(i) Symbol for some elements as proposed by Dalton.

(ii) The following rules are followed while assigning symbol to an elements:
(a) Chemical symbols usually consist of one or two letters
(b) The symbols of most elements correspond to the first letter (which is capitalized) of their English name. For example, the symbol for oxygen is "O" and that for hydrogen is "H".

Properties of metalloids :
(i) Metalloids exhibit the properties of both metals and non-metals.
(ii) Unusually they look like metals but behave largely like non-metals,
(iii) They can form alloys like metals.

(i) Hardness:
1. Metals are generally hard and shiny elements except Na.
Ex: Gold
2. Non-metals are generally soft and dull elements except diamond.
Ex: Sulphur.
(ii) Physical nature:
1. All the metals are solids at room temperature except mercury.
2. Non-metals can be gases, solids and liquids.
Ex: H₂, O₂, CI₂ - Gases
I₂, S₈ P₄ -Solids
Br₂ - Liquid .
(iii) Malleability:
1. Metals are malleable. They can be bent or beaten into sheets and drawn into wires.
Ex: Iron, Gold
2. Non-metals are non-bendable and non-ductile.
(iv) Conductivity:
1. Metals are good conductors of heat and electricity
Ex: Copper, Nickel
2. Non-metals are poor conductors of heat and electricity except graphite.

i) A compound is formed only when the constituent elements combine in a fixed proportion.
ii) Compound exhibits entirely different properties of their constituent elements.
iii) A compound cannot be broken down by physical methods.
iv) A compound can be separated into its constituent elements by chemical methods only.

Water is considered as compound because,
(i) Water is composed of two elements: H₂ and O₂
(ii) The ratio of Hydrogen and Oxygen by mass in any sample of pure water is the same.
(iii) The properties of water are different from its constituent elements: Hydrogen and Oxygen.
(iv) Water can be decomposed only by chemical methods into Hydrogen and Oxygen.

Hydrogen is considered as an element because,
(i) Hydrogen cannot be broken into new substance by ordinary physical or chemical means. (i.e) by applying heat, light.
(ii) Hydrogen has one kind of atoms.

When air inside the hot air balloon is heated with a burner, it expands. The expansion causes the density of the air inside the balloon to decrease. Hence, the air inside the balloon has a lower density that the air outside of the balloon. This difference is density allows the hot-air balloon to float.

On earth the three most abundant atom are iron, oxygen, and silicon.

The smallest unit of an element that exhibits the properties of the element is called as 'atom'.