Elements and Atomic Number Study Notes

Elements and Atomic Number

Definition of an Atom

• Atoms are the fundamental units of matter that define elements.
• Modern atomic theory posits that:
  • Atoms of one element are identical.
  • Atoms of different elements are different.

Characteristics of Atoms

• Protons: The fundamental characteristic that differentiates atoms of different elements is the number of protons in their nucleus.
  • Example:
  • Hydrogen (H) has 1 proton.
  • Iron (Fe) has 26 protons.

Atomic Number (Z)

• The atomic number is defined as the number of protons in an atom.
  • Each element has a unique atomic number:
  • Hydrogen has an atomic number of 1.
  • Iron has an atomic number of 26.
• The atomic number also indicates the number of electrons in a neutral atom, leading to the principle of electrical neutrality where:
  • Number of protons (positive charge) = Number of electrons (negative charge).
• Example:
  • Helium has atomic number Z = 2:
  • 2 protons
  • 2 electrons

Ions

• When atoms gain or lose electrons, they become charged (ions).
• This results in an unequal number of protons and electrons, breaking electrical neutrality.

Mass Number (A)

• The mass number is defined as the total number of protons and neutrons in an atom's nucleus: $A = Z + N$ where:
  • A = mass number
  • Z = atomic number (number of protons)
  • N = number of neutrons
• Atoms of the same element can have different numbers of neutrons, resulting in different mass numbers, known as isotopes.

Isotopes

• Isotopes are variants of elements which have the same number of protons but different numbers of neutrons.
• Example:
  • Common carbon isotopes have 6 protons and 6 neutrons (mass number = 12).
• Isotopes are crucial to determine properties and applications of elements in various fields.

Practical Examples

Carbon Isotope Example

• For a carbon atom:
  • Atomic number: 6
  • Neutrons: 6
  • Therefore, mass number: 6 + 6 = 12.

Uranium Isotope Example

• An isotope of uranium:
  • Atomic number: 92
  • Mass number: 235
  • Therefore:
    • Protons = 92
    • Neutrons = Mass number - Protons = 235 - 92 = 143.

Additional Example

Tin Isotope Example

• A tin atom:
  • Protons: 50
  • Neutrons: 68
  • Therefore:
    • Atomic number = 50
    • Mass number = 50 + 68 = 118.

Summary of Key Points

• Atoms are comprised of protons, neutrons, and electrons.
• Each element has a distinct atomic number corresponding to its number of protons.
• Isotopes of an element differ in their neutron count.
• Elements are represented using their atomic symbol.