Counting Atoms
In the chemical formula H2O, how many hydrogen atoms are present?
2 hydrogen atoms
Is molecular oxygen (O2) an element or a compound? Explain briefly.
It is an element. Although it has more than one atom, it is only one TYPE of atom (oxygen).
What is a molecule? Give a one-sentence, brief definition.
A molecule is two or more atoms bonded together
Name the three main subatomic particles found in an atom.
Protons, neutrons, electrons.
A soft drink with dissolved CO2 and sugar that looks the same throughout — name the type of mixture.
Homogeneous mixture.
How many total atoms are in one molecule of CO2?
Three atoms - one carbon and 2 oxygen
Is sodium chloride (NaCl) an element or compound? Name the atoms that make it up.
It is a compound. It has 1 Na (sodium) atom and 1 Cl (chlorine) atom.
Is N2 a molecule, an element, a compound, or any combination of the three?
N2 is a molecule and also an element (made of two nitrogen atoms); not a compound because both atoms are the same type (nitrogen).
Which subatomic particle has a positive charge and where is it located?
Protons; located in the nucleus.
Cereal in milk — classify and give one observable feature that makes it a homogenous or heterogenous.
Heterogeneous mixture; you can clearly see the different parts (cereal and milk), and it is not uniformly mixed throughout.
In the formula C6H12O6, how many carbon atoms are there?
Six carbon atoms
List two differences between elements and compounds in terms of atoms and chemical formulas.
Elements contain only one kind of atom and are shown with a single chemical symbol (like Fe).
Compounds contain two or more different atoms chemically bonded and shown with a chemical formula (like H2O)
How does a molecule differ from an atom? Give an example.
A molecule is a group of at least 2 bonded atoms; an atom is a single particle. Example: A single oxygen atom is O; an oxygen molecule is O2.
What is the approximate location and charge of electrons?
Electrons are tiny, negatively charged particles that orbit the nucleus in circles called orbitals.
A mixture that shows different compositions in samples taken from different locations — what classification fits this description?
Heterogenous mixture.
For 2H2O (two molecules of water), how many hydrogen atoms are there in total?
4 total hydrogen atoms
Give an example of a compound and write its chemical formula; identify the types of atoms in that compound.
Example: Water, H2O; contains hydrogen and oxygen atoms.
Draw (or describe) a molecule that contains 3 atoms and is a compound; give its chemical formula.
Example: Carbon dioxide, CO2 (3 atoms: one carbon, two oxygen).
Example: Water, H2O (three atoms: 2 hydrogens and one oxygen).
How does the number of protons in an atom relate to its chemical symbol on the periodic table?
The number of protons equals the element’s atomic number.
A brass alloy is made of copper and zinc; classify it (element/compound/homogeneous/heterogeneous) and explain why.
Homogeneous mixture (alloy); the atoms of copper and zinc are mixed uniformly but not chemically bonded together.
In the chemical formula Al2(SO4)3, how many oxygen atoms are present? (Show how you counted them.)
12 oxygen atoms. 4 x 3 = 12
Explain why iron (Fe) and iron oxide (Fe2O3) are not the same substance, using atoms and molecules in your answer.
Iron (Fe) is an element made of only iron atoms. Iron oxide (Fe2O3) is a compound made of iron and oxygen atoms chemically combined; it has different properties.
Explain why all compounds are made of molecules but not all molecules are compounds, with an example.
All compounds are molecules because they are bonded atoms of different elements (e.g., NaCl), but molecules like O2O2 are not compounds because they contain only one element.
What is an "isotope" of an element?
Isotopes of an element have the same number of protons, but different numbers of neutrons.
You observe bubbling when zinc is added to hydrochloric acid. How does this evidence help classify whether a chemical change produced a new compound or mixture?
Bubbling (gas production) indicates a chemical reaction forming new substances (like hydrogen gas). A chemical reaction indicates a chemical change producing new substances (likely a compound).