Atomic Structure — Practice · Grade 8 Science

Chapter: Properties of Matter

What this practice covers

By the end of Atomic Structure you should be able to:

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

Vocabulary used: atom, proton, neutron, electron, nucleus, atomic number, mass number, isotope, ion, model, evidence, system boundary, assumption.

Problem 1

Which subatomic particle has a negative charge?
  1. proton
  2. neutron
  3. electron
  4. nucleus
Answer and explanation

electron

Electrons carry negative charge and are described by an electron cloud, not literal planet-like orbits.

Problem 2

Where are protons and neutrons located in an atom?
  1. In the nucleus
  2. Orbiting the nucleus
  3. Outside the atom entirely
  4. Only in charged atoms
Answer and explanation

In the nucleus

Protons and neutrons are packed together in the central nucleus.

Problem 3

A sodium (Na) atom has atomic number 11 and mass number 23. How many neutrons does it have?
Answer and explanation

12

Neutrons = mass number − atomic number = 23 − 11 = 12.

Problem 4

A magnesium atom has 12 protons and 12 neutrons. What is its mass number?
Answer and explanation

24

Mass number = protons + neutrons = 12 + 12 = 24.

Problem 5

Explain why an atom is electrically neutral overall.
Answer and explanation

A neutral atom has the same number of protons (positive) as electrons (negative), so the charges cancel out and the atom has no overall charge.

Problem 6

An atom of chlorine has 17 protons and a mass number of 37. How many neutrons does it have?
Answer and explanation

20

Neutrons = mass number − atomic number = 37 − 17 = 20.

Problem 7

An atom has 8 protons, 9 neutrons and 10 electrons. Identify the element, mass number and charge. Explain the reasoning in your own words.
Answer and explanation

An oxygen-17 ion with charge −2.

Atomic number 8 identifies oxygen.

Mass number is 8+9=17; 8 positive and 10 negative charges give a net charge of −2.

Response criteria

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

Problem 8

A neutral atom has 13 protons and 14 neutrons. State its mass number and electron count and explain both. Show the reasoning, not just the conclusion.
Hints

Check that changing neutron count leaves both proton count and charge unchanged.

Answer and explanation

Mass number=13+14=27; there are 13 electrons because neutrality requires equal positive and negative charges.

Problem 9

Atoms A and B have 9 and 10 protons but both have 10 neutrons. Are they isotopes? Explain.
Hints

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

Answer and explanation

No. Isotopes share proton count. These have different atomic numbers and are different elements.

Problem 10

Draw two water molecules and a larger repeating salt patch. Compare total atom counts with a formula ratio, and explain why a two-ball salt molecule is an incomplete crystal model.
Hints

Use the illustrated study titled Molecules and extended structures. Keep the observations, model and conditions separate.

Answer and explanation

Two water molecules contain four H and two O atoms. A salt patch represents part of an extended arrangement; its bulk Na:Cl ratio is one-to-one rather than a claim that the crystal is a collection of separate NaCl molecules. Retain element/ion labels and note that colors, bonding lines and distances are conventions.

Two water molecules contain four H and two O atoms. A salt patch represents part of an extended arrangement; its bulk Na:Cl ratio is one-to-one rather than a claim that the crystal is a collection of separate NaCl molecules. Retain element/ion labels and note that colors, bonding lines and distances are conventions.

Response criteria

  • Distinguish molecule from extended arrangement.
  • Count water atoms correctly.
  • Explain formula ratio and a model limit.

Problem 11

Proton count fixes element identityProton count fixes element identityC6 p⁺ · 6 n · 6 e⁻O8 p⁺ · 8 n · 8 e⁻Carbon: atomic number 6Oxygen: atomic number 8Count protons to identify the element; electron count tells net charge.
Carbon and oxygen are compared with their proton, neutron and electron counts labeled. Six protons identifies carbon; eight identifies oxygen. The cloud and nucleus sizes are deliberately schematic and electrons are not drawn on fixed planetary tracks. A neutral atom balances proton and electron counts.
For this unit’s illustrated case, a particle has 8 protons, 8 neutrons and 10 electrons. Is it carbon, oxygen or a new element? Explain its charge.
Answer and explanation

It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

Read p⁺ as positively charged protons, n as neutral neutrons and e⁻ as negatively charged electrons. Element identity and net charge answer different questions. Changing only electrons cannot turn oxygen into carbon.

It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

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 12

An atom has 12 protons, 13 neutrons and 10 electrons. State atomic number, mass number and net charge, and explain whether losing electrons changed its element.
Answer and explanation

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Read proton count for atomic number.

Add 12+13 for mass number; compare 12 positive with 10 negative charges.

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

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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