describe an atom in terms of protons, neutrons and electrons, use atomic number and mass number to find each, and ask testable questions about why elements behave differently.
Instruction
The phenomenon. Miners in the Sierra Nevada foothills pulled gold out of streams in 1849. Gold nuggets and flakes from that era, now in museum cases in Sacramento, still shine. The iron picks, pans and shovels the miners used are brown with rust, and many have crumbled. Both metals sat in the same wet ground for the same years. Why does one element react with air and water while another does not? That question organizes this unit. To answer it we need a model of what an element is made of.
The nuclear atom. Every atom has a tiny, dense nucleus made of protons (charge +1) and neutrons (no charge), surrounded by much lighter electrons (charge −1). Protons and neutrons each have a mass of about 1 atomic mass unit (u); an electron has about 1/1,836 of that. So almost all of an atom’s mass is in the nucleus, but almost all of its volume is the space where its electrons are found. If the nucleus of a gold atom were the size of a marble, the atom would be the size of a football stadium.
Atomic number defines the element. The number of protons is the atomic number, Z. Every gold atom has 79 protons; an atom with 78 protons is platinum, not gold. In a neutral atom the number of electrons equals the number of protons, because the charges must balance. The mass number, A, counts protons plus neutrons. So neutrons = A − Z.
Worked example. Gold-197: Z = 79, so 79 protons and 79 electrons; neutrons = 197 − 79 = 118. Iron-56: Z = 26, so 26 protons and 26 electrons, and 56 − 26 = 30 neutrons. Notice that gold has about three times as many protons as iron. Size of the nucleus is not what makes gold unreactive, though; the explanation lies in how its electrons are arranged, which we develop over the next three lessons.
Isotopes. Atoms of one element can have different numbers of neutrons. Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons. They are isotopes of carbon: same element, same chemistry, different mass. The atomic mass printed on the periodic table (12.011 for carbon) is an average over the isotopes found in nature, weighted by how common each is.
Ions. When an atom gains or loses electrons it becomes an ion. A sodium atom that loses one electron has 11 protons and 10 electrons, a net charge of +1 (Na+). A chlorine atom that gains one has 17 protons and 18 electrons, charge −1 (Cl−). Chemical reactions change electrons, never protons: that is why reactions cannot turn iron into gold.
Asking good questions. A scientific question can be tested with evidence. “Why is gold pretty?” cannot. “Does iron rust faster in salt water than in fresh water?” can. As you build the question board today, sort questions into ones we can investigate in the lab, ones we can answer with data from NIST or OpenStax, and ones that need a model before they can be answered.
Formative check
Work through these before moving on. They are not graded — they tell you, and your teacher, whether the standard below has landed yet.
Gold and iron tools sat in the same wet Sierra ground since 1849. Which statement best predicts what you would find today?
Iron-56 has protons, neutrons and electrons in a neutral atom.
Which change turns an atom into a different element?
Simulation & tools
Build an atom, an isotope and an ion
Open the Atom screen. (1) Build a neutral lithium-7 atom and record the protons, neutrons and electrons the sim needs. (2) Add one neutron: record what changes in the element name, the mass number and the charge. (3) Remove one electron: record the charge. (4) Answer: which particle count decides the element, which decides the isotope, and which decides the charge? No device? Complete the particle table for Li-7, Li-6 and Li+ on paper using Z = 3.
Open in a new tab ↗ · PhET Interactive Simulations, University of Colorado Boulder · CC BY 4.0
Write three questions about the Gold Rush phenomenon: one we could answer with a lab, one we could answer with published data, and one that needs a model of the atom first. Explain your sorting in one sentence each.
How confident are you that you can describe an atom’s particles and use atomic number and mass number to find protons, neutrons and electrons?
Practice
Work these on paper or in your notebook, then open Check your answer. Aim for all of Fluency and Application; try at least one Challenge.
Printable version: this unit’s practice workbook (PDF)
Fluency
Build speed and accuracy with the core skill.
- How many protons, neutrons and electrons are in a neutral atom of carbon-14?
Check your answer
Answer: 6 protons, 8 neutrons, 6 electronsProtons = electrons = Z = 6; neutrons = A − Z = 14 − 6 = 8. - How many protons, neutrons and electrons are in a neutral atom of sodium-23?
Check your answer
Answer: 11 protons, 12 neutrons, 11 electronsProtons = electrons = Z = 11; neutrons = A − Z = 23 − 11 = 12. - How many protons, neutrons and electrons are in a neutral atom of gold-197?
Check your answer
Answer: 79 protons, 118 neutrons, 79 electronsProtons = electrons = Z = 79; neutrons = A − Z = 197 − 79 = 118. - How many protons, neutrons and electrons are in a neutral atom of uranium-235?
Check your answer
Answer: 92 protons, 143 neutrons, 92 electronsProtons = electrons = Z = 92; neutrons = A − Z = 235 − 92 = 143. - How many protons, neutrons and electrons are in a neutral atom of lithium-7?
Check your answer
Answer: 3 protons, 4 neutrons, 3 electronsProtons = electrons = Z = 3; neutrons = A − Z = 7 − 3 = 4. - An ion has 12 protons and 10 electrons. What is its charge, and what element is it?
Check your answer
Answer: +2; magnesium (Mg2+)Charge = protons − electrons = 12 − 10 = +2; Z = 12 is magnesium. - An ion has 16 protons and 18 electrons. Give its symbol and charge.
Check your answer
Answer: S2−Z = 16 is sulfur; 16 − 18 = −2.
Application
Use the skill in context. Show your reasoning.
- Carbon-12 and carbon-14 are both carbon. Explain what is the same and what is different about their atoms, and why they react the same way chemically.
Check your answer
Answer: Same: 6 protons and 6 electrons. Different: 6 vs 8 neutrons, mass numbers 12 vs 14. Chemistry depends on electrons, which are the same.Isotopes differ only in the nucleus’ neutron count; the electron arrangement that controls reactions is identical. - A student says iron could be turned into gold by a chemical reaction if we found the right one. Use atomic number to explain why no chemical reaction can do this.
Check your answer
Answer: Iron has 26 protons and gold 79; chemical reactions only move electrons, so they cannot change the number of protons.Element identity = proton number. Changing it is a nuclear process, not a chemical one. - A proton has mass about 1 u and an electron about 1/1,836 u. In a neutral gold-197 atom, estimate what fraction of the atom’s mass the electrons contribute.
Check your answer
Answer: About 0.022% (79 electrons ≈ 0.043 u out of about 197 u)79/1836 = 0.0430 u; divided by 197 u gives 0.000218, about 0.02%.
Challenge
Stretch problems. Expect to think before you write.
- An atom has mass number 40 and 22 neutrons. A second atom has mass number 40 and 20 neutrons. Identify both, and say whether they are isotopes of each other.
Check your answer
Answer: The first has Z = 18 (argon-40); the second has Z = 20 (calcium-40). They are not isotopes: different proton numbers mean different elements.Z = A − neutrons: 40 − 22 = 18, 40 − 20 = 20. Same mass number does not make atoms isotopes.
Review
Keep earlier skills sharp.
- Convert 60 °C to kelvin.
Check your answer
Answer: 333.15 KK = °C + 273.15. - Convert 1250 g to kg.
Check your answer
Answer: 1.25 kg - What is the mass of 5.0 cm3 of aluminum (density 2.7 g/cm3)?
Check your answer
Answer: 13.5 gMass = density × volume.