# 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.

## Atoms

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:

### ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$

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

## Isotopes

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

${}_{6}{}^{12}\mathrm{C}$ and ${}_{6}{}^{13}\mathrm{C}$ are two isotopes of carbon.
They have both 6 protons but different numbers of neutrons (6 and 7 neutrons).

## Ions

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