Particles in the atom
Introduced| English | Meaning |
|---|---|
| atom/ˈætəm/ | The smallest unit of an element that retains its chemical identity, consisting of a nucleus and electrons. |
| nucleus/ˈnjuːklɪəs/ | The small, dense central part of an atom, containing protons and usually neutrons. |
| proton/ˈprəʊtɒn/ | A positively charged subatomic particle found in an atom's nucleus. |
| neutron/ˈnjuːtrɒn/ | An electrically neutral subatomic particle found in most atomic nuclei. |
| electron/ɪˈlektrɒn/ | A negatively charged subatomic particle found outside an atom's nucleus. |
| shell/ʃel/ | A main electron energy level in an atom. |
| proton number/ˈprəʊtɒn ˈnʌmbə/ | The number of protons in an atomic nucleus, which identifies its element. |
| nucleon number/ˈnjuːklɪən ˈnʌmbə/ | The total number of protons and neutrons in an atomic nucleus. |
| ion/ˈaɪɒn/ | An atom or group of atoms with a net electric charge from losing or gaining electrons. |
A tiny centre holds nearly all the mass
- An atom 原子 is mostly empty space. A small dense nucleus 原子核 holds protons 质子 and neutrons 中子; electrons 电子 occupy the space around it in shells 壳层.
- Separate three questions: where is the mass, where is the charge, and how many particles are present? A large drawing of a shell does not mean that most of the mass is there.
Mass and charge are distributed differently
| Particle | Relative charge | Relative mass |
|---|---|---|
| proton | +1 | 1 |
| neutron | 0 | 1 |
| electron | −1 | about 1/1836 |
- Nearly all the mass is in the nucleus because protons and neutrons are much heavier than electrons. The positive charge is also in the nucleus; negative charge is associated with electrons outside it.
- An electron's mass is small, not literally zero. A neutral atom has equal positive and negative charges even though those charges occupy different regions.
The drawing separates the regions; the nucleus and shells are not drawn to the same scale.
Explore the atom
Tap each part. A tiny dense nucleus of protons and neutrons holds the mass; light electrons occupy shells around it.
A neutron has a relative charge of +1 and a proton has no charge.
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False. It is the other way round: a proton has a relative charge of +1 and a neutron has no charge. Electrons carry −1.
Which particle has a relative charge of 0 and a relative mass of 1?
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A neutron is neutral (charge 0) with relative mass 1; the proton is +1 mass 1; the electron is −1 and almost massless.
Match each particle to its relative charge and mass.
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Protons and neutrons (mass 1 each) sit in the nucleus; the much lighter electrons occupy shells around it.
Count the nucleus first, then the electrons
- Proton number 质子数 $Z$ counts protons; nucleon number 核子数 $A$ counts protons plus neutrons. Therefore neutrons $=A-Z$. Changing the number of electrons makes an ion 离子 without changing $A$ or $Z$.
- Start a neutral atom with $Z$ electrons. Subtract electrons for a positive charge and add them for a negative charge. Treat the superscript charge separately from the nucleon number.
For $^{27}_{13}\mathrm{Al}^{3+}$: protons $=13$, neutrons $=27-13=14$, electrons $=13-3=10$. Check the charge from protons minus electrons: $13-10=+3$. The ion has not lost three protons.
The number of neutrons in an atom is:
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A counts protons + neutrons and Z counts protons, so neutrons = A − Z.
How many electrons does the ion ²⁷₁₃Al³⁺ have?
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A neutral Al atom has 13 electrons; the 3+ ion has lost 3, leaving 13 − 3 = 10.
Determine the numbers of protons, neutrons and electrons in ³⁴₁₆S²⁻ and ⁴⁰₂₀Ca²⁺. Show a charge check for each. Are these two species isotopes of the same element?
S²⁻ has 16 protons, 34 − 16 = 18 neutrons and 16 + 2 = 18 electrons; 16 − 18 = −2. Ca²⁺ has 20 protons, 40 − 20 = 20 neutrons and 20 − 2 = 18 electrons; 20 − 18 = +2. They have equal electron counts but different proton numbers, so they are different elements, not isotopes of one element.
Compare beams at the same velocity
- For separate beams entering the same electric field at the same velocity, protons bend towards the negative plate and electrons towards the positive plate. Neutrons continue straight because they have no charge.
- Protons and electrons have equal charge magnitude, so the force magnitudes are equal. Electrons accelerate more because of their much smaller mass; under these stated conditions their path bends much more.
Schematic directions for equal entry velocities; the relative curvatures are not a quantitative scale.
Separate beams of protons, neutrons and electrons enter the same electric field at the same velocity. Which is deflected most?
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The electron and proton have equal charge magnitude and equal force magnitude in the field. Under equal entry velocities the much lighter electron accelerates more and bends more; the neutron has no electric force.
Separate proton, neutron and electron beams enter vertically upwards between two vertical plates. The left plate is positive and the right plate negative. Entry velocities are equal. On paper sketch and label the three paths, then explain their directions and relative bending here.
The proton bends right towards the negative plate. The electron bends left towards the positive plate and much more, because the equal charge magnitude gives equal force magnitude but the electron has much smaller mass. The neutron continues vertically upwards because it has no charge. Keep a common starting direction and label each path; retain the sketch for teacher review.
Check what is being held constant
- A positive ion and a proton can bend in the same direction but by different amounts. With the same charge and entry velocity, a heavier particle accelerates less in the same field.
- Do not rank deflections using mass alone if charge or entry velocity also changes. The same-velocity condition is part of the comparison, rather than an optional detail.
Direction depends on the sign of charge. The amount of bending also depends on mass, charge magnitude and the motion through the field. A neutral particle has no electric force in this comparison; it does not bend weakly towards either plate.
Two positive ions have the same charge and enter the same electric field at the same velocity. Ion Q has twice the mass of ion P. Compare the direction and degree of bending, and explain why greater mass does not mean greater electric force.
Both bend towards the negative plate. The electric force is the same because charge and field are the same; Q accelerates less because it has greater mass, so it bends less during its passage through the field. Mass affects the response to the force, rather than increasing the electric force here.
You've got it
- The small nucleus contains nearly all the mass and all positive charge; electrons carry negative charge outside it.
- Count protons from $Z$, neutrons from $A-Z$, then adjust electrons for the ion charge.
- For equal entry velocities in the same field: electron and proton bend to opposite plates, the electron more; the neutron remains straight.
- Name the comparison conditions before explaining a different deflection.