1.1 Particles in the atom and atomic radius
- Syllabus
- 9701–2028–2029
- Topic
- 1.1
- Level
- AS
Learning objectives
1.1.1Atomic structure and nucleus• Understand: atoms are mostly empty space around a small dense nucleus (protons/neutrons); electrons occupy shells.1.1.2Protons, neutrons and electrons• Identify/describe: protons, neutrons and electrons in terms of their relative charges and masses1.1.3Terms: atomic and proton number, mass• Know terms: atomic and proton number, mass and nucleon number1.1.4Distribution of mass and charge within an atom• Describe the distribution of mass and charge within an atom1.1.5Behaviour of beams of protons, neutrons• Describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field1.1.6Determine the numbers of protons, neutrons• Determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic/proton number, mass/nucleon number and charge1.1.7Trends: the variations in atomic radius• State/explain trends: the variations in atomic radius and ionic radius across a period and down a group
Most of an atom is empty space
An atom has a very small, dense nucleus containing protons and neutrons. Electrons occupy shells in the much larger region around the nucleus, so most of the atom’s volume is empty space.
The nucleus is tiny compared with the whole atom but contains almost all its mass. The electron shells describe allowed energy regions around the nucleus; they do not fill the space with solid matter.
“Mostly empty” does not mean the atom has no structure. The positive nucleus and negative electrons are held together by electrostatic attraction, while the shell model records where electrons may be found rather than fixed planetary paths.
Compare the three subatomic particles
Protons, neutrons and electrons are distinguished by relative charge and relative mass. Relative values compare the particles on a convenient scale; they are not their masses in grams or charges in coulombs.
| Particle | Relative charge | Relative mass |
|---|---|---|
| proton | +1 | 1 |
| neutron | 0 | 1 |
| electron | −1 | 18361 |
A proton and an electron have equal-magnitude opposite charges. A neutron is uncharged. Because an electron has far less mass than either nuclear particle, electron mass is usually negligible when accounting for an atom’s mass.
Atomic number and mass number count different particles
Atomic number and proton number are two names for the number of protons in an atom’s nucleus. Mass number and nucleon number are two names for the total number of protons plus neutrons in that nucleus.
| Quantity | Symbol | What it counts |
|---|---|---|
| atomic (proton) number | Z | protons |
| mass (nucleon) number | A | protons + neutrons |
The atomic number identifies the element. The mass number describes one particular nuclide and is always a whole number; it is not the relative atomic mass shown in many Periodic Tables, which can be a weighted mean.
Mass and charge are distributed differently in an atom
Nearly all the mass of an atom and all its positive charge are concentrated in the nucleus. The surrounding electron region contributes negative charge but very little mass.
Protons and neutrons each have relative mass about 1, whereas an electron has relative mass 18361. Neutrons add mass but no charge; protons add mass and positive charge; electrons add negative charge outside the nucleus.
A neutral atom has equal numbers of protons and electrons, so its total charge is zero even though positive and negative charge occupy different regions. Zero overall charge does not mean that charge is absent.
How particle beams move in an electric field
An electric field exerts a force on charged particles. For proton, neutron and electron beams moving at the same velocity, the direction of bending reveals charge and the amount of bending reflects mass as well as charge.
| Beam | Charge | Path between charged plates | Relative deflection |
|---|---|---|---|
| proton | +1 | toward the negative plate | small |
| neutron | 0 | straight, with no deflection | none |
| electron | −1 | toward the positive plate | large |
The proton and electron experience forces in opposite directions because their charges have opposite signs. Their charge magnitudes are equal, but the electron’s mass is much smaller, so it accelerates and bends much more strongly. The comparison depends on the stated equal velocity and electric-field arrangement.
Count particles in atoms and ions
Use A for mass number, Z for atomic number and q for the signed ionic charge. Read the nuclear counts first; only then use the charge to adjust the electron count.
\text{protons}=Z,\qquad \text{neutrons}=A-Z,\qquad \text{electrons}=Z-q
For 2040Ca2+, A=40, Z=20 and q=+2. It therefore has 20 protons, 40−20=20 neutrons and 20−(+2)=18 electrons. A positive ion has lost electrons; a negative value of q makes Z−q larger because electrons were gained.
Charge changes only the electron count. If a calculation changes the proton number, it has changed the element rather than formed an ion.
Explain trends in atomic and ionic radius
Radius reflects the balance between nuclear attraction, the distance of the outer occupied shell and shielding by inner electrons. Use these causes—not a memorised arrow alone—to explain each trend.
| Direction | Atomic radius | Why |
|---|---|---|
| across a period | generally decreases | proton number rises while electrons enter the same principal shell, so nuclear attraction strengthens |
| down a group | increases | an extra occupied shell increases distance and shielding |
A cation is smaller than its atom because losing electrons reduces electron–electron repulsion and may remove an outer shell. An anion is larger because added electrons increase repulsion within the electron region. Down a group, ions of comparable charge become larger as shells are added.
Across one period, radii generally decrease within the cation series and within the anion series as nuclear charge increases. There is a large jump between the last cation and first anion because the anions occupy an additional outer shell. Compare like species before applying a trend.