Atomic structure, isotopes and electron shells

What are the three main subatomic particles in an atom?

Protons (positive), neutrons (neutral), and electrons (negative)

Where is most of an atom’s mass located and why?

In the nucleus, because protons and neutrons are much heavier than electrons

What distinguishes isotopes of the same element?

They have the same number of protons but different numbers of neutrons

Define atomic number and mass number.

Atomic number = number of protons; Mass number = protons + neutrons

What is the relative atomic mass?

: The weighted average mass of an element’s isotopes, using carbon‑12 as a reference (1/12 of its mass = 1 u)

How do you calculate relative atomic mass of chlorine with 75% Cl-35 and 25% Cl-37?

(0.75 × 35) + (0.25 × 37) = 35.5

What are electron shells?

Regions around the nucleus where electrons orbit in specific energy levels

How does electronic configuration relate to periodic table positions?

Shell count = period; electrons in outer shell = group number (for Groups I–VII)

What is the significance of a full outer electron shell?

Atoms are most stable with full outer shells (noble-gas configuration)

How can electrons change energy levels?

They absorb or emit specific quanta of electromagnetic radiation to move shells