Hello.this os Dr.Ice Cream here.This is my chemistry class.Feel free to ask questions that you do not understand.
Let's start easy.We will begin on oxygen.Sorry if some words are hard to read or confuse you.
Oxygen is a chemical element with the chemical symbol O and atomic number 8. Oxygen is the most common element on Earth, and the second most common element in Earth's atmosphere (second to nitrogen). It composes around 49% of the mass of Earth's crust and 28% of the mass of Earth as a whole, and is the third most common element in the universe. On Earth, it is usually covalently or ionically bonded to other elements.
Unbound oxygen (also called molecular dioxygen, O2, a diatomic molecule) first appeared in significant quantities on Earth during the Paleoproterozoic era (between 2.5 billion years ago and 1.6 billion years ago) as a product of the metabolic action of early anaerobes (archaea and bacteria). The presence of large amounts of free oxygen may have driven most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms; roughly three quarters of the free element being produced by algae in the oceans, and one quarter from terrestrial plants.
What Time is it?
It's time to get harder.
The halogens or halogen elements are a series of nonmetal elements from Group 17 of the periodic table, made up by fluorine(F), chlorine(Cl), bromine(Br), iodine(I), and astatine(At). The undiscovered element 117, temporarily named ununseptium, may also be a halogen.