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9th grade · Science · Unit 3 of 10

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.

Arizona Academic Standards learning standard: HS.P1U1.1 · atomic structure, electron shells and the periodic table

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 Arizona

In the Arizona Academic Standards, this is unit 3 of 10 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: Arizona Department of Education

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

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

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

  2. Question 2

    How many electron shells (energy levels) are used by a neutral sodium atom?

    • 3
    • 4
    • 2
    • 5

  3. Question 3

    Chlorine (Cl) has atomic number 17. How many electrons does a neutral atom of it have?

    answer box

  4. Question 4

    When an atom gains an electron, it becomes…

    • a neutron
    • a negative ion
    • a different element
    • a positive ion

  5. Question 5

    Metals are generally…

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

  6. Question 6

    The elements in Group 18 (the noble gases) rarely react because…

    • they are very heavy
    • their outer shells are already full
    • they are liquids
    • they have no electrons

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