Lesson 1 – Introduction to Atoms

 Atoms are made of even smaller particles called neutrons, protons, and electrons.  An atom consists of a nucleus surrounded by one or more electrons.  The number of protons equals the number of electrons.  As a result, the positive charge from the protons equals the negative charge from the electrons. The charges balance, making the atom neutral.

 Electrons move within a sphere-shaped region surrounding the nucleus.  Most of an atom’s volume is the space in which electrons move.

 Although electrons occupy most of an atom’s volume, they don’t account for much of its mass.  Together, the protons and neutrons make up nearly all the mass of an atom.  Atoms are too small to be measured in everyday units of mass, such as grams or kilograms, so scientists use units known as atomic mass units (amu)

 An element can be identified by the number of protons in the nucleus of its atom.

 Every atom of an element has the same number of protons.  Each element has a unique atomic number – the number of protons in its nucleus.

 Although all atoms of an element have the same number of protons, their number of neutrons can vary.  An isotope is identified by its mass number, which is the sum of the protons and neutron in the nucleus of an atom.

 Because atoms are so small, scientists create models to describe them.  Scientists use models to study objects and events that are too small, too large, too slow, too fast, too dangerous, or too far away to see.  In chemistry, models of atoms are used to explain how matter behaves.

 The chemical elements can be organized into something like a calendar. The name of the “chemists” calendar Is the periodic table.  A Russian scientist, Dmitri Mendeleev discovered a set of patterns that applied to all the elements.

 He noticed that a pattern of properties appeared when he arranged the elements in order of increasing atomic mass.  To find a pattern, Mendeleev wrote each element’s melting point, density, and color on individual cards. Along with the atomic mass and number of chemical bonds it could form.  Mendeleev found that the properties of elements repeated.  Mendeleev found that arranging the known elements strictly by increasing atomic mass did not always group similar elements together.  Mendeleev published the first periodic table in 1869.