Science grade 8 – Combination of Atoms

elements rarely occur in pure form, usually in combination with other elements
Do elements occur in pure form in the Earth’s crust?

a substance made of atoms of 2 or more elements joined by chemical bonds
what is a compound?

a molecule is a group of atoms held together by a chemical bond
what is a molecule?

a molecule that exists by itself is the smallest unit of matter
what is the smallest unit of matter that can exist by itself and keep the substance’s chemical properties?

a chemical formula shows the number of atoms of each element required to make one molecule of a compound. example: H2O = 2 H + 1 O
what is a chemical formula?

the reaction of compounds and elements example: methane + oxygen = carbon dioxide + water
What is a chemical equation?

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the number of atoms of each element on the right side must equal the number on the left of the equation
What does it mean for a chemical equation to be balanced?

atoms form chemical bonds by transfering electrons or sharing electrons
In what two ways do atoms form bonds?

an atom or molecule that has gained or lost one or more electrons and has a – or + charge
What is an Ion?

the attraction between opposite charged ions, which happens when electrons are transfered
What is an Ionic bond?

Ionic compound
what is the compound called when electrons are transfered?

the bond formed when atoms share one or more pairs of electrons
What is a covalent bond?

when bonded atoms have an unequal attraction for the shared electrons
What is a Polar Covalent Bond?

a mixture forms when 2 or more substances combine that are not chemically combined
What is a Mixture?

a unevenly dispersed mixture if heterogenous
Heterogenous Mixture?

uniformly dispersed mixture is homogenous
Homogenous Mixture?

a solution composed of 2 or more metals. example: steel
What is an alloy?

Negative Ions are formed how?
Negative Ions form when a valence level is more than half full and the atom takes on electrons

What is an Isotope?
An isotope is an atom of the same element but with different number of neutrons. Example: Hydrogen1, Hydrogen2

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