8th grade · Science · Unit 3 of 10
Atoms & Elements
Elements, compounds and chemical symbols
What your child practices
Elements versus compounds and mixtures, chemical symbols (H, O, Na, Cl, Fe, Au…), counting atoms in formulas like H₂O and C₆H₁₂O₆, metals and non-metals, and the basics of the periodic table.
Minnesota K-12 Academic Standards benchmark: 8P.1.1.1.1; 8P.3.1.1.1 · the periodic table, and the atoms in simple molecules
How we explain it
Children can open this short lesson before they practice.
- An element has only one kind of atom. A compound has two or more elements joined chemically. A mixture is substances mixed but not joined.
- Rows of the periodic table are periods; columns are groups. Metals are on the left, non-metals on the right.
- In a formula, the small number after a symbol counts the atoms just before it.
Example: How many atoms are in one molecule of H₂O? H₂ means 2 hydrogen atoms. O with no number means 1 oxygen atom. 2 + 1 = 3. Answer: 3
Atoms & Elements in Minnesota
In the Minnesota K-12 Academic Standards, this is unit 3 of 10 in 8th grade Science, after Light & Radiation and before Circuits & Static.
Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
Water is H₂O. What does the small 2 tell you?
💧- Hydrogen weighs 2 times as much as oxygen
- There are 2 water molecules
- There are 2 oxygen atoms
- There are 2 hydrogen atoms for every oxygen atom
There are 2 hydrogen atoms for every oxygen atom
Hint: A small number written after a symbol counts the atoms of the element just before it.
Question 2
What is the chemical symbol for sodium?
⚛️sodium- Sd
- So
- Na
- S
Na
Hint: Most symbols are the first one or two letters of the name (O for oxygen, Ca for calcium). Some come from Latin names: Na (natrium) for sodium, Fe (ferrum) for iron, Au (aurum) for gold, Ag (argentum) for silver, K (kalium) for potassium, Pb (plumbum) for lead, Hg (hydrargyrum) for mercury, Sn (stannum) for tin.
Question 3
Which property is typical of a solid non-metal, such as sulphur?
- shiny and bendable
- dull and brittle
- a good conductor of electricity
- easy to hammer into thin sheets
dull and brittle
Hint: Non-metals are usually poor conductors, and as solids they tend to be dull and break easily.
Question 4
Elements in the same column (group) of the periodic table…
- all have the same mass
- were all discovered in the same year
- have similar chemical properties
- are all gases
have similar chemical properties
Hint: Groups are like families: sodium and potassium, in the same group, both react strongly with water.
Question 5
How many atoms in total does the formula CaCO₃ (calcium carbonate (limestone)) show?
CaCO₃calcium carbonate (limestone)answer box
5
Hint: A small number after a symbol tells how many of that atom there are. A symbol with no small number means 1. Add them all up.
Question 6
Sodium is a soft, very reactive metal. Chlorine is a poisonous green gas. Chemically combined, they make…
🧂- table salt (sodium chloride)
- water
- sugar
- baking soda
table salt (sodium chloride)
Hint: A compound can have very different properties from the elements in it. NaCl is safe to eat in small amounts.