The history of the periodic table

An element contains all the same types of atoms.

The modern periodic table is based on the Russian chemist Dmitri Mendeleev's ideas. Mendeleev arranged elements into groups (vertical columns) and periods (horizontal rows) based on their relative atomic mass and patterns in their properties. Mendeleev left gaps for undiscovered elements and predicted properties of missing elements. When new elements were discovered, they fitted Mendeleev's predictions. Data about properties of elements in the periodic table can be used to work out trends and to make predictions.

Johann Döbereiner noticed 'triads' that linked patterns of the relative atomic masses for three elements. John Newlands noticed an 'octaves' pattern, where every eighth element has similar properties. Scientists rejected Döbereiner's triads and Newlands' octaves because most of the elements did not fit their 'patterns'.

When elements are heated they emit coloured flames. Some elements emit distinctive flame colours such as lithium salts which produce a red flame. The coloured light produced can be split into a line spectrum that is unique to each element. The discovery of some elements such as helium happened because of the development of spectroscopy.

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Inside the atom

Atoms have a tiny, central nucleus that contains protons and neutrons. Electrons travel around the outside of the atom in shells. All the atoms of a particular element have the same number of protons. Atoms have the same number of protons and electrons. The modern periodic table arranges atoms in order of their proton number.

Number of protons + Number of neutrons = Relative atomic mass


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Sorting electrons & Finding elements in the Period

Electrons are arranged in shells around the nucleus. The first shell is closest to the nucleus and can hold 2 electrons. The second shell can hold 8 electrons. The electron arrangement of chlorine can be written as 2.8.7. 

The number of electrons in an atom is the same as the number of protons. For the first 20 elements, the third shell holds 8 electrons. Electrons in different shells have different energy levels. The closer the electron shell to the nucleus, the lower the energy level. 

In the periodic table, elements are arranged in order of proton number. A row across the table is called a period. The number of electrons in the outer shell increases across a period. As you move from left to right along a period, the elements change from metals to non-metals. Properties such as melting points change across a period. These are called trends. Elements in group 0 have full electron shells and are inert- this means they are very unreactive

The number of electrons in the outer shell of an atom is the same as its group number on the periodic table. Atoms of elements with up to 3 electrons in their outer shells are metals. Atoms of elements with 5 or more electrons in their outer shell are non-metals. Elements with a full outer shell are inert gases.

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Group 1 - the alkali metals

A group is a vertical column in the Periodic Table. Group 1 is called the alkali metals. All elements in group 1 are metals and have one electron in the outer shell of their atoms. Group 1 elements are soft metals that can be cut with a knife. The freshly cut surface is shiny but tarnishes quickly is moist air by reacting with oxygen. The physical properties of group 1 (melting point/boiling point/density) show trends down the group. 

The reactivity of group 1 elements is linked to the single electron in the outer shell. They all form ions with a 1+ charge by losing an electron. The outer electron is easiest to lose is the atom is bigger (because the electron is further from the nucleus). So reactivity increases down the group 1 elements as the atoms get bigger.

Group 1 elements all react with water. For example, lithium and sodium fizz and move around on the surface of the water. The reaction gets more violent as you move down group 1. For example, potassium explodes and rubidium explodes more violently. In the reaction, hydrogen gas is formed which pops when lit. The reaction also makes a metal hydroxide which is an alkali and turns pH indicator blue. Group 1 metals are flammable and their hydroxides are harmful and corrosive. When handling group 1 metals, they should be kept away from water and naked flames. 


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Reactions of Group 1 elements

Metal + Water --> Metal hydroxide + Hydrogen                    

(M represents any group 1 metal)     2M(s) + 2H2O (l) --> H2 (g) + 2MOH (aq)

Metal + Chlorine --> Metal chloride

(M represents any group 1 metal)     2M (s) + Cl2 (g) --> 2MCl (s)

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Group 7 - the halogens

The elements in group 7 are called the halogensThe halogens become darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green, and bromine is red-brown. Iodine crystals are shiny purple - but easily turn into a dark purple vapour when they are warmed up. The halogens all contain diatomic molecules which means they have two atoms joined together in each molecule. The formulae of the halogens are: Chlorine: Cl2; Bromine:  Br2; Iodine: I2. The physical properties of the halogens show a trend down the group. For example, melting points and boiling points increase. 

Group 7 elements are corrosive and toxic. They need to be used in a fume cupboard. They react with alkali metals and with other metals such as iron to form metal halides. eg. Potassium + Iodine --> Potassium iodide. Halogens are less reactive down the group. For example, sodium reacts vigourously in chlorine but less violently in iodine. Displacement reactions happen when a more reactive halogen takes the place of a less reactive halogen in a compound. Chlorine is more reactive than bromine and displaces bromine from potassium bromide solution. 

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Reactivity of group 7 & Ionic compounds

All halogen atoms have 7 electrons in their outer shell. The trend in reactivity is linked to the number of electron shells in the atom. For non-metals, the smaller the atom (the fewer the electron shells), the more reactive the element. The reactivity of group 7 decreases (gets slower) down the group - this is the opposite pattern to group 1.

What are ionic compounds?

Compounds of a group 1 element and a group 7 element (eg. sodium chloride) are solids with high melting points. These compounds are ionic compounds beacuse they contain charged particles (ions) that are arranged in a regular pattern called a crystal lattice:


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Forming ions, formulae and charges:

Ionic compounds are soluble in water and conduct electricity when they are melted. When an ionic crystal melts or dissolves in water, the ions are free to move. An ionic compound conducts electricity when molten or in a solution but not when soild. This is because the ions must move to the electrodes to complete the circuit. 

Forming ions, formulae and charges:

When a group 1 metal atom becomes an ion it loses one electron from its outer shell. Therefore, all group 1 metals become ions with a positive charge eg. Na becomes Na+.

When a group 7 metal atom becomes an ion it gains one elctron in its outer shell. Therefore, all group 7 metals become ions with a negative charge eg. Cl becomes Cl-. 

The ions have the same electron arrangement as an atom in group 0.

In the formulae for an ionic compound, the number of positive and negative charges must balance. For example, Na+ and Cl- have one positive and one negative charge and so form the compound NaCl.

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