An element is a substance
that cannot be broken down to other substances by chemical
reactions.
Of the 92 natural elements, about 20–25% are essential
elements that an organism needs to live a healthy life and
reproduce.
A compound is a substance consisting of two or more
different elements combined in a fixed ratio.
An element’s properties depend
on the structure of its atoms
Atomic number&Atomic mass
Isotopes: different atomic forms of the same element are called
isotopes of the element(contains more neurons)
A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy。
fallout from nuclear accident
valence electron(on valence shell)
The reactivity of an atom arises from the presence of
unpaired electrons in one or more orbitals of the atom’s
valence shell.
electron orbitals
No more than 2 electrons can occupy a single orbita
