All practice pages/Interatomic theory
Protons, neutrons and electrons, isotope symbols, average atomic mass, and how to read a group off the table. The in-class sheet, generated practice and rounds of ten.
Finished Monday 21 September
Reading a periodic table square, isotopes and ions, naming an ion, atomic mass as a weighted average, then the group names and the properties of metals and nonmetals. The whole sheet has been worked through; compounds is what lecture is on now.
The atomic number is the element. Change the neutrons and you have an isotope of the same element; change the electrons and you have an ion of it. Only a change in protons makes it something else.
Protons come from the atomic number, which sits below the symbol on the table as a whole number. Neutrons are the mass number minus the atomic number. Electrons equal protons in a neutral atom, and the charge is protons minus electrons, so a 2+ ion has lost two of them.
This is why the mass on the table is never a whole number, and why saying "an atom of chlorine weighs 35.45 u" is wrong. No chlorine atom weighs that. Three quarters of them weigh about 35 u and the rest about 37 u, and 35.45 is the average of the crowd.
The periodic law: the properties of the elements are periodic functions of their atomic numbers. Mendeleev and Meyer found that periodicity independently, both ordering the elements by increasing atomic mass, and Mendeleev went further, leaving gaps in his table and predicting the properties of elements nobody had found yet. Moseley settled the ordering in 1913, showing that it follows atomic number rather than mass, and every table since has been built that way. The lecture dates Mendeleev's table to 1871, the year of his revised version, and the book puts the first one at 1869. How that argument ran.
Every square carries the atomic number Z above the symbol and the atomic mass in amu below it, which is the weighted average of that element's isotopes. Those are the three things a printed table gives you; the mass number A is not one of them. The far end of the table, element 104 onwards, is made an atom at a time, so those squares are coloured by the column they sit in rather than by chemistry anyone has been able to test.
| Group | Also called | Name |
|---|---|---|
| 1A | group 1 | alkali metals |
| 2A | group 2 | alkaline earth metals |
| 3B to 2B | groups 3 to 12 | transition metals |
| 5A | group 15 | pnictogens |
| 6A | group 16 | chalcogens, the oxygen family |
| 7A | group 17 | halogens |
| 8A | group 18 | noble gases |
A group is a column and a period is a row. The old A and B labels and the 1 to 18 numbering both turn up, so read whichever the question uses.
The metalloids sit along the staircase that steps down the right of the table: boron, silicon, germanium, arsenic, antimony, tellurium and astatine, seven of them. They behave partly like each side, which is what makes silicon a semiconductor. Polonium is the next square along the staircase, but the book counts it as a metal, so leave it off the list. Astatine is the awkward one: it sits in group 17 with the halogens and the book still shades it a metalloid, so it answers to both.
Seven rows, eighteen columns. The rows are periods, the columns are groups, and the columns fall into three parts: the A groups on the two wings, the B groups in the middle, and the two rows printed underneath that really belong inside the table.
Two numbering systems are in use and a question can be written in either. The older American one splits the columns into A groups and B groups, so the halogens are group 7A. The newer international one runs straight across, 1 to 18, so the same column is group 17. The clickable table further up the page prints 1 to 18, and the square you pick names it both ways.
Where those two rows start is not settled. The textbook, and the table further up this page, begin them at lanthanum and actinium, so 57 to 71 and 89 to 103. The lecture slide begins them at cerium and thorium, so 58 to 71 and 90 to 103, which is why it calls the block fourteen columns wide rather than fifteen. Nothing else changes: the upper row is the lanthanides and the lower one the actinides either way. Answer with the book's numbers on the homework, since the homework comes from the book.
The Chapter 2 Atoms and the Periodic Table sheet, question by question. Fill in every box on a card, then check it.
A new atom, ion or element every time, asked the way the sheet asks it.