the charge and mass of the negative subatomic particles—the electrons
the positively charged subatomic particles—the protons (p+)—are much more massive than electrons
highly concentrated in the atom’s nucleus
A third subatomic component that bears no electrical charge—the neutrons (n0)—have masses similar to protons and co-locate with protons in the nucleus.
When describing the properties of tiny objects such as atoms, we use appropriately small units of measure, such as the atomic mass unit (amu) and the fundamental unit of charge (e).
it has been defined with regard to the most abundant isotope of carbon, atoms of which are assigned masses of exactly 12 amu.
Thus, one amu is exactly of the mass of one carbon-12 atom.
e = 1.602 × 10−19 C.
The number of protons in the nucleus of an atom is its atomic number (Z).
the defining trait of an element
The total number of protons and neutrons in an atom is called its mass number (A).
atomic number (Z) = number of protons mass number (A) = number of protons + number of neutrons A – Z = number of neutrons
Isotopes of an element are atoms with the same atomic number but different mass numbers
differ from each other only in the number of neutrons within the nucleus
When the numbers of these subatomic particles are not equal, the atom is electrically charged and is called an ion.
Atomic charge = number of protons − number of electrons
An atom that gains one or more electrons will exhibit a negative charge and is called an anion.
Positively charged atoms called cations are formed when an atom loses one or more electrons.
A chemical symbol is an abbreviation that we use to indicate an element or an atom of an element.