Atomic Structure and Particles

Overview

This lecture introduces atoms as the fundamental building blocks of matter, explores their structure, and explains how atomic number and mass relate to elements.

Structure of Atoms

• Atoms are made of a central nucleus surrounded by electrons in shells (rings).
• The nucleus contains protons and neutrons, tightly packed together.
• Electrons orbit the nucleus and constantly move.
• Most of the atom is empty space, with a tiny nucleus at the center.

Subatomic Particles: Mass and Charge

• Protons have a relative mass of 1 and a +1 (positive) charge.
• Neutrons have a relative mass of 1 and no charge (neutral).
• Electrons have a much smaller mass (about 1/2000 of a proton) and a −1 (negative) charge.
• The number of protons equals the number of electrons in a neutral atom, making atoms overall neutral.

Ions and Atomic Charge

• Atoms become ions by gaining or losing electrons.
• If electrons are gained, the atom becomes a negative ion (more electrons than protons).
• If electrons are lost, the atom becomes a positive ion (more protons than electrons).

Elements and Nuclear Symbols

• Each type of atom is called an element, shown in the periodic table.
• The elemental symbol is one or two letters (e.g., O for oxygen, Li for lithium).
• The atomic number (bottom left) is the number of protons in the atom.
• The mass number (top left) is the total number of protons and neutrons.
• Number of neutrons = mass number − atomic number.
• The atomic number also tells the number of electrons in a neutral atom.

Key Terms & Definitions

• Atom — the smallest unit of matter, made of a nucleus and electrons.
• Proton — positively charged particle in the nucleus (relative mass 1).
• Neutron — neutral particle in the nucleus (relative mass 1).
• Electron — negatively charged particle orbiting the nucleus (very small mass).
• Ion — a charged atom formed by gaining or losing electrons.
• Element — a type of atom defined by its number of protons.
• Atomic Number — number of protons in an atom.
• Mass Number — total number of protons and neutrons in an atom.