Atoms and Molecules | General Chemistry 1

Atoms and molecules are studied in this chapter: elements in chemistry, states of matter, atomic theory, difference between atoms, isotopes and ions, chemical nomenclature of compounds.

Elements in Chemistry

Elements: substances that cannot be broken down into simpler substances. They can be divided into two broad classes: metals (good conductors of electricity and heat, malleable, ductile) and nonmetals.

Compounds: substances with two or more elements. A compound or molecule consisting of just two atoms is called diatomic molecule.

Chemical symbols are abbreviations used to designate the elements.


H = Hydrogen, He = Helium, Li = Lithium


7 elements are found as diatomic molecules in nature: H2, N2, O2, F2, Cl2, Br2, I2

States of Matter

The states of matter are solid, liquid and gas:

A solid (s) ⇒ fixed volume + fixed shape.
The particles form an ordered lattice / network of atoms or molecules

A liquid (l) ⇒ definite volume but not a specific shape. It fills the container.
The particles are held together by forces but are still free to randomly move

A gas (g) ⇒ fills the entire volume of the container (no definite shape).
The particles rapidly move about the full volume of the container

Phase changes:

Condensation (gas → liquid), Freezing (liquid → solid)
Melting (solid → liquid),  Boiling (liquid → gas),
Sublimation (solid → gas), Deposition (gas → solid)

The Atomic Theory

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

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


Atom = smallest constituent unit of matter that constitutes a chemical element.

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: outside nucleus, charge = -1, mass = 5.5 x 10-4 amu << 1 amu
⇒ most of the mass of an atom is concentrated in its nucleus.

Nuclear Notation:


X = chemical symbol of the element
Z = atomic number = number of protons
Each element is characterized by a unique atomic number.
A= mass number = total number of protons and neutrons



Atoms of the same element always have the same number of protons Z.


All carbon atoms have 6 protons.


However, atoms can have a different number of neutrons.
Isotopes = atoms of the same element containing different numbers of neutrons.
Most elements occur in nature as mixtures of isotopes.
The number of neutrons N = A - Z


C612 and C613 are two isotopes of carbon. 
They have both 6 protons but different numbers of neutrons (6 and 7 neutrons).


Ions = charged particles
⇒ atoms or molecules with a charge due to the loss or gain of one or more electrons
⇒ number of protons (charge +1) ≠ number of electrons (charge -1)

Ions positively charged = cations.
Ions negatively charged = anions.


Na+ = singly charged sodium ion = sodium cation
Cl- = singly charged chloride ion = chloride anion


The number of electrons an atom loses is related to its position on the periodic table
⇒ an atom will gain or lose electrons to form ions with the same number of electrons as the nearest noble gas.

Polyatomic Ions Naming

Ions can be polyatomic.
Names of polyatomic ions (general rules):
- end in -ate
- with 1 oxygen less than the most common ionic form: end in -ite
- with 2 oxygens less than the most common ionic form: prefixe hypo- + end in -ite
- with 1 oxygen more than the most common ionic form: prefixe per-
- with hydrogen attached: prefixe hydrogen


ClO3- = chlorate = most common form
ClO2- = chlorite
ClO- = hypochlorite
ClO4- = perchlorate
HPO42- = hydrogen phosphate

Chemical Nomenclature

Naming binary compounds:

1 metal element + 1 nonmetal element or polyatomic ion:

Name of the compound = name of the metal + name of the nonmetal, with the suffix ide or the name of the polyatomic ion. If the metal has a variable charge metal, the charge is indicated in bracket with roman numbers.


CaS = calcium sulfide
CaBr2 = calcium bromide
Fe2(SO4)3 = iron(III) sulfate


2 nonmetals elements:

Prefixes are used to indicate the number of atoms of a given element in a molecule (mono- = 1, di- = 2, tri- = 3, tetra- = 4 …). The prefix mono- is generally omitted. The final a or o of the prefix is combined with a name beginning with a vowel.

CO2 = carbon dioxide
PCl5 = phosphorus pentachloride

Acids Naming

Binary acids (one anion and one hydrogen):
prefix hydro- + first syllable of the anion + suffix -ic + acid.


HCl = hydrochloric acid


Acids having oxygen in the compound:
polyatomic ions with an end in -ate finish in -ic for the acids
polyatomic ions with an end in -ite finish in -ous for the acids


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