CHEMICAL ELEMENTS

Chemical Elements, substance made up only of atoms that all have the same atomic number. The atomic number is the number of protons in the atom’s nucleus: element atomic number 1 hydrogen has one proton in the nucleus and the element atomic number 92 uranium has 92 protons. Ninety-four natural elements have been detected in the universe. More than 110 elements have been identified, with some created only in laboratories as artificial elements. Under certain conditions, one element may be changed into another element through processes that add or remove protons from a nucleus.

CHEMICAL ELEMENTSAlthough the number of protons in the nucleus

of a particular chemical element is always the same, the number of neutrons can vary, creating isotopes of that element that have different atomic masses and physical properties. The chemical properties of an element are mainly determined by the number of electrons in the outermost shell of an atom of that element. The number of electrons is the same as the number of protons in the nucleus of an element. The unit for atomic weight of the elements is one-twelfth of the weight of the carbon-12 atom, which is arbitrarily set at 12.

CHEMICAL ELEMENTSAtoms of a single element may combine to form

molecules of that element—two atoms of oxygen combine to form oxygen molecules, eight atoms of sulfur combine to form sulfur molecules. Atoms of different elements may combine to form molecules of chemical compounds—two hydrogen atoms combine with one oxygen atom to form molecules of water. Unlike chemical compounds, elements cannot be broken down into simpler substances by ordinary heat, light, electricity, or chemical reactions. Like other forms of matter, most elements can exist as gases, liquids, or solids, depending on the pressure and temperature.

CHEMICAL ELEMENTS

Scientists use one- and two-letter symbols for each element. In some cases these symbols are based on old Greek or Latin names for the elements and don’t correspond to their common English names. For example, hydrogen is H, but gold is Au, from Latin aurum.

ORIGIN OF ELEMENTSThe process of building up the nucleus of an element from

protons and neutrons is called nucleosynthesis. This process only takes place under extreme conditions of pressure and temperature. The first period of nucleosynthesis occurred in the early moments of the universe following the big bang, creating hydrogen and helium. When the first stars formed, another type of nucleosynthesis began in the cores of stars, fusing hydrogen into helium, and releasing energy. The fusion processes continued to create the elements carbon (6 protons), nitrogen (7 protons), and oxygen (8 protons) on up to iron (26 protons). The fusion of iron does not release energy and stars that reach this stage suffer a core collapse, usually exploding as supernovas. When supernovas explode, the extreme conditions can create even heavier elements, up to uranium.

ABUNDANCE OF ELEMENTSThe relative abundance of elements in the universe

reflects how the elements were formed. The big bang created the protons that form all the hydrogen nuclei in the universe. Hydrogen accounts for about 73 percent of the ordinary matter in the universe by mass, helium about 25 percent, and all other elements less than 2 percent. By number of atoms, about 90 percent of the atoms in the universe are hydrogen, about 9 percent are helium, and all the other elements account for less than 1 percent. Processes in stars, and particularly supernova explosions, determined the abundance of other elements. Oxygen (8 protons) is the most abundant, followed by carbon (6 protons), nitrogen (7 protons), neon (10 protons), silicon (14 protons), magnesium (12 protons), and iron (26 protons).

ABUNDANCE OF ELEMENTSThe particular abundance of elements

found on Earth reflects the processes that formed our planet from material that became the Sun and the solar system. For example, Earth was not massive enough to retain helium from the original material that formed the solar system. The helium now found in Earth is the result of radioactive decay of minerals that release alpha particles, which have 2 protons and 2 neutrons, the same as helium nuclei.

CLASSIFICATION OF ELEMENTSThe particular abundance of elements

found on Earth reflects the processes that formed our planet from material that became the Sun and the solar system. For example, Earth was not massive enough to retain helium from the original material that formed the solar system. The helium now found in Earth is the result of radioactive decay of minerals that release alpha particles, which have 2 protons and 2 neutrons, the same as helium nuclei.