Use the periodic law and an element's position to identify its family, classify it as metal, nonmetal or metalloid, and predict its ionic charge

Learning goal: State the periodic law and explain what makes a group of elements a family; use an element's place on the periodic table to tell whether it is a metal, nonmetal or metalloid and whether it belongs to the alkali metals, alkaline earth metals, halogens or noble gases; describe trends in simple observable properties within a family; and use position or an electron dot diagram to identify the ionic charge of elements in groups 1, 2, 17 and 18.

Why the table has the shape it has. Line the elements up by atomic number and a pattern of properties keeps coming back: a very reactive metal, then a less reactive metal, and some elements later a reactive nonmetal and then an unreactive gas, after which the pattern starts again. This repetition is the periodic law. The periodic table is built to display it: each horizontal row, a period, runs through one cycle, and each vertical column, a group or family, collects the elements that fall at the same point in every cycle. Elements in the same group have similar chemical and physical properties because, in the Bohr picture, they have the same number of valence electrons: every group 1 element has one, every group 17 element has seven, every group 18 element has a full outer shell. That is why the table works as a prediction tool. When a new element was made or discovered, its position told chemists what to expect before its properties were measured, and the trends within a family were used to predict values that had not yet been observed.

Metals, nonmetals and metalloids. Middle school introduced the properties of metals (shiny, malleable, ductile, good conductors of heat and electricity, mostly solids at room temperature) and nonmetals (dull, brittle when solid, poor conductors, many of them gases). On the table, metals occupy the left and center and nonmetals the upper right. A staircase line separates them, and the elements that touch that line are the metalloids: boron, silicon, germanium, arsenic, antimony and tellurium. Metalloids have some properties of each class; silicon, for example, is shiny like a metal but brittle, and conducts electricity poorly at room temperature yet better when warmed or when small amounts of other elements are added, which is the basis of computer chips. Hydrogen sits in group 1 for its single valence electron but is a nonmetal gas, not an alkali metal, and its position is a reminder that the column tracks the number of valence electrons, while the metal or nonmetal class is a separate property that must be read from the staircase line.

Four named families. The alkali metals are group 1 below hydrogen: lithium, sodium, potassium, rubidium, cesium and francium. They are soft, silvery metals that react vigorously with water, and the reactions become more violent down the group. The alkaline earth metals are group 2: beryllium, magnesium, calcium, strontium, barium and radium. They are harder and less reactive than their group 1 neighbors, though still reactive metals. The halogens are group 17: fluorine, chlorine, bromine, iodine and astatine. They are reactive nonmetals that readily form 1− ions and combine with metals to make salts; at room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid, a trend in state that runs down the group. The noble gases are group 18: helium, neon, argon, krypton, xenon and radon. They are colorless gases that are almost entirely unreactive, because their outer shell is already full; they are used where an unreactive atmosphere is needed, such as inside filament bulbs and around welding.

Trends within a family. Simple observable properties change steadily down many families, and the trend can be used to estimate an unmeasured value. Among the alkali metals the melting point falls down the group: lithium melts at about 180 °C, sodium at about 98 °C, potassium at about 63 °C, rubidium at about 39 °C and cesium at about 28 °C, so cesium would melt in a warm room. Density generally rises down the same group, though not perfectly smoothly: potassium is slightly less dense than sodium, which is a useful warning that trends are patterns, not laws without exception, and a predicted value must be checked by measurement. Reactivity increases down group 1 (the reaction with water becomes more violent) and decreases down group 17, so fluorine is the most reactive halogen and cesium the most reactive alkali metal among those that can be handled. Other trends across the table, such as atomic radius, electronegativity and ionization energy, are studied in chemistry and are not needed to use the table at this level.

Ionic charge from position. Groups 1, 2 and 17 have characteristic ionic charges that will be used to predict the formulas of compounds. Draw the electron dot diagram and the charge follows: group 1 atoms show one dot and lose it to form 1+ ions; group 2 atoms show two dots and form 2+ ions; group 17 atoms show seven dots and gain one to form 1− ions; group 18 atoms show eight (helium two) and form no ion. Hydrogen is the special case: with one electron it is counted as 1+ when predicting formulas with nonmetals such as chlorine and oxygen (HCl, H₂O), although in those molecules it actually shares its electron in a covalent bond rather than giving it up. Because the group fixes the charge, sodium, potassium and rubidium all form 1+ ions and all react with chlorine in the same one-to-one ratio.

A worked model: two elements compared. Take sodium (group 1, period 3) and chlorine (group 17, period 3). Sodium is a soft, shiny metal that conducts electricity, has one valence electron and forms Na⁺; chlorine is a yellow-green gas that does not conduct, has seven valence electrons and forms Cl⁻. They are opposites in almost every observable property, yet sit in the same period; what they share is a period number, meaning their valence electrons are in the same shell. Now take sodium and potassium (group 1, periods 3 and 4). Both are soft reactive metals forming 1+ ions; potassium reacts faster with water and melts at a lower temperature. Same group, similar behavior, with a steady shift down the column. The table encodes both comparisons: read across for change, read down for family resemblance.

Investigation: sort unknown elements with a flow chart, safely. Materials: small sealed samples or clear photographs and data cards for eight elements chosen by the teacher (for example copper, carbon as graphite, sulfur, aluminum, silicon, zinc, iodine in a sealed vial, tin), a simple conductivity tester (battery, bulb, two probes), a hammer and a scrap board for a malleability test on teacher-approved samples only, a hand lens, goggles. Steps: build a flow chart with the questions "Does it conduct electricity?", "Is it shiny?", "Does it bend or flatten rather than shatter when tapped?" leading to metal, nonmetal or metalloid; test each sample and record each answer; classify; then compare your classification with the sample's position on the periodic table. Record: a table of sample, three test results, your classification and the table's classification. A result that supports the claim: samples the chart calls metals sit left of the staircase and samples it calls nonmetals sit to the right, with silicon giving mixed answers (shiny but brittle, weakly conducting) that place it on the line. Safety: goggles; iodine and sulfur stay sealed and are observed only, because iodine vapor is irritating; no alkali metals or halogens are handled by students; wash hands after handling any sample. What the model cannot show: graphite is a known exception to the flow chart, a nonmetal that conducts electricity, so a chart built only on conductivity and luster would misplace carbon; its position on the table settles it. A conductivity bulb gives yes or no, not a measurement, so it cannot rank a metalloid between a metal and a nonmetal, and appearance tests can be fooled by tarnish or coatings.

Worked example 1 — identify the family from position. An element is in group 2, period 4. Group 2 makes it an alkaline earth metal; period 4 places it as calcium. Predictions: a reactive metal, harder and less reactive than potassium beside it in group 1, forming a 2+ ion, with two dots in its electron dot diagram. An element in group 18, period 3 is argon: a noble gas, unreactive, no ion. An element in group 17, period 4 is bromine: a halogen, a nonmetal, forming Br⁻, and a liquid at room temperature in line with the trend from gaseous chlorine above it to solid iodine below.

Worked example 2 — use a trend to estimate a value. The melting points of lithium, sodium, potassium and cesium are about 180, 98, 63 and 28 °C. Rubidium lies between potassium and cesium in the group, so the trend predicts a melting point between 63 and 28 °C. The measured value, about 39 °C, fits. The prediction is only a range, and the density of the same family shows why: potassium is less dense than sodium even though the overall trend is upward, so a value estimated from a trend must be labeled as an estimate until measured.

Worked example 3 — metal, nonmetal or metalloid from tests. A gray, shiny sample conducts electricity, dents when tapped and is in the lower center of the table: metal (tin, for example). A yellow, dull, brittle sample that does not conduct and sits in group 16: nonmetal (sulfur). A gray, shiny but brittle sample that conducts only weakly and sits on the staircase in group 14: metalloid (silicon). The mixed result is the signature of a metalloid, and the position on the staircase line confirms it.

Worked example 4 — ionic charge from a dot diagram. Strontium is in group 2, so its dot diagram shows Sr with two dots; it loses both and forms Sr²⁺. Iodine is in group 17 with seven dots; it gains one and forms I⁻. Krypton in group 18 has eight dots and forms no ion. Rubidium in group 1 has one dot and forms Rb⁺. In each case the charge is decided by the group, so an element's column is enough to predict it without any other data.

Representation task: a labeled sketch of the table. Draw a blank periodic table outline for periods 1 to 4 of the main groups (groups 1, 2 and 13 to 18) and shade metals, nonmetals and metalloids in three colors. Label the four named families, mark hydrogen as a nonmetal, and write beside groups 1, 2, 17 and 18 the ionic charge each forms. Under the sketch write the periodic law in your own words and one sentence saying why the columns, not the rows, hold the elements that behave alike.

Independent performance: an evidence-based classification key. Write a dichotomous key (a series of paired yes/no choices) that classifies any element sample as a metal, nonmetal or metalloid using only tests a student can do safely: appearance, conductivity and response to tapping. Success criteria: the key has at least three decision points, each with a specific observable test and both outcomes; it correctly classifies aluminum, sulfur, silicon and copper; it includes a note on where the key can be fooled (a tarnished metal, a coated sample) and what the tester would do about it; and it ends with a paragraph explaining, using the words periodic law, group, valence electrons and family, why the sample's position on the table is expected to agree with the key.