The Periodic Table — Practice · Grade 8 Science

Chapter: Properties of Matter

What this practice covers

By the end of The Periodic Table you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: periodic table, period, group (family), valence electron, metalloid, model, evidence, system boundary, assumption.

Problem 1

Elements in the same column (group) of the periodic table tend to have similar…
  1. numbers of neutrons
  2. numbers of valence electrons
  3. atomic masses
  4. numbers of protons
Answer and explanation

numbers of valence electrons

Elements in a group share the same number of valence electrons, which drives similar chemical behavior.

Problem 2

Sulfur is in Group 16. How many valence electrons does it have?
Answer and explanation

6

For main-group elements, the group number (16) tells you the valence electron count: 16 − 10 = 6.

Problem 3

Which region of the periodic table contains elements that are generally shiny, malleable, and good conductors?
  1. Nonmetals (upper right)
  2. Metals (left and center)
  3. Metalloids (along the zigzag line)
  4. Noble gases (Group 18) only
Answer and explanation

Metals (left and center)

Metals, found on the left and center of the table, are typically shiny, malleable, and conduct electricity well.

Problem 4

Explain why elements in the same group have similar chemical properties.
Answer and explanation

They have the same number of valence electrons, and valence electrons mostly determine how an atom bonds and reacts with other atoms.

Problem 5

As you move left to right across a period, atomic size generally…
  1. increases
  2. decreases
  3. stays the same
  4. becomes zero
Answer and explanation

decreases

Atomic size generally decreases across a period because added protons pull the electron cloud inward.

Problem 6

An element is in Group 2. How many valence electrons does it have?
Answer and explanation

2

Group 2 elements (alkaline earth metals) all have 2 valence electrons.

Problem 7

Element X has atomic number 12 and element Y has atomic number 20. Both are in group 2. What can you infer? Explain the reasoning in your own words.
Answer and explanation

They are different elements with some similar chemical properties; the table alone does not give identical reaction rates.

Their proton counts are 12 and 20, so they are different elements.

Their shared main group suggests similar outer-electron patterns and some similar reactions.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

An element has atomic number 16. A student says it contains 16 neutrons. Is this supported? Show the reasoning, not just the conclusion.
Hints

Check the group labels; neighboring position alone is not the comparison rule.

Answer and explanation

No. Atomic number specifies 16 protons. Neutron count requires isotope mass number or other evidence.

Problem 9

Two main-group elements share a column but have different densities. Does this contradict the group pattern?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Similar valence patterns suggest some related chemical behavior; they do not require identical physical properties such as density.

Problem 10

Read atomic number separately from average atomic massRead atomic number separately from average atomic mass11NaSodium22.99 u (average)Atomic number → protonsA common Na-23 atom has 11 protons and 12 neutrons; the decimal is an average.
This sodium element card separates atomic number 11 from average atomic mass about 22.99 u. The atomic number fixes proton count; a neutral sodium atom also has eleven electrons. A specified Na-23 isotope has twelve neutrons. The decimal average is not the neutron count, total charge or an exact number of particles in every atom.
For this unit’s illustrated case, how many protons and electrons does a neutral Na-23 atom have, and why is rounding 22.99 not a definition of every sodium isotope?
Answer and explanation

It has 11 protons and 11 electrons. Na-23 has 12 neutrons because its specified mass number is 23. The decimal table value is an isotope-weighted average, not an isotope definition.

The unit u is an atomic-mass unit used for masses at the atomic scale. Mass number is a count, whereas the listed average mass is a measured average for an isotope mixture. Keep both distinct from atomic number.

It has 11 protons and 11 electrons. Na-23 has 12 neutrons because its specified mass number is 23. The decimal table value is an isotope-weighted average, not an isotope definition.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

A learner reads atomic number 17 and average atomic mass about 35.45 from a chlorine card and reports 18.45 neutrons in one atom. Correct the interpretation for a specified chlorine-37 atom.
Answer and explanation

Chlorine-37 has 17 protons and 37−17 = 20 neutrons. The decimal 35.45 is an isotope-average mass, not one atom’s particle count.

Use the specified isotope mass number, not the decimal average.

Subtract atomic number from mass number.

Chlorine-37 has 17 protons and 37−17 = 20 neutrons. The decimal 35.45 is an isotope-average mass, not one atom’s particle count.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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