Elements, Molecules, and Compounds

Hydrogen is a pure substance represented by the chemical symbol H. Which of the following best describes hydrogen?

• Hydrogen is an element made of two or more kinds of atoms.
• Hydrogen is a compound made of only one kind of atom.
• Hydrogen is a compound made of two or more kinds of atoms.
• Hydrogen is an element made of only one kind of atom.

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The element oxygen, represented by the symbol O, is classified as

• a pure substance
• a compound
• a mixture
• a solution

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Carbon (C) and hydrogen (H) are pure substances. Each is made of only one type of atom, but carbon atoms are different from hydrogen atoms. Carbon and hydrogen chemically combine to form methane (CH4).

• methane is an element and carbon and hydrogen are compounds.
• carbon, hydrogen, and methane are all elements.
• carbon, hydrogen, and methane are all compounds.
• carbon and hydrogen are elements and methane is a compound.

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What is a compound?

• an atom with more electrons than protons

• a substance in which the atoms of two or more different elements are combined together

• an atom with more neutrons than protons

• a substance in which the atoms of three or more elements are combined together

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The chemical combination of two or more different kinds of atoms in fixed amounts is called a(n)

• orbit
• compound
• mixture
• element

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Atoms combine to form _______. 

• electrons
• molecules
• nuclei
• neutrons

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Where are protons and neutrons located? 

• in the nucleus
• in the electron cloud
• in the valence shell

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Which of these compounds has the most atoms?

• CO2
• C6,H12,O6
• O2
• Na,Cl,He,O3

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At room temperature, oxygen is a gas and hydrogen is a gas.  When they combine, they form a 

• solid
• liquid
• gas
• solution

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A water molecule contains

• one hydrogen atom and two oxygen atoms
• two hydrogen atoms and one oxygen atom
• two hydrogen atoms and two oxygen atoms
• two hydrogen atoms and no oxygen atoms

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