Atomic Structure: Protons, Neutrons, Electrons and Electron Shells — Answers
Answer sheet with model answers and success criteria for the Year 9 chemical sciences worksheet.
Success TutoringWorksheetsAnswer sheetYear 9 · Science · Chemical SciencesAtomic Structure: Protons, Neutrons, Electrons and Electron Shells — Answers
Part A: Inside the atom
An atom has a tiny, dense nucleus containing protons and neutrons. Electrons move around the nucleus in energy levels called shells. Almost all of the mass of an atom is in its nucleus.
The atomic number is the number of protons. In a neutral atom, the number of electrons equals the number of protons. The mass number is the total number of protons and neutrons, so neutrons = mass number − atomic number.
For the first 20 elements, electrons fill the shells in order: up to 2 in the first shell, up to 8 in the second, up to 8 in the third, and then the fourth shell starts. Calcium, with 20 electrons, is 2, 8, 8, 2.
Example: an atom of sodium has atomic number 11 and mass number 23. It has 11 protons, 23 − 11 = 12 neutrons and 11 electrons, arranged 2, 8, 1.
- Proton
- Neutron
- Electron
- No charge, relative mass 1, found in the nucleus
- Charge −1, very tiny mass (about 1/1836 of a proton), found in shells around the nucleus
- Charge +1, relative mass 1, found in the nucleus
Answer: Proton → Charge +1, relative mass 1, found in the nucleus; Neutron → No charge, relative mass 1, found in the nucleus; Electron → Charge −1, very tiny mass (about 1/1836 of a proton), found in shells around the nucleus
- AThe proton
- BThe neutron
- CThe electron
- DThe nucleus as a whole
Answer: The proton
Every atom of an element has the same number of protons. Change the number of protons and you have a different element.
- AIt has equal numbers of protons and electrons
- BIt has equal numbers of protons and neutrons
- CNeutrons cancel out the charge of the protons
- DElectrons have no charge
Answer: It has equal numbers of protons and electrons
Each proton (+1) is balanced by one electron (−1). Neutrons have no charge, so they do not affect the balance.
Part B: Count the particles
For each neutral atom, write the number of protons, neutrons and electrons, and the electron arrangement.
Answer: 8 protons, 8 neutrons, 8 electrons; 2, 6
Answer: 9 protons, 10 neutrons, 9 electrons; 2, 7
Answer: 12 protons, 12 neutrons, 12 electrons; 2, 8, 2
Answer: 13 protons, 14 neutrons, 13 electrons; 2, 8, 3
Answer: 15 protons, 16 neutrons, 15 electrons; 2, 8, 5
Answer: 17 protons, 18 neutrons, 17 electrons; 2, 8, 7
Answer: 18 protons, 22 neutrons, 18 electrons; 2, 8, 8
Answer: 19 protons, 20 neutrons, 19 electrons; 2, 8, 8, 1
Part C: Symbols and shells
Symbols always start with a capital letter; a second letter is lower case. Some symbols come from Latin names, such as Na (natrium) for sodium and K (kalium) for potassium.
- Sodium
- Potassium
- Chlorine
- Magnesium
- Sulfur
- Silicon
- Calcium
- Nitrogen
- Cl
- Si
- Ca
- S
- K
- Na
- N
- Mg
Answer: Sodium → Na; Potassium → K; Chlorine → Cl; Magnesium → Mg; Sulfur → S; Silicon → Si; Calcium → Ca; Nitrogen → N
Answer: Sulfur (16 electrons, so atomic number 16).
Answer: Carbon (6 electrons, so atomic number 6).
Answer: 2, 8, 8, 2
Answer: 2, 5
Part D: Isotopes, groups and ions
Isotopes are atoms of the same element with different numbers of neutrons.
Answer: Carbon-12: 6 protons and 6 neutrons. Carbon-14: 6 protons and 8 neutrons.
Answer: Both have 17 protons and 17 electrons (so they react in the same way). Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons, so they have different masses.
Answer: False
Isotopes have the same number of protons (otherwise they would be different elements) but different numbers of neutrons.
Answer: True
Elements with the same number of outer-shell electrons are in the same group of the periodic table and have similar chemical properties. These three are all reactive metals in group 1.
Answer: True
Noble gases have stable outer shells (8 electrons, or 2 for helium), so they rarely react with other elements.
Answer: 11 protons and 10 electrons. It has one more proton than electrons, so its charge is +1 (written Na+). Its arrangement is now 2, 8.
Part E: Draw it
Model answer: A small circle in the centre labelled 12p and 12n (the nucleus). Three rings around it: the first ring with 2 electrons, the second with 8 electrons and the third (outer) ring with 2 electrons, giving 2, 8, 2 and 12 electrons in total.
Success criteria:
- Nucleus labelled with 12 protons and 12 neutrons
- Three shells drawn
- Electrons shown as 2, 8, 2
- Total of 12 electrons
Part F: The first 20 elements
Model answer: Hydrogen H 1: 1 · Helium He 2: 2 · Lithium Li 3: 2, 1 · Beryllium Be 4: 2, 2 · Boron B 5: 2, 3 · Carbon C 6: 2, 4 · Nitrogen N 7: 2, 5 · Oxygen O 8: 2, 6 · Fluorine F 9: 2, 7 · Neon Ne 10: 2, 8 · Sodium Na 11: 2, 8, 1 · Magnesium Mg 12: 2, 8, 2 · Aluminium Al 13: 2, 8, 3 · Silicon Si 14: 2, 8, 4 · Phosphorus P 15: 2, 8, 5 · Sulfur S 16: 2, 8, 6 · Chlorine Cl 17: 2, 8, 7 · Argon Ar 18: 2, 8, 8 · Potassium K 19: 2, 8, 8, 1 · Calcium Ca 20: 2, 8, 8, 2. Pattern: the number of outer-shell electrons goes up by one with each element across a row (period), and starts again at one when a new shell begins (lithium, sodium, potassium). Elements in the same column (group) have the same number of outer-shell electrons; helium is the exception, with a full first shell of 2, which is why it sits with the other noble gases.
Success criteria:
- All 20 symbols correct, with correct capital and lower-case letters
- Electron arrangements follow the 2, 8, 8, 2 pattern
- Describes the repeating pattern in outer-shell electrons
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.