Atomic Structure and Isotope Calculations: Study Guide for Unit 2

Atomic Number (Z)

This is the number of protons (p+) in an atom's nucleus. It is a whole number and is the identity of the element.

Mass Number (A)

This is the total number of protons plus neutrons (n0). It is always a whole number and identifies a specific isotope.

Atomic Mass

This is the weighted average mass of all the element's naturally occurring isotopes. This is the decimal number found on the periodic table.

Protons (p+)

Equal to the Atomic Number.

Neutrons (n0)

Calculated by subtracting the Atomic Number from the Mass Number: Neutrons=Mass Number−Atomic Number.

Electrons (e−)

In a neutral atom, e− equals p+. In an ion, e− is different from p+.

Isotopes

Isotopes are atoms of the same element (same number of protons) but with a different number of neutrons.

Mass Numbers of Isotopes

Because the number of neutrons is different, they have different Mass Numbers (e.g., Carbon-12 vs. Carbon-13).

Calculating Atomic Mass

The decimal number on the periodic table is the weighted average. You calculate it using the formula: Atomic Mass=∑(Isotope Mass×Fractional Abundance).

Ions

An ion is an atom that has gained or lost electrons, giving it an overall positive or negative charge.

Cation

A positively charged ion formed when an atom loses electrons (metals tend to form these). Example: Na+1 (Sodium cation) means one electron was lost.

Anion

A negatively charged ion formed when an atom gains electrons (nonmetals tend to form these). Example: O−2 (Oxide anion) means two electrons were gained.

Dalton's Atomic Theory

All elements are composed of indivisible particles called atoms.

Atoms of the Same Element

Atoms of the same element are identical; atoms of different elements are different.

Chemical Reactions

Chemical reactions involve the separation, joining, or rearrangement of atoms; atoms are never changed into other elements in a chemical reaction.

Thomson and the Electron

Used the Cathode Ray Tube experiment. Discovered the electron, a subatomic particle with a negative charge.

Rutherford and the Nucleus

Used the Gold Foil Experiment. Key Observations and Conclusions: Most alpha particles passed straight through ⟹ The atom is mostly empty space.

Deflection of Particles

A few particles deflected at large angles ⟹ The atom has a tiny, dense, positively charged center called the nucleus.

Number of Neutrons

Calculated as A−Z.

Difference Between Anion and Cation

Anion (gains e−, negative charge) and Cation (loses e−, positive charge).

Weighted Average Atomic Mass

The process for calculating the weighted average atomic mass.