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

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

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.

Atomic Structure

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.

Atomic Numbers

Nuclear notation:

 

 

X: Element symbol representing a specific element.
Z: Atomic number = number of protons in the nucleus
     Each element is characterized by a unique atomic number.
A: Mass number = number of protons + number of neutrons = number of nucleons.


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

 

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 = 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

Cations & Anions

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

Naming Ions

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
O2- = 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

Check your knowledge about this Chapter

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