Periodic Table

Elements The Greeks thought that all matter was made from four elements – earth, wind, fire and water This was believed for approximately 2000 years In 1661 an Irish scientist called Boyle described elements as “primitive and simple substances”

An element is a substance that cannot be split into simpler substances by chemical means In the 1800’s an English scientist called Davy made some of the most important discoveries of the history of the elements Davy passed electricity through a number of compounds splitting them into the elements they were made from

In 1914 an English scientist called Moseley used X-rays to see how many protons were in the nucleus of an atom He discovered that the main difference between each element is the number of protons in their nuclei

History of the Periodic Table Dobereiner’s Triads – Found that the atomic weight of bromine (80) was approximately halfway between the atomic weights of chlorine (35.5) and iodine (127). He found the same pattern in calcium, strontium and barium, and in sulfur, selenium and tellurium. He called each of these groups a triad

A triad is a group of three elements with similar chemical properties where the atomic weight of the middle element is the approximate average of the other two

Newland’s Octaves – Newland arranged all the elements in order of increasing atomic weight He noticed that the properties of the elements repeated every eighth element Newland’s Octaves are groups of elements arranged in order of increasing atomic weight where the first and the eighth element of each group have similar properties

Mendeleev’s Periodic Table – Arranged the elements in different orders and found that if arranged in order of increasing atomic weight every eighth element had similar properties. Placed all elements with similar properties in vertical columns. The properties occurred periodically so he called the table the Periodic Table.

Mendeleev left gaps in his table so the elements could go into the correct group From these gaps he could predict the properties of elements yet to be discovered Mendeleev reversed the order of some elements to make sure they were in the correct group

Moseley & the Atomic Number – Moseley put all the elements from Mendeleev’s table in order of increasing atomic number and they all fell into their correct places this way

The Modern Periodic Table A list of all known elements in order of increasing atomic number There are more elements There are no gap Need to know the name and symbol of elements 1-36

Elements with similar properties placed in vertical columns called groups Elements between Groups II and III are called the d-block elements The transition metals are part of the d-block elements

The horizontal rows across the table are called periods These are named n=1, n=2, n=3…etc. The number “n” is equal to the number of shells in the atoms of the elements in that period The number of elements in a period is equal to the maximum number of electrons allowed in that shell.

Groups in the periodic table Name: Group I Alkali Metals Group II Alkaline Earth Metals Group VII Halogens Group VIII Noble Gases

Properties of Groups At room temperature most of the elements are solids Two are liquids – bromine (Br) & mercury (Hg) Eleven are gases – hydrogen (H), nitrogen (N), oxygen (O), fluorine (F), chlorine (Cl) & all of Group VIII

Group I – Alkali Metals Very reactive Reactivity increases down group Physical: - soft metals (can cut with knife) - low densities (less than water)

Chemical: - Metal reacts with air to form the metal oxide, e.g., 4Na(s) + O2(g) → 2Na2O(s) - Burn readily in air to form metal oxide, e.g., 4Li(s) + O2(g) → 2Li2O(s)

React vigorously with water forming an alkaline solution and hydrogen gas 2Na(s) + H2O(l) → 2NaOH(aq) + H2(g) sodium water sodium hydroxide hydrogen metal gas - Stored under oil to prevent metals reacting with atmospheric oxygen and water

Group II - Alkaline Earth Metals Reactive elements Reactivity increases down group Physical: - harder than alkali metals

Chemical: - do react with water but less vigorously than alkali metals, e.g., Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)

Group VII - Halogens Very reactive non-metals Reactivity decreases down the Group Physical: - low melting & boiling points (F & Cl are gases, Br is a liquid and I is a solid)

Chemical: - form compounds with hydrogen that dissolve in water to form acidic solutions, e.g., H2(g) + Cl2(g) → 2HCl(g) hydrogen chlorine hydrogen chloride gas gas gas 2HCl(g) + H2O(l) → HCl(aq) hydrogen chloride water hydrochloric gas acid

- react vigorously with alkali metals to form white salts, e.g., 2Na(s) + Cl2(g) → 2NaCl(s) sodium chlorine sodium metal gas chloride salt

Group VIII - Noble Gases Physical: - all gases at room temperature - boiling point increases down group Chemical: - inert gases so do not readily react (stable due to full outer shell)