Structure of the information cell
Each cell of the table contains the basic "passport" of a chemical element. The data is distributed according to a strict template:
- In the upper left corner, the atomic number is indicated, which corresponds to the nuclear charge.
- In the upper right corner, the exact atomic mass of the element is given.
- In the center are the chemical symbol and its Russian name.
- In the lower part, the electronic configuration of the valence shell is recorded (population of outer sublevels).
For example, the carbon cell (6 C) contains an atomic mass of 12.0107 and the configuration 2–2. For lithium (3 Li), the mass is 6.941, and the outer shell structure is written as 1–0.
Structure of short periods (1–3)
The first three horizontal rows are called short periods. In them, the sequential filling of s- and p-orbitals takes place.
- First period: includes only the 1s orbital. This is where hydrogen (1 H) with a mass of 1.00794 and configuration 1 is located, as well as the noble gas helium (2 He).
- Second period: filling of the 2s and 2p orbitals occurs. The row begins with the alkali metal lithium (3 Li) and ends with neon (10 Ne).
- Third period: 3s and 3p orbitals are filled. Includes elements from sodium (11 Na) to argon (18 Ar).
Features of long periods (4–7)
Starting from the fourth period, the filling of d-orbitals begins, so the rows expand significantly. The order of electron distribution starts to depend on subgroup affiliation.
- 4th period: For main groups (A), the order 4s–3d–4p is characteristic (e.g., gallium 31 Ga has the configuration 2–10–1). For secondary groups (B), the order changes to 3d–4s–4p. This is where the first d-elements appear: scandium, titanium, copper, and zinc.
- 5th period: The 5s, 4d, and 5p orbitals are filled similarly. The row extends from rubidium (37 Rb) to xenon (54 Xe).
- 6th period: The 4f-sublevel appears. The configuration of main groups is built according to the scheme 4f–6s–5d–6p (e.g., lead 82 Pb with a mass of 207.2 and configuration 14–2–10–2).
- 7th period: The heaviest row. The 5f, 7s, 6d, and 7p orbitals are filled. Includes elements up to number 118, some of which (113, 115, 117, 118) are listed without names in the table.
Differences between main (A) and subgroups (B)
Vertical columns share a similar valence shell structure, but differ significantly depending on the subgroup. This is clearly visible when comparing the elements:
- Groups VA and VB: Nitrogen (7 N) and phosphorus (15 P) from the main subgroup have a 2–3 configuration in the outer shell. Subgroup elements differ: vanadium (23 V) has a 2–3–0 structure, while tantalum (73 Ta) has 14–2–3–0.
- Groups VIA and VIB: Oxygen (8 O) and sulfur (16 S) with a 2–4 configuration belong to subgroup A. Chromium (24 Cr) with a 1–5–0 configuration and tungsten (74 W) belong to subgroup B.
- Triads (VIIIA and VIIIB): Instead of classic groups, specific metal triads are identified here. The table includes the iron triad (Fe, Co, Ni), the palladium triad (Ru, Rh, Pd), and the platinum triad (Os, Ir, Pt). Heavy transactinides, namely hassium, meitnerium, and darmstadtium, also belong to VIIIB.
Extracted rows: f-elements
To avoid disrupting the visual compactness of the table, elements with a filling f-sublevel are placed in a separate lower block.
- Lanthanides: They are part of the 6th period (6s–4f–6d orbitals). The series begins after lanthanum (57 La) and includes elements from cerium (58 Ce, mass 140.116) to lutetium (71 Lu).
- Actinides: They belong to the 7th period (7s–5f–6d orbitals). They start after actinium (89 Ac). They include elements such as thorium (90 Th), uranium (92 U, configuration 2–3–1), and mendelevium (101 Md).
To avoid confusing the subgroups: in main (A) groups, the configuration ends with s- and p-sublevel electrons, whereas in subgroup (B) elements, a zero appears at the end of the valence shell entry (e.g., 2-5-0 for manganese), since the outer shell is fully formed and the penultimate d-sublevel is being filled.
Class and exam questions
What do the numbers at the very bottom of an element's cell mean?
These numbers indicate the population of the outer sublevels, i.e., the electron configuration of the valence shell. They reflect the exact distribution of electrons participating in the formation of chemical bonds.
Why are lanthanides and actinides located below the main table?
They represent large families of f-elements from the 6th and 7th periods. They are placed in the lower block solely to preserve the logical structure of the main and subgroups without breaking the table.
What is the difference between subgroup orbitals in the 4th period?
In main subgroups (A), electrons fill the orbitals in the 4s–3d–4p order. In subgroups (B), this order changes to 3d–4s–4p, which affects the chemical properties of the resulting d-elements.
What are triads in the Periodic Table?
In groups VIIIA and VIIIB, elements are grouped in threes: the iron triad (iron, cobalt, nickel), the palladium triad, and the platinum triad. They share a similar electronic structure and atomic mass.
What else to study on this topic
- Detailed analysis of the electronic configurations of the platinum and palladium triads
- Comparison of the structures of halogens (Group VIIA) and manganese subgroup elements (Group VIIB)
- Electronic configuration of heavy actinides using plutonium and americium as examples
- Anomalies in orbital filling order for subgroup elements
- 7th period transactinides: rutherfordium, dubnium, seaborgium, and bohrium
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Structure of Periods and Groups of the Periodic Table" and 3,009 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in "Chapter 5. Structure of Matter"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.