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9th grade · Science · Unit 3 of 7

Atoms & Electrons

Shells, valence and the periodic table

What your child practices

Protons, neutrons and electrons, drawing electron shells for the first 20 elements, valence electrons, periodic table groups and periods, and ions and isotopes.

Minnesota K-12 Academic Standards benchmark: 9C.3.1.1.1 · using the periodic table to predict the properties of elements

How we explain it

Children can open this short lesson before they practice.

  1. Atomic number = number of protons. A neutral atom has the same number of electrons.
  2. Shells hold 2, then 8, then 8 electrons for the first 20 elements. Outer-shell electrons are valence electrons.
  3. Ions form when atoms gain or lose electrons. Isotopes have different numbers of neutrons.

Example: Magnesium has atomic number 12. How are its electrons arranged in shells? A neutral atom has 12 electrons. Fill 2, then 8, then the rest: 12 − 10 = 2. Answer: 2, 8, 2

Atoms & Electrons in Minnesota

In the Minnesota K-12 Academic Standards, this is unit 3 of 7 in 9th grade Science, after Reproduction and before Electrons & Compounds.

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.

Source: Minnesota Department of Education (MDE)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Sodium (Na) has atomic number 11. How many electrons does a neutral atom of it have?

    answer box

  2. Question 2

    Sulphur (S) has atomic number 16. How many electrons does a neutral atom of it have?

    answer box

  3. Question 3

    In the periodic table, a period (row) tells you…

    • the color of the element
    • the number of neutrons
    • the element's state at 0 °C
    • the number of electron shells in the atom

  4. Question 4

    Group 1 elements, the alkali metals, are very reactive because they have…

    • a full outer shell
    • no neutrons
    • one valence electron that is easily lost
    • eight valence electrons

  5. Question 5

    What is an isotope?

    • Atoms of the same element with different numbers of neutrons
    • A type of ion
    • A kind of molecule
    • Atoms with different numbers of protons

  6. Question 6

    Metals are generally…

    • gases at room temperature
    • always magnetic
    • good conductors of electricity and malleable
    • brittle and poor conductors

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