Atoms, Elements and the Periodic Table

The Building Blocks of Matter

• Imagine trying to break a piece of pure gold in half, over and over again. Eventually, you would reach a point where you cannot divide it any further without changing what it is.
• This indivisible piece is an atom, defined precisely as the smallest part of an element that can exist. All substances are made of these tiny particles.
• An element is a pure substance made of only one type of atom, and it cannot be broken down into anything simpler by chemical means.
• If atoms of two or more elements chemically combine in fixed proportions, they form a compound.

Chemical Symbols: The Alphabet of Chemistry

• There are about 100 different elements, and writing out their full names every time would be exhausting. Instead, scientists use a universal chemical symbol for each one.
• These symbols are one- or two-letter codes found on the Periodic Table, sometimes based on their English or Latin names (like Fe for iron from the Latin Ferrum).
• A crucial rule for chemical symbols is case sensitivity:
  • If a symbol has one letter, it must be uppercase (e.g., C for Carbon, O for Oxygen).
  • If it has two letters, the first must be uppercase and the second must be lowercase (e.g., Na for Sodium, Mg for Magnesium).
• A standalone symbol represents exactly one atom of that element. For example, 'He' means one atom of helium.

Worked Example: Correct vs. Incorrect Notation

• Sodium is correctly written as Na. Writing NA is incorrect and will not be accepted.
• Co represents the element cobalt, whereas CO represents a compound (carbon monoxide) containing atoms of both carbon and oxygen.

The First 20 Elements

• Elements are arranged in the Periodic Table in order of increasing atomic (proton) number. Elements in the same group have the same number of electrons in their outer shell, giving them similar chemical properties.
• While you are provided with a Periodic Table in the exam, familiarising yourself with the first 20 elements helps you locate them much faster.

Atomic Number	Element Name	Symbol	Atomic Number	Element Name	Symbol
1	Hydrogen	H	11	Sodium	Na
2	Helium	He	12	Magnesium	Mg
3	Lithium	Li	13	Aluminium	Al
4	Beryllium	Be	14	Silicon	Si
5	Boron	B	15	Phosphorus	P
6	Carbon	C	16	Sulfur	S
7	Nitrogen	N	17	Chlorine	Cl
8	Oxygen	O	18	Argon	Ar
9	Fluorine	F	19	Potassium	K
10	Neon	Ne	20	Calcium	Ca

Group 1: The Alkali Metals

• Group 1 elements are known as the alkali metals. This group includes Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), and Francium (Fr).
• All alkali metals have exactly one electron in their outermost shell, which they lose in chemical reactions to form 1+ ions (such as $Na^+$).
• They are uniquely soft metals that can be cut with a knife, have low melting points, and low densities (lithium, sodium, and potassium are less dense than water).
• Reactivity in Group 1 increases as you go down the group. This means lithium is the least reactive, whilst francium is the most reactive.
• They react vigorously with water to form a metal hydroxide (an alkaline solution) and hydrogen gas:

$$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$$

Group 7: The Halogens

• Group 7 elements are called the halogens. This group consists of non-metals including Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At).
• All halogens have seven electrons in their outermost shell. They react by gaining one electron to form 1- ions (like the chloride ion, $Cl^-$).
• Halogens naturally exist as a diatomic molecule, meaning two atoms of the same element are chemically bonded together (e.g., $F_2$, $Cl_2$, $Br_2$).
• Unlike Group 1, halogen reactivity decreases as you go down the group. Fluorine is the most reactive, and astatine is the least reactive.
• A more reactive halogen can take the place of a less reactive one in a compound. This is known as a displacement reaction.

Worked Example: Halogen Displacement

• Reaction: Adding chlorine solution to potassium bromide solution.
• Equation: $Cl_2(aq) + 2KBr(aq) \rightarrow 2KCl(aq) + Br_2(aq)$
• Observation: The colourless solution turns orange.
• Explanation: Chlorine is higher up in Group 7 than bromine, meaning it is more reactive and successfully displaces bromine from its salt compound.