Chemistry - Atomic Structure

Nucleus

• the center of the atom

• composed of protons and neutrons

Protons

• found in the nucleus

• determines the identity of an atom/what element an atom is

• heavy in mass (about 1 amu)

• positively charged (+1)

Atomic Number

the number of protons

identifies an element

• all atoms of the same element will have the same atomic number

• no two elements have the same atomic number

Neutrons

• found in the nucleus

• neutral charge (0)

• heavy in mass (about 1 amu)

• holds the protons together in the nucleus — because the protons of an atom all have a positive charge, they would repel from one another without the neutrons

Electrons

• located outside the nucleus

• in constant motion — gives an atom that “fuzzy edge”

• negatively charged (-1)

• virtually no mass; smallest of the 3 subatomic particles

• responsible for many of the properties of individual atoms and are involved in all chemical reactions

Neutral Atoms

• contains the same number of electrons as protons

• have a neutral charge (0)

Mass Number

• the total number of protons and neutrons in an atom

• most of the mass depends on the nucleons 

Number of Neutrons

rarely obvious

will have to do a math problem to find the number of neutrons;

• Mass Number - Atomic Number = Number of Neutrons

• not given a specific mass number, use the periodic table’s average atomic mass and round to the nearest whole number

• given a specific mass number, use that as your mass; not the periodic table

Isotopes

• atoms of the same element with different numbers of neutrons

• only way to tell them apart is by their masses

Nuclear Reaction

• difference in mass/amount of neutrons is important to consider

• examples are radioactive decay and fission

Isotope Symbols

X = element symbol

A = Mass Number

Z = Atomic Number

Isotope Names

another way to refer to isotopes

simply add the mass number to the name of the element

• example: boron-11

Average Atomic Mass

• shown on each square of the periodic table

• equation: %/100 * amu + %/100 * amu