The Atomic Theory | General Chemistry 1

Elements:

Elements are fundamental substances that cannot be broken down into simpler substances. They are categorized as metals and nonmetals based on their properties. Metals are characterized by their conductivity, malleability, and ductility.

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

Atoms:

Atoms are the smallest constituent unit of matter that still retains the properties of matter. They are composed of protons, neutrons, and electrons.

Compounds:

Compounds are substances composed of two or more different elements. Some compounds, known as diatomic molecules, consist of two atoms of the same element bonded together.

The atomic theory was formulated by John Dalton in the early 19th century. Here are the key 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 consist of at least two atoms of different elements. These atoms combine to form molecules, the resulting particle.
• In a chemical reaction, atoms are rearranged, separated, or recombined to form new compounds. No atoms are created or destroyed.

Electrons:

Electrons are negatively charged subatomic particles discovered by Thomson in 1897 by deflecting cathode rays (electrons) through electric and magnetic fields. The charge-to-mass ratio (e/m) of electrons was then determined, revealing that electrons have a much smaller mass compared to protons and carry a negative charge.

Millikan's oil drop experiment provided a more precise measurement of the electron's charge, allowing for the determination of the absolute charge of the electron: e = 1.602177 x 10^-19 C.

Protons and neutrons:

Subatomic particles found in the nucleus of an atom:

• Protons have a positive charge and are located in the nucleus, determining the atom's identity and atomic number.
• Neutrons have no charge and are also located in the nucleus, contributing to the mass of the atom without affecting its charge.

The nucleus of an atom was discovered by Rutherfold in 1911. Rutherford's experiment involved bombarding thin gold foil with α particles and observing their scattering patterns. Most of the α particles passed through the foil, indicating that atoms are mostly empty space. Some α particles were deflected, suggesting the presence of a dense, positively charged nucleus.

Summary of atomic structure:

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

⇒ Despite their small size, atoms contain most of their mass in the nucleus.

Nuclear notation:

Isotopes:

Atoms of the same element with different numbers of neutrons. Most elements exist naturally as mixtures of isotopes.
 

All carbon atoms have the same number of protons Z = 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

Ion:

An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a positive or negative charge ⇒ its number of protons (positive charge) is different from its number of electrons (negative charge). 

The number of electrons gained or lost by an atom is related to its position in the periodic table ⇒ Atoms tend to gain or lose electrons to achieve the electron configuration of the nearest noble gas.

Cation vs. anion:

• The loss of an electron from an atom or molecule results in the formation of a cation, which carries a net positive charge.
• The gain of an electron by an atom or molecule leads to the formation of an anion, which carries 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

Monatomic ions:

• Monatomic cations:

Monatomic cations are named by adding the word "ion" to the name of the element. For elements with multiple possible charges (especially the transition metals), Roman numerals are used to indicate the charge.
 

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

• Monatomic anions:

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

Cl^- = chloride ion
O^2- = oxide ion

Oxoanions:

Oxoanions are 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