neutrons are neutral particles with no electrical charge, also with a mass of one unit (one a.m.u.).protons are small, positively charged particles with, by definition, a mass of one unit (one a.m.u.).There are three types of subatomic particles that are found in the atoms of all but one element. the physical and chemical properties of all elements are the result of the origination of substructure of their atoms.the unique chemical properties of just a few atoms determine the form and function of all living things.The three most abundant elements on earth are oxygen, silicon and aluminum in living organisms the six most abundant elements are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. there are 92 natural elements and 13 more that have been created in a laboratory.all the atoms in a pure substance or element are identical to one another but different to the atoms in a different element.each element is made up of identical particles of matter called atoms.a pure or elementary substance cannot be broken down into smaller or simpler constituents without loosing all its original properties such pure substances are called elements.common substances, such as air, water, wood and cloth are mixtures of materials and can be chemically or physically broken down into simpler substances.Make sure that you have written down the following definitions, explanations and important concepts in your notes.Īn atom is the smallest unit of a pure substance or element that can exist and still retain the properties of the original substance or element. Use this section to check up on the accuracy of your lecture notes. It is obvious, then, that the number of protons in a nucleus (or, the nuclear charge, or the atomic number) determines the chemical properties of an element.Science at a Distance Science at a Distance The number of electrons in an atom, in turn, is determined by the nuclear charge. The reason is that an element's chemical properties depend on the number and arrangement of electrons in its atoms.

When atomic number, rather than atomic mass, is used to construct a periodic table, these problems disappear. Certain pairs of elements (tellurium and iodine constitute one example) appear to be misplaced when arranged according to their masses. Although he was essentially correct, the periodic table constructed on this basis had three major flaws.

Mendeleev had said that the properties of the elements vary in a regular, predictable pattern when the elements are arranged according to their atomic masses. Moseley's discovery made possible a new understanding of the periodic law first proposed by Dmitri Mendeleev in the late 1850s. Moseley hypothesized that the regular change in wavelength from element to element was caused by an increase in the positive charge on atomic nuclei in going from one element to the next heavier element.

He discovered that the wavelength of the reflected x rays decreases in a regular predictable pattern with increasing atomic mass. Moseley bombarded a number of chemical elements with x rays and observed the pattern formed by the reflected rays.

The concept of atomic number evolved from the historic research of Henry Gwyn-Jeffreys Moseley in the 1910s. In nuclear chemistry, an element's atomic number is written to the left and below the element's symbol since an element's atomic number can always be determined simply by knowing its symbol, however, the former is often omitted from a nuclear symbol, as in 16O, where the superscript represents the atomic mass. It is always the smaller whole number found in association with an element's symbol in the table. The atomic number of an element can be read directly from any periodic table.

Oxygen atomic number