Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Periodic Table

Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and physical properties. Elements, such as oxygen, iron, and gold, are the most basic chemical substances and cannot be broken down by chemical reactions. All other substances are formed from combinations of elements. The periodic table provides a means of arranging all the known elements and even those yet to be discovered.

See Alphabetic list of Chemical Elements


The elements within the modern periodic table are arranged from left to right, top to bottom, in order of increasing atomic number. An element’s atomic number is the number of protons in its nucleus. There are 92 naturally occurring elements, ranging from hydrogen, which has atomic number 1, to uranium, whose atomic number is 92. The periodic table also includes artificially created elements, whose atomic numbers are higher than 92. These additions must be prepared through nuclear reactions. The most recent element to be synthesized was an element with 114 protons in the nucleus of each of its atoms. None of the artificially created elements with atomic numbers higher than 109 have yet been officially named.

Whereas the ordering of the elements is completely determined by their atomic numbers, the arrangement into vertical columns, called groups, is determined by a number of factors. These factors include chemical properties, physical properties, and the number of electrons thought to exist in the outer shells of the element’s atoms. (The electrons that surround the nucleus of an atom are arranged in concentric shells.)

The placement of elements into groups within the periodic table is not completely clear-cut. Some scientists disagree about minor differences in the placement of elements such as hydrogen and helium. Helium, which does not react with other elements, is usually placed in group 18, which houses the noble gases. This group also includes neon, argon, and krypton, all of which are also very unreactive. Scientists who group the elements based primarily on the number of outer-shell electrons place helium with elements such as magnesium, calcium, and barium among the alkaline earth metals of group 2. Elements in group 2 have two electrons in their outermost shell.

The periodic table has been published in various shapes and sizes, but the most commonly used modern form begins with a column of group 1 metals on the left-hand side, followed by a column of group 2 alkaline earth metals. These columns are followed by a block of 40 elements divided into ten columns of four elements each. The groups in this block, collectively called the transition metals, are numbered 3 through 12. Groups 13 through 18 make up the right-hand side of the table. A diagonal dividing line separates the nonmetals in the upper-right portion of this block, such as oxygen, carbon, and nitrogen, from the metals such as tin and lead in the lower left portion.

There is an additional block of 28 elements, divided into two rows of 14 elements each, that is usually placed beneath the main table. These are the rare earth elements, whose properties are all remarkably similar. They are so similar to one another that chemists have difficulty separating them when they occur together as mixtures. This additional block really belongs between the first block, consisting of groups 1 and 2, and the transition metal block. For convenience it is placed at the bottom of the table rather than in its proper place. Otherwise the periodic table would be very wide and would not lend itself to being represented on wall charts.

Scientists refer to the horizontal rows in the periodic table as periods. Periods vary in length. Moving through the table from top to bottom, the successive periods contain 2, 8, 8, 18, 18, 32, and 32 elements. These numbers correspond to the maximum number of electrons that can be accommodated in the largest electron shell in an atom of any element belonging to that period.