The modern periodic table
Atomic number and protons
The atomic number of an A substance made of one type of atom only. was originally just its position on the periodic table. After the discovery of Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1., scientists realised that the The number of protons in the nucleus of an atom. Also called the proton number. of an element is the same as the number of protons in its The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei..
In the modern A tabular representation of all known elements in order based on atomic number, eg all the noble gases are found on the right of the periodic table., the elements are arranged according to their atomic number - not their The mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom..
In the periodic table the elements are arranged into:
- horizontal rows, called A horizontal row in the periodic table., in order of increasing atomic number
- vertical columns, called A vertical column in the periodic table containing elements with similar chemical properties., where the elements have similar The characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point.
Learn more on groups in the periodic table in this podcast.
Listen to the full series on 大象传媒 Sounds.
Group numbers
Group numbering has changed over time:
- in older periodic tables, group numbers 1-7 and 0 are given for the main groups
- in newer periodic tables, 鈥業UPAC鈥 group numbers 1-18 are given for all the groups
Hydrogen, H, is placed above the top of group 1 - it is not a member of group 1.
Metals and non-metals in the table
The Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. elements are found on the left-hand side of the periodic table and the Element that is a poor conductor of electricity and heat, and which forms acidic oxides. elements are found on the right. You can imagine a zig-zag line, starting at B-Al-Si, separating metals from non-metals.
Resolving pair reversals
Mendeleev did not know about Atoms of an element with the same number of protons and electrons but different numbers of neutrons., but their existence is an explanation for pair reversals in his table. The positions of iodine and tellurium were reversed in Mendeleev鈥檚 table because:
- iodine has one naturally occurring isotope, 127I
- the most If there is a lot of something, it is described as being abundant. tellurium isotopes are 128Te and 130Te
The high relative abundance of these tellurium isotopes gives tellurium the greater relative atomic mass. The atomic number of tellurium is 52 and the atomic number of iodine is 53, so these elements are in the correct order in the modern periodic table.