Atomic Structure
All matter is made of atoms. Understanding atomic structure — protons, neutrons, and electrons — is the foundation for chemistry. The number of protons in an atom defines which element it is.
What You Need to Know
Key Concept Diagram
An atom consists of a nucleus (containing protons and neutrons) surrounded by electrons in shells
Protons carry a positive charge; electrons carry a negative charge; neutrons are neutral
The atomic number is the number of protons in an atom — it defines the element
The mass number is the total number of protons and neutrons in the nucleus
Key Vocabulary
Atom
The smallest unit of an element that retains the chemical properties of that element
Proton
A positively charged particle found in the nucleus of an atom
Neutron
A neutral particle (no charge) found in the nucleus of an atom
Electron
A negatively charged particle orbiting the nucleus of an atom in electron shells
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What determines which element an atom belongs to?
Question 2
A neutral atom has 8 protons. How many electrons does it have?
Question 3
Where are protons and neutrons located in an atom?
Key Concepts Summary
- ●An atom consists of a nucleus (containing protons and neutrons) surrounded by electrons in shells
- ●Protons carry a positive charge; electrons carry a negative charge; neutrons are neutral
- ●The atomic number is the number of protons in an atom — it defines the element
- ●The mass number is the total number of protons and neutrons in the nucleus