Physical Science / Unit 3: Atoms and the Periodic Table
3.1 Structure of the Atom
Notes
An atom is the smallest unit of an element that keeps the element's properties.
- The nucleus at the center holds protons (positive charge, +1) and neutrons (no charge).
- Electrons (negative charge, -1) move around the nucleus in a region called the electron cloud.
- Protons and neutrons each have a mass of about 1 atomic mass unit (amu). An electron's mass is about 1/1,836 of that, so almost all of an atom's mass is in the nucleus.
- A neutral atom has equal numbers of protons and electrons.
Practice
Which particle has a negative charge?
Electrons carry a -1 charge.
Where is most of an atom's mass found?
Protons and neutrons are much heavier than electrons and sit in the nucleus.
A neutral atom has 9 protons. How many electrons does it have?
Charges must balance, so electrons equal protons.
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