atoms consists of a nucleus and electrons

nucleus = protons (2 in Helium) and neutrons (usually 2 in Helium), protons have a positive electrical charge of 1

all atoms of a single element have the same number of protons = atomic number

protons and neutrons have mass, and the total is the atomic weight

electrons have no mass but do have a negative electrical charge of 1

What is the charge of an atom that has lost two electrons?

What is the charge of an atom that has gained one electron?

Atoms that have gained or lost electrons are called ions.

atoms that have the same atomic number (number of protons), but have different numbers of neutrons, and therefore are only very slightly different, chemically. Some are unstable (radioactive decay).

• important isotopes in biology:
  ¹⁴C, ¹³C, ¹²C     ¹⁸O, ¹⁶O     ³H, ²H
  

Plasma: atoms (nuclei) without electrons (e.g., cosmic rays)

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