Understanding Atoms: Mass, Number, Ions, and Isotopes

Understanding Atoms: Key Concepts

Atom: The smallest unit of a chemical element that retains its identity and properties, and cannot be split by chemical processes.

Mass Number: Represents the sum of protons and neutrons present in the atomic nucleus. Also known as the nucleon number.

Atomic Number: The positive integer that equals the total number of protons in an atom's nucleus.

Ion: A particle formed when a neutral atom or group of atoms gains or loses one or more electrons.

Cation: An ion (either atom or molecule) with a positive electric charge.

Anion: An ion (either atom or molecule) with a negative electric charge.

Isotope: Used to indicate that all isotopes of the same element are found in the same place in the periodic table.

Atomic Mass: The mass of an atom at rest. The SI unit in which it is often expressed is the unified atomic mass unit. The atomic mass can be considered as the total mass of protons and neutrons in a single atom at rest.

Atomic mass calculation: (mass of isotope 1 x % of isotope 1 + mass of isotope 2 x % of isotope 2) / 100

Historical Perspectives on Atomic Structure

Dalton: Published his ideas on the construction of matter:

• Atoms: Each chemical element has identical atoms.
• Chemical Compounds: Formed by the joining of atoms of different elements.

Dalton believed atoms were indivisible and solid.

Thomson: Proposed that atoms are formed by electrons with a negative electric charge distributed throughout a sphere of positively charged material, so that overall the atom is electrically neutral (Plum Pudding Model).

Thomson discovered subatomic particles, consisting of positive and negative particles (electrons and protons).

Rutherford: Proposed that atoms are made up of protons, neutrons, and electrons. In each atom, the number of protons and electrons is the same, making the atom electrically neutral.

Rutherford's model consisted of a nucleus (containing protons and neutrons) where most of the atom's mass is concentrated, with electrons orbiting around it.

Key Equations

Mass Number (A): The sum of protons and neutrons. For a neutral atom, the number of protons equals the number of electrons.

A = Z + N, where N = A - Z (N = number of neutrons, Z = atomic number)