‘A’ Level Chemistry

‘A’ Level Chemistry

AMath Example 1

Chemistry Example 1


Atomic Structure & the Periodic table

Key points:

(1)        Atom – the most basic building block of elements/compounds

(2)        An atom is very small. 12 g of carbon contains 6 X 1023 atoms


(3)        In fundamental sense, the three important subatomic particles (components) of an  atom are protons, neutrons and electrons.


Protons Neutrons Electrons
Relative Mass 1 1 0
Relative Charge +1 0 -1

(5)        In essence,
(a)        The mass of an atom is contributed by the total number of protons and neutrons.
(b)        Both the protons and neutrons are located at the centre of the atom, nucleus.
(c)        In an uncharged atom, the number of protons and electrons are the same.
(d)       During a chemical reaction, electrons might be gained or lost by an atom, BUT the number of protons and neutrons remained the same.


(6)        The periodic table is arranged in vertical column called group, and horizontal row called period.


(7)        Atoms in the same group have similar chemical properties, though they might differ in their relative reactivity in a chemical reaction.


(8)        Atoms in the same group have the same number of outermost electrons.


Dissecting the periodic table

Atom Proton Number Neutron Number Electron number Atomic Mass
Lithium 3 4 3 7

Atom, molecule, ion and isotopes

Atom: Sodium atom, Carbon atom, Hydrogen atom

A molecule is made up of 2 or more atoms chemically combined together.
Eg. H2 (2 hydrogen atoms), CO2 (1 carbon atom & 2 oxygen atoms), O2 (2 oxygen atoms), NaCl (1 sodium atom & 1 chlorine atom)

An ion is a charged atom. It contains an unequal number of protons and electrons.
Cl (A chlorine atom gains one more electron. Proton number = 17, Electron number = 18)

Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Since the number of electrons is the same, isotopes have the same chemical properties.

Writing electronic configuration

Note: At your level, it is sufficient to know that electrons are arranged in shell radiating out from the nucleus. The first shell can contain 2 electrons and subsequent shells can contain 8 electrons.

Hence, the electronic configuration of,

Electronic configuration

Metals, Non-metals & Nobel gases

Metals have 3 or less outermost electrons (Group 1-3).

Non-metals have outermost electrons between 4 to 7 (Group 4-7).

Nobel gases are highly unreactive as they have a complete shell of outermost electrons (either 2 or 8).

Posted in Chemical A

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