Atoms and the Periodic Table
Grade 8 · Science · Study Guide
Explore atomic structure, subatomic particles, and how the periodic table organizes all known elements.
Atoms are the basic building blocks of all matter. Each atom consists of a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in energy levels called shells. The number of protons defines the element and is called the atomic number.
The periodic table organizes all 118 known elements by increasing atomic number. Elements in the same column (group or family) share similar chemical properties because they have the same number of electrons in their outer shell. Elements in the same row (period) have the same number of electron shells.
Metals, nonmetals, and metalloids are the three main categories on the periodic table. Metals (left side) are good conductors, shiny, and malleable. Nonmetals (right side) are poor conductors and vary in state. Metalloids (staircase line) have properties of both.
| Particle | Charge | Location | Mass (amu) |
|---|---|---|---|
| Proton | Positive (+1) | Nucleus | ~1 |
| Neutron | Neutral (0) | Nucleus | ~1 |
| Electron | Negative (-1) | Electron shells | ~0 (negligible) |
| Feature | What It Tells You |
|---|---|
| Period (row) | Number of electron shells |
| Group (column) | Number of valence electrons; similar properties |
| Atomic number | Number of protons |
| Atomic mass | Average mass of all isotopes |
Key terms
- Atomic Number
- The number of protons in an atom's nucleus; uniquely identifies each element.
- Atomic Mass
- The average mass of an element's atoms, measured in atomic mass units (amu).
- Electron Shell
- An energy level around the nucleus where electrons are found.
- Isotope
- Atoms of the same element with different numbers of neutrons.
Practice questions
What determines which element an atom is?
- The number of neutrons
- The number of electrons
- The number of protons
- The atomic mass
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The atomic number (number of protons) is unique to each element and defines what element it is.
Elements in the same group (column) of the periodic table have similar:
- Atomic masses
- Numbers of neutrons
- Chemical properties
- Numbers of electron shells
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Same-group elements have the same number of valence electrons, giving them similar reactivity and chemical behavior.
Isotopes of an element have different numbers of protons.
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Isotopes have the same number of protons (same element) but different numbers of neutrons.
The number of protons in an atom's nucleus is called the ___ number.
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The atomic number identifies each element uniquely on the periodic table.
Oxygen has an atomic number of 8 and an atomic mass of approximately 16. How many neutrons does a typical oxygen atom have?
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Neutrons = atomic mass (rounded) minus the atomic number.
Fun facts
- If an atom were the size of a football stadium, the nucleus would be the size of a pea at the center — atoms are almost entirely empty space.
- Hydrogen is the most abundant element in the universe, making up about 75% of all normal matter.
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