# 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

Element:

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

Atom:

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

Compound:

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

Isotopes:

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: ${}_{6}{}^{12}\mathrm{C}$ (6 neutrons) and ${}_{6}{}^{13}\mathrm{C}$ (7 neutrons)
${}_{6}{}^{12}\mathrm{C}$ and ${}_{6}{}^{13}\mathrm{C}$ are two isotopes of carbon

## Ions - Cations & Anions

Ion:

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

Oxoanions:

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

Oxoacids:

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

An element is the simplest form of a substance and cannot be broken down into simpler substances while an atom is the part of an element. 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.

An element is a substance that cannot be broken down into simpler substances while a compound is a substance with 2 or more elements.

An atom is the smallest constituent unit of matter which still retains the properties of matter. It consists of protons (charge = +1, mass ~ 1 amu) and neutrons (charge = 0, mass ~ 1 amu) in the nucleus, and electrons (charge = -1, mass = 5.5 x 10-4 amu) outside of the nucleus.

The existence of neutron was proved by the English physicist James Chadwick who bombarded a thin sheet of beryllium with α particles. A very high-energy radiation was emitted by the metal that was not deflected by either electric or magnetic fields. This experiment proved the existence of a third type of subatomic particle, which Chadwick named neutrons because they were electrically neutral.

The atomic theory states that all matter is made up of extremely small particles called atoms, which cannot be subdivided, created, or destroyed. Atoms of a given element are identical in size, mass, and other properties while atoms of different elements are different. This theory provides an explanation of both the Law of constant composition and the law of conservation of mass.

• 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

The atomic number (Z) is the number of protons in an atom of an element. Each atom of an element has the same number of protons.

The number of protons is equal to the atomic number (Z) of the element. To find the number of neutrons, subtract the number of protons from the mass number (A). To find the number of electrons, subtract the charge of the element from the number of protons.

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

An ion is 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 charge of an atom is the number of protons minus the number of electrons.

To find the number of electrons, subtract the charge of the ion from the number of protons. If the charge is positive, there are more protons than electrons in the ion. If the charge is negative, there are more electrons than protons.

A cation is an ion with a net positive charge due to the loss of an electron, while an anion is an ion with a net negative charge due to the gain of an electron.

Monatomic cations are 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.

Monatomic anions are named by changing the ending of the element's name to -ide and adding the word ion.

An oxoanion is a polyatomic anion that contains one or more oxygen atoms and a central atom of another element. Oxoanions 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

An oxoacid is an acid that produces hydrogen ions and the corresponding oxoanions when dissolved in water.

An oxoacid which produces an oxoanion ending in -ate is called [oxoanion]-ic acid while an oxoacid which produces an oxoanion ending in -ite is called [oxoanion name]-ous acid .