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What are the three subatomic particles in an atom?
Protons: Positive charge (+1), found in the nucleus
Neutrons: Neutral charge (0), found in the nucleus
Electrons: Negative charge (-1), found in energy shells
What is the difference between atomic number and mass number?
Atomic number (Z): Number of protons (also equals electrons in a neutral atom).
Mass number (A): Total number of protons + neutrons.
Formula: Mass number = Protons + Neutrons
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons.
Example: Carbon-12 and Carbon-14
How are electrons arranged in an atom?
Electrons occupy energy shells:
1st shell: Max 2 electrons
2nd shell: Max 8 electrons
3rd shell: Max 8 electrons
Example: Sodium (11 electrons) → 2,8,1
How does atomic size change across a period and down a group?
Across a period: Atomic size decreases (more protons pull electrons closer).
Down a group: Atomic size increases (more shells are added).
Why are noble gases (Group 0) unreactive?.
They have a full outer shell of electrons, making them stable and unreactive.
