How the periodic table is arranged
Elements are placed in order of atomic number (the number of protons in the nucleus).
- A period is a row. The period number = the number of electron shells. Sodium (2,8,1) has 3 shells, so it is in period 3.
- A group is a column. For groups 1 to 7, the group number = the number of electrons in the outer shell. Group 0 has a full outer shell.
Elements in the same group react in similar ways because they have the same number of outer electrons. That is why the table is so useful: knowing one element lets you predict its family.
Development of the periodic table
Before protons were discovered, chemists ordered elements by atomic mass. Early tables were incomplete and some elements were put in the wrong groups.
In 1869 Dmitri Mendeleev did three clever things:
- He grouped elements by similar properties, even if this meant swapping the mass order (for example tellurium and iodine).
- He left gaps for elements not yet discovered.
- He predicted the properties of those missing elements.
When gallium (1875) and germanium (1886) were found and matched his predictions, scientists accepted his table. Later, the discovery of protons, neutrons and isotopes explained why order by atomic number, not mass, is correct.
Metals and non-metals
Most elements are metals. They are found on the left and towards the bottom. Non-metals are on the right and towards the top. A staircase line separates them; elements near it (like silicon) have mixed properties.
- Metals: shiny, conduct heat and electricity, can be bent or hammered, usually high melting point; their atoms lose electrons to form positive ions; metal oxides are basic.
- Non-metals: dull, poor conductors, brittle when solid, many are gases; they gain or share electrons; their oxides are usually acidic.
Why? Metals on the left have few outer electrons, so losing them is easy. Non-metals on the right have nearly full outer shells, so they gain or share.
Group 0, Group 1 and Group 7
Group 0: noble gases
Helium, neon, argon, krypton, xenon. Full outer shell (helium 2, others 8), so they are very unreactive and exist as single atoms. Boiling point increases down the group as atoms get heavier.
Group 1: alkali metals
Lithium, sodium, potassium, rubidium. One outer electron. Soft, low density; they react with water to make a metal hydroxide (an alkali) and hydrogen: 2Na + 2HโO โ 2NaOH + Hโ. They react with oxygen and chlorine too. Reactivity increases down the group: the outer electron is further from the nucleus and more shielded, so it is lost more easily. Lithium fizzes, sodium melts into a ball, potassium bursts into lilac flame.
Group 7: halogens
Fluorine, chlorine, bromine, iodine. Seven outer electrons; they exist as pairs of atoms (Clโ). Going down, melting and boiling points rise (chlorine gas, bromine liquid, iodine solid) and reactivity decreases: the outer shell is further from the nucleus, so gaining one electron is harder. A more reactive halogen displaces a less reactive one from a solution of its salt: Clโ + 2KBr โ 2KCl + Brโ.
Key formulas and definitions
- Order: atomic number (protons), not mass
- Period = row = number of shells; Group = column = outer electrons (groups 1-7)
- Group 1: 2M + 2HโO โ 2MOH + Hโ; reactivity increases down
- Group 7: more reactive halogen displaces less reactive one; reactivity decreases down
- Group 0: full outer shell โ unreactive; boiling point increases down
Worked examples
1. Magnesium has electron arrangement 2,8,2. Which group and period is it in?
2 outer electrons โ group 2. 3 shells โ period 3.
2. Why did Mendeleev's table become accepted?
He left gaps and predicted properties of unknown elements. When gallium and germanium were found with properties close to his predictions, it showed the table was right.
3. Predict what happens if chlorine water is added to potassium iodide solution.
Chlorine is more reactive than iodine, so it displaces it: Clโ + 2KI โ 2KCl + Iโ. The solution turns brown as iodine forms.
4. Rubidium is below potassium in group 1. Predict its reaction with water.
Even more vigorous than potassium (it may explode), giving rubidium hydroxide and hydrogen, because its outer electron is further from the nucleus and lost more easily.
Common mistakes
- Saying the modern table is ordered by atomic mass. It is ordered by atomic number.
- Mixing up the trends: group 1 gets MORE reactive down; group 7 gets LESS reactive down.
- Thinking group 0 elements have 0 outer electrons. They have full outer shells.
- Calling hydrogen an alkali metal just because it sits above group 1. It is a non-metal gas.