Atoms are the building block of all matter.
Each atom has a central nucleus containing protons and neutrons,
surrounded by a cloud of electrons. Atoms can lose, gain, or share electrons.
- Nucleus: contains protons (+) and neutrons (no charge)
- Atomic number = # of protons
- Atomic mass ("weight") = # of protons + neutrons
- Isotopes of the same element differ in # of neutrons (hence, in mass)
- Electrons have negative charge and almost no mass.
- They occupy shells (orbitals) at different energy levels around the nucleus.
- Shell #1 is full when it has 2 electrons.
- Shell n can hold a maximum of 2n2 electrons, but
- the outermost shell can only have a maximum of 8 electrons ("octet rule").
- A full outer shell means a stable configuration.
- Ions and bonds: electrically charged atoms are called ions.
Periodic table
- Metallic elements tend to lose electrons and form positive ions.
- Nonmetals tend to gain electrons and form negative ions.
- Covalent bonds: Certain atoms can also share electrons, forming covalent bonds.
- H forms 1 bond; O forms 2;
N forms 3; C forms 4.
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