C. Subatomic Particles
➢ Atom
- Smallest unit of an element
- Building blocks of physical world
➢ Subatomic Particles
- Protons
■ Packed with neutrons in nucleus
■ Positively charged
■ Most atoms have same amount of protons as electrons, making them electrically neutral - Neutrons
■ Packed with protons in nucleus
■ No charge
■ Isotopes
● Same element with different amount of neutrons in nucleus
● Vary in mass
● Radioactive isotopes decay spontaneously, giving off particles and energy - Electrons
■ Negatively charged
■ Spin around nucleus
■ Very small; effectively massless
■ Electrons on an atom differ in their amounts of potential energy
■ Electron’s state of potential energy is called its energy level, or electron shell
■ Valence electrons are those in the outermost shell, or valence shell
■ Chemical behavior of an atom is mostly determined by the distribution of electrons in electron shells
● Valence shell most important
● Elements with full valence shells are chemically inert
● Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms - Atoms of different various elements differ in number of subatomic particles
- $\text{Atomic number=protons in nucleus}$
- $\text{Mass Number= protons+neutrons}$
■ Average of all isotopes - Atomic mass+atom’s weighted average total mass
