine an electron.

Gold Foil Experiment
After Thomson determined the ratio of the charge to the mass of an electron from studying cathode rays, he proposed the plum pudding model, in which he compared electrons to the raisins embedded in plum pudding. This model of the atom was disproved by the gold foil experiment. The gold foil experiment led to the discovery of the nucleus of an atom. Scientists at Rutherford's laboratory bombarded a thin gold foil with high-speed helium ions. Much to their surprise, some of the ions were reflected by the foil. The scientists concluded that the atom has a hard central core, which we now know to be the nucleus.


Problems that Rutherford's Model Had with Spectral Lines
Rutherford's model allowed for the electrons of an atom to be in an infinite number of orbits based on Newton's laws of motion.
Rutherford believed that electrons could orbit the nucleus at any distance from the nucleus and that electrons could change velocity and direction at any moment. But, according to Rutherford's model, the electrons would lose energy and spiral into the nucleus. Unfortunately, if this was in fact true, then every atom would be unstable. Rutherford's model also does not correspond to the spectral lines emitted from gases at low pressure. The spectral lines are discrete bands of light at specific energy levels. These spectral lines indicate that electrons must be at specific distances from the nucleus. If electrons could be located at any distance from the nucleus, then these gases should emit continuous spectra instead of spectral lines.