Chemistry · Atomic structure and the periodic law · Grade 7

Elements and the periodic table

Why do 118 elements fit into one table, and what does it tell us about the atom?

The situation

On the Mendeleev table on the chemistry-room wall there are 118 cells: each holds a symbol, a number and a name. Gold is mined at Muruntau, copper at Almalyk and uranium in the Navoi region — they too sit in cells of this table. Why are the elements arranged in precisely this order? What do the numbers in a cell say about the atom, and can the properties of an element be predicted from the table?

Topic

Elements and the periodic table *(sample)*. Mendeleev's table in 3D: an element in every cell, its atom and its use in everyday life

Key ideas

Chemical element
Period and group
Atomic structure

Formulas

Z = N (p^{+}) = N (e^{-})

The atomic number is the number of protons in the nucleus; a neutral atom has the same number of electrons.

A = Z + N

Mass number: the sum of protons and neutrons ( $N$ ).

N_{\max} = 2 n^{2}

Electrons that fit in shell $n$ : K — 2, L — 8, M — 18, N — 32.

A_{r} = \sum_{i} x_{i} A_{r, i}

Relative atomic mass is the average mass of the natural isotopes ( $x_{i}$ are the fractions).

Theory

A chemical element and its symbol

A chemical element is a type of atom with the same nuclear charge (number of protons). Letter symbols were proposed in 1813–1814 by J. Berzelius: the first letter, or two letters, of the Latin name — Na (*natrium*), Fe (*ferrum*), Cu (*cuprum*), Au (*aurum*), Hg (*hydrargyrum*). The atomic number $Z$ is the number of protons; a neutral atom also has $Z$ electrons. Today 118 elements are known: almost all of those up to uranium ( $Z$ = 92) occur in nature, and those after it were made artificially (as were technetium and promethium).

How the table is built

The horizontal rows are periods (7): the period number equals the number of electron shells in the atom. The vertical columns are groups (18). In the short-form table of many textbooks there are 8 groups, each divided into A (main) and B (subsidiary) subgroups; for elements of subgroup A the group number equals the number of electrons in the outer shell (Na — I A, 1; Cl — VII A, 7). The lanthanides and actinides are in separate rows below the table. Metals are on the left and at the bottom, non-metals at the top right, and along the stair-step boundary are the metalloids (B, Si, Ge, As, Sb, Te).

The structure of the atom and electron shells

At the centre of an atom is the nucleus — protons (+) and neutrons (no charge) — and around it are the electrons (−) in shells: shell $n$ holds at most $2 n^{2}$ electrons, and the outer shell no more than 8. The model on the stage is the Bohr model, a simplified picture: in reality the electrons are in s, p, d, f orbitals (the configuration on the card, for example Fe: [Ar] 3d⁶ 4s²). The sizes are schematic: the real nucleus is about 100 000 times smaller than the atom.

The periodic law and trends

In 1869 D. I. Mendeleev showed that the properties of the elements depend periodically on atomic mass; the modern statement is that they depend on the nuclear charge. Across a period from left to right the nuclear charge increases while the number of shells stays the same: the electrons are pulled in more strongly — the atomic radius decreases, and the ionisation energy and electronegativity increase, and the metallic properties weaken. Down a group the shells increase in number — the radius grows and it becomes easier to remove an electron. Apart from period 1, every period begins with an alkali metal and ends with a noble gas.

Isotopes and relative atomic mass

Isotopes are atoms with the same number of protons but different numbers of neutrons: ³⁵Cl (17 p⁺, 18 n⁰) and ³⁷Cl (17 p⁺, 20 n⁰). Their chemical properties are almost identical because their electrons are the same. The $A_{r}$ in the table is the average mass of the mixture of natural isotopes (relative to 1/12 of the mass of a ¹²C atom): for chlorine $0.7576 \cdot 34.969 + 0.2424 \cdot 36.966 \approx 35.45$ . A number in square brackets, for example Tc [97], is the mass number of the longest-lived isotope of an element that has no stable isotope.

About the sources and the model

Atomic masses are the IUPAC/CIAAW (2024) abridged standard values; ionisation energies are from NIST; covalent radii are from Cordero et al. (2008); electronegativity is on the Pauling scale. The properties of the elements after element 104 are mostly theoretical. The trend columns are proportional to the value. In Uzbekistan: gold — Muruntau (Navoi region); copper, molybdenum, zinc, silver — the Almalyk Mining and Metallurgical Combine; uranium — the Navoi region; potash fertiliser — Dehkanabad; phosphorite — Kyzylkum.

Did you know?

In 1869 D. I. Mendeleev left empty cells in his table for elements not yet discovered and predicted their properties: “eka-aluminium” — gallium (1875), “eka-boron” — scandium (1879) and “eka-silicon” — germanium (1886) were discovered later.
Element 118, oganesson, was named in 2016 after the physicist Yuri Oganessian — the second element named after a scientist who was alive at the time (the first was seaborgium, after Glenn Seaborg).

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Atomic structure and isotopes — Grade 7
Electron shells and orbitals — Grade 8