2.2 Atomic structure and the Periodic Table
- Syllabus
- 0620–2026–2027
- Topic
- 2.2
- Level
- —
Learning objectives
2.2.1Structure of the atom as a central• Describe structure of the atom as a central nucleus containing neutrons and protons surrounded by electrons in shells2.2.2Relative charges and relative masses• State the relative charges and relative masses of a proton, a neutron and an electron2.2.3Proton number/atomic number as the• Define proton number/atomic number as the number of protons in the nucleus of an atom2.2.4Mass number/nucleon number as the• Define mass number/nucleon number as the total number of protons and neutrons in the nucleus of an atom2.2.5Electronic configuration of elements• Determine the electronic configuration of elements and their ions with proton number 1 to 20, e.g. 2,8,32.2.6Periodic table shell rules: (a) Group• State periodic table shell rules: (a) Group VIII noble gases have full outer shells (b) outer-shell electrons equal group number in Groups I-VII (c) occupied shells equal period number
Locate the particles in an atom
An atom has a tiny central nucleus containing protons and neutrons. Electrons occupy shells around the nucleus.
| Region | Particles | Structural meaning |
|---|---|---|
| nucleus | protons and neutrons | contains almost all the atom's mass and has an overall positive charge |
| shells around nucleus | electrons | occupy most of the atom's volume |
An atom is electrically neutral because it has equal numbers of positively charged protons and negatively charged electrons.
Electrons are not in the nucleus, and neutrons do not cancel an atom's charge: neutrons have no charge.
State subatomic charges and masses
Relative charge compares electrical charge; relative mass compares particle mass using a proton or neutron as approximately one unit.
| Particle | Relative charge | Relative mass |
|---|---|---|
| proton | +1 | 1 |
| neutron | 0 | 1 |
| electron | −1 | 1/1840 |
Because an electron's relative mass is extremely small, nearly all atomic mass comes from the protons and neutrons in the nucleus.
A neutron is neutral, not negative. Protons and electrons have equal charge magnitude but very different masses.
Use proton number to identify an element
Proton number, also called atomic number, is the number of protons in the nucleus of one atom of an element. It is written as Z.
| Information | Consequence |
|---|---|
| proton number Z | number of protons |
| neutral atom | number of electrons = Z |
| same element | every atom has the same Z |
| different Z | different element |
Forming an ion changes only the number of electrons. It does not change the proton number, so the ion remains the same element.
Atomic number is not the total number of particles in the nucleus and does not change when an atom gains or loses electrons.
Use mass number to count nucleons
Mass number, also called nucleon number, is the total number of protons and neutrons in the nucleus of one atom. It is written as A.
| Quantity | Relation |
|---|---|
| mass number | A = protons + neutrons |
| proton number | Z = protons |
| neutron number | neutrons = A − Z |
In nuclide notation ᴬ_ZX, the upper number is mass number A and the lower number is proton number Z. Both are whole-number counts for one atom.
Electrons are not nucleons and are not included in mass number. Two different elements can have the same mass number but must have different proton numbers.
Write electron configurations for atoms and ions
For proton numbers 1–20, place electrons into shells from the inside out. The school-model capacities used here are 2, then 8, then 8, before the fourth shell begins.
| Step | Action |
|---|---|
| 1 | use proton number Z to count electrons in a neutral atom |
| 2 | fill shells in order and write totals separated by commas, such as 2,8,3 |
| 3 | for a positive ion, subtract the number of electrons shown by the charge |
| 4 | for a negative ion, add the number of electrons shown by the charge |
| 5 | check that the electron total and the written ionic charge agree |
| Particle | Electron count | Configuration |
|---|---|---|
| Na | 11 | 2,8,1 |
| Na⁺ | 10 | 2,8 |
| S | 16 | 2,8,6 |
| S²⁻ | 18 | 2,8,8 |
| Ca²⁺ | 18 | 2,8,8 |
Charge = number of protons − number of electrons. Particles with the same electron configuration are isoelectronic, but they can have different proton numbers and charges.
Losing electrons makes a positive ion; gaining electrons makes a negative ion. Never change the number of protons to create an ion.
Read group and period from electron shells
For the main-group elements in this objective, electron configuration links directly to position in the Periodic Table.
| Feature | Electron-shell rule |
|---|---|
| Group I–VII | group number = number of outer-shell electrons |
| Group VIII noble gases | outer shell is full |
| period | period number = number of occupied electron shells |
2,8,1 has three occupied shells and one outer electron, so it is Period 3, Group I. The configuration 2,8,7 is Period 3, Group VII. The configuration 2,8,8 is Period 3, Group VIII.
Helium is a noble gas with configuration 2. Its first shell is full with two electrons, so it belongs to Group VIII despite not having eight outer electrons.
Do not use the total number of electrons as the group number or count empty shells for the period. The simple outer-electron group rule is for Groups I–VII, with noble gases treated by their full outer shell.