The Atomic Theory | General Chemistry 1

Atoms are studied in this chapter: elements in chemistry, atomic theory, atomic structure, difference between isotopes and ions, cations vs. anions, chemical nomenclature of ions and acids

Atoms and Elements


A substance that cannot be broken down into simpler substances. The elements can be divided into 2 broad classes: metals (good conductors of electricity and heat, malleable, ductile) and nonmetals. Only 7 elements are found as diatomic molecules in nature: H2, N2, O2, F2, Cl2, Br2, I2; the other elements are monoatomic. Chemical symbols are abbreviations used to denote the elements

H = Hydrogen, He = Helium, Li = Lithium



The smallest constituent unit of matter that still retains the properties of matter. An atom consists of protons, neutrons, and electrons: 

  • Proton: located in nucleus, charge = +1, mass ~ 1 amu = 1.661 x 10-27 kg
  • Neutron: located in nucleus, charge = 0, mass ~ 1 amu
  • Electron: located outside the nucleus, charge = -1, mass = 5.5 x 10-4 amu << 1 amu

⇒ most of the mass of an atom is concentrated in its nucleus



A substance with 2 or more elements. A compound or molecule made up of only 2 atoms is called a diatomic molecule

The Atomic Theory

The atomic theory was formulated by John Dalton. Here are the main points:

  • Matter is made up of atoms, small and indivisible particles
  • All atoms of the same element are identical and have the same mass
  • Atoms of different elements vary in size, mass and chemical behavior
  • Chemical compounds are made up of at least 2 atoms of different elements. The resulting particle is called a molecule
  • In a chemical reaction, atoms are rearranged, separated, or recombined to form new compounds but no atom is created or destroyed

Atomic Structure

Nuclear Notation:



X: Element symbol
Z: Atomic number = number of protons
Each element is characterized by a unique atomic number
A: Mass number = number of protons + neutrons = number of nucleons

Number of neutrons = A-Z
Number of electrons = number of protons - charge 



Atoms of the same element containing different numbers of neutrons. Most elements occur in nature as mixtures of isotopes

All carbon atoms have the same number of protons = 6
However, they can have a different number of neutrons: C612 (6 neutrons) and C613 (7 neutrons)
C612 and C613 are two isotopes of carbon

Ions - Cations & Anions


An atom or a molecule that has gained or lost one or more electrons giving it a negative or positive charge: its number of protons (positive charge) is different from its number of electrons (negative charge). The number of electrons an atom gains or loses is related to its position in the periodic table ⇒ an atom will gain or lose electrons to form ions with the same number of electrons as the nearest noble gas


Cation vs. anion:

The loss of an electron in an atom or molecule yields a cation, an ion with a net positive charge, while the gain of an electron yields an anion, an ion with a net negative charge

The atomic number of sodium Na is Z = 11 ⇒ 11 protons
Na is neutral ⇒ 11 electrons
Cation Na+ is positively charged ⇒ loss of 1 electron

Naming Ions

Monatomic ions:

Monatomic cations:

  • named by adding the word ion to the name of the elements
  • if there are more than one possible charge (especially for the transition metals), a Roman numeral is added behind the name of the elements to indicate the charge

Mg2+ = magnesium ion
Fe2+ = iron(II) ion
Fe3+ = iron(III) ion

Monatomic anions:

  • named by changing the ending of the element's name to -ide and adding the word ion

Cl- = chloride ion
O2- = oxide ion



Polyatomic anions that contain one or more oxygen atoms and one central atom of another element. They often occur in series of 2 or more ions with the same central atom but different numbers of oxygens

The most common form of the oxoanions in a series is named by changing the ending of the central element's name to -ate and adding the word ion

  • the ion with 1 less oxygen than the most common ionic form is called -ite ion
  • the ion with 2 fewer oxygens than the most common ionic form is called hypo- ... -ite ion
  • the ion with 1 more oxygen than the most common ionic form is called per- ... -ate ion

ClO3- = chlorate ion = most common form
ClO2- = chlorite ion
ClO- = hypochlorite ion
ClO4- = perchlorate ion

Naming Acids

Binary acids:

Acids formed by an anion and a hydrogen H+. They are named:

  • by adding the prefix hydro- in front of the name of the anion
  • by changing the -ide ending on the anion to -ic
  • by adding the word acid at the end

HCl = hydrochloric acid



Acids that produce hydrogen ions and the corresponding oxoanions when dissolved in water. An oxoacid based on an -ate ion is called ... -ic acid while an oxoacid based on an -ite ion is called ... -ous acid 

NO3- = nitrate ⇒ HNO3 = nitric acid
NO2- = nitrite ⇒ HNO2 = nitrous acid