Atomic Structure

atom:  the smallest piece of an element that retains the properties of that element.

nucleus:  a dense region in the center of an atom.  The nucleus is made of protons and neutrons, and contains almost all of an atom’s mass.

proton:  a subatomic particle found in the nucleus of an atom.  It has a charge of +1, and a mass of 1 atomic mass unit (amu).

neutron:  a subatomic particle found in the nucleus of an atom.  It has no charge (is neutral), and has a mass of 1 amu.

electron:  a subatomic particle found outside the nucleus of an atom.  It has charge of −1 and a mass of 0 amu (really about ¹/₂₀₀₀ amu).  Atoms can gain, lose, or share electrons in chemical reactions.

charge:  positive and negative charges cancel each other out, so the charge of an atom is the difference between how many positive charges (protons) it has, and how many negative charges (electrons) it has.  For example, an chlorine atom with 17 protons (+17) and 18 electrons (−18) would have a charge of −1.  (The difference is 1, and it’s negative because it has more negatives than positives.)

neutral atom: an atom with a charge of zero (positives = negatives).

ion:  an atom or molecule that has a positive or negative charge, because it has either more negatives (electrons) than positives (protons), or more positives (protons) than negatives (electrons).

atomic number:  the identity of an atom is based on the amount of (positive) charge in its nucleus.  (This works because because the nucleus cannot be given to or shared with another atom.)  The atomic number is the number of protons in the nucleus.  Each element has a unique atomic number.

mass number:  the mass of an atom is the mass of its nucleus.  (The electrons are so small that we can ignore their mass.)  Because protons and neutrons each have a mass of 1 amu, the mass number for the atom is just the number of protons + neutrons that the atom has.

isotopes:  atoms of the same element (same atomic number = same # of protons), but that have different numbers of neutrons (and therefore different mass numbers) from each other.

Isotopes are described by their mass numbers.  For example, carbon-12 has 6 protons and 6 neutrons, which gives it a mass number of 12.  Carbon-14 has 6 protons and 8 neutrons, which gives it a mass number of 14.

element symbol:  a one- or two-letter abbreviation for an element.  (New elements are given temporary three-letter symbols.)  The first letter in an element symbol is always capitalized.  Other letters in an element symbol are always lower case.  This is important to remember.  For example, Co is the element cobalt, but CO is the compound carbon monoxide, which contains the elements carbon and oxygen.

chemical symbol:  a shorthand notation that shows information about an element, including its element symbol, atomic number, mass number, and charge.  For example, the symbol for a sodium-23 ion with a +1 charge would be:

This notation shows the element symbol for sodium (Na) in the center, the atomic number (11, because it has 11 protons) on the bottom left, the mass number (23, because it has 11 protons + 12 neutrons = 23 amu) on the top left, and the charge (+1, which means it lost one of its electrons in a chemical reaction) on the top right.