Introduction to the Chemistry of Life and Atomic Structure

• Protons: Positively charged particles ($+1$) located in the nucleus (the dense center of the atom) with a mass of $1$.
• Neutrons: Particles with no charge (neutral) located in the nucleus with a mass of $1$.
• Electrons: Negatively charged particles ($-1$) that travel in the space surrounding the nucleus. They have a negligible mass.

In a neutral atom, the number of protons and electrons are equal, resulting in a net charge of zero.

Exam Tip: Remember that while protons and neutrons contribute significantly to an atom's mass, electrons are so light that their mass is considered negligible when calculating the atom's overall mass.

Identifying Elements: Atomic Number and Mass Number

The identity and physical characteristics of an element are determined by specific numerical values found in the periodic table of elements. The periodic table organizes elements by their atomic number and relative atomic mass, providing insight into how they will react to form molecules.

Two key values define an atom:

1. Atomic Number: The number of protons in the nucleus. This number is unique to each element and defines what the element is.
2. Mass Number (Atomic Mass): The total sum of protons and neutrons in the nucleus.

Because the mass of an atom is essentially the sum of its heavy particles, you can calculate the number of neutrons using this formula: $\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$

Isotopes and Radioactivity

Sometimes, atoms of the same element can vary in their composition. Isotopes are different forms of the same element that possess the same number of protons (and thus the same atomic number) but have a different number of neutrons.

For example, Carbon-12 and Carbon-14 are both carbon because they both have 6 protons, but Carbon-14 has more neutrons, making it heavier. If an isotope is unstable and spontaneously loses particles or energy to become more stable, it is classified as a radioactive isotope or radioisotope.

Real-World Application: Scientists use the decay of radioisotopes like Carbon-14 for "carbon dating." Because Carbon-14 decays at a predictable rate (its half-life), researchers can calculate the age of fossils up to approximately 50,000 years old.