Atomic Structure

Alkali Metal

Alkali metals are elements in Group 1 of the periodic table. They are the most reactive metals because their atoms can easily give away their one outer-level electron. Alkali metals are so reactive that in nature they are found only combined with other elements.

Alkaline-Earth Metal

Alkaline-earth metals are elements in Group 2 of the periodic table. They are less reactive than alkali metals because alkaline-earth metals have two outer-level electrons. It is more difficult for atoms to give two electrons than to give one when joining with other atoms.

Ion

An atom that has gained or lost one or more electrons and has a negative or positive charge.

Period

Each horizontal row of elements (from left to right) on the periodic table is called a "period." The physical and chemical properties of elements in a row follow a repeating, or periodic, pattern as you move across the period.

Group

Each vertical column of elements (from top to bottom) on the periodic table is called a "group." Elements in the same group often have similar chemical and physical properties, so is also called a "family."

Halogen

Halogens (Group 17) are very reactive nonmetals because their atoms need to gain only one electron to have a complete outer level. The atoms of halogens combine readily with other atoms, especially metals, to gain that missing electron. The reaction of a halogen with a metal makes a salt, such as sodium chloride.

Isotope

Isotopes are atoms that have the same number of protons, but have a different numbers of neutrons. Atoms that are isotopes of each other are always the same element, because isotopes always have the same number of protons. But, they have different numbers of neutrons, which gives them different atomic masses. Isotopes of an element share most of the same chemical and physical properties. And, most elements contain a mixture of two or more isotopes. Some isotopes of an element have special properties because they are unstable. An unstable atom has a nucleus that will change over time. This type of isotope is radioactive. Radioactive atoms fall apart after a certain amount of time. When they do, they give off smaller particles, as well as energy.

Electron

Negatively charged (-1) particles that are present in every atom. Electrons orbit the atom's nucleus in electron clouds. Electrons have very little mass (0 amu).

Neutron

Neutrons are particles in the nucleus of the atom that have no electrical charge (0). Neutrons are slightly more massive than protons, but still have amu=1.

Nobel Gas

Noble gases are stable (non-reactive) nonmetals and are in Group 18 of the periodic table. The atoms of these elements have a full set of electrons in their outer level. So, they do not need to lose or gain any electrons. Under normal conditions, they do not react with other elements. Noble gases are found in small amounts and are relatively rare.

Periodic

Periodic means "happening at regular intervals." If something is periodic, then it has a predictable, repeating pattern (like days of a week).

Proton

Protons are positively charged (+1) particles in the nucleus of an atom. Protons have amu=1. The number of protons in an atom is equal to the atomic number of that element, and can be used to uniquely identify the element.

Electron Cloud

Region in the atom where electrons are likely to be found. In the current model of the atom, electrons surround the nucleus of an atom in electron clouds. The size of electron clouds determines the size (diameter and volume) of the atom.

Atomic Mass

The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element. A weighted average accounts for the percentages of each isotope that are present.

Nucleus

The center of the atom, which is small, dense and positively charged. The nucleus contains protons and neutrons, which account for most of the atom's mass. The diameter of the nucleus is about 1/100,000th the diameter of the atom.

Mass Number

The mass number is the sum of the protons and neutrons in an atom. Electrons are not included in an atom's mass number because they have so little mass. The mass number is similar, but not exactly equal, to an element's atomic mass.

Atomic Number

The number of protons in the nucleus of an atom is the atomic number of that atom. All atoms of an element have the same atomic number.

Periodic Law

The periodic law states that the repeating chemical and physical properties of elements change periodically with the elements' atomic numbers. The periodic law is responsible for element groups (or families) on the periodic table.

Atom

The smallest particle into which an element can be divided and still be the same substance.

Atomic Mass Unit (amu)

The unit used to measure the mass of particles in an atom. Protons and neutrons have amu=1. Electrons have amu=0. 1 amu ≈ 1.7 x 10-24 g