Unit 1 Notes
Adnan Chowdhury 1
An element is a substance that cannot be broken down by chemical means into other
substances.
All elements are made up of atoms.
An atom is the smallest part of an element that can take part in a chemical change.
Atoms consists of three fundamental particles.
1. Proton
2. Neutron
3. Electron
Protons and neutrons stay in the nucleus and hence are called nucleons.
Most of the mass of the atom is concentrated in the nucleus.
The diameter of the nucleus (10
-14
m) is small compared to the whole atom (10
-10
m).
Electrons are found in electron shells or orbits. They are in continuous circular motion around
the nucleus.
The orbitals take up most of the volume of the atom.
Subatomic Particle Relative Mass Relative Charge Symbol
Proton 1u +1e
1
1p
Neutron 1u 0
1
0n
Electron
1
2000
u
-1e
0
-1e
1u = 1.67 10
-27
kg
1e = 1.6 10
-19
C
Unit 1 Notes
Adnan Chowdhury 2
Atomic Number / Proton Number (Z):
This is the total number of protons in the nucleus of an atom of an element.
Mass Number / Nulceon Number (A):
This is the total number of protons and neutrons in the nucleus of an atom an element.
Nuclide:
When the mass number and the atomic number of a nucleus is stated, it is called a nuclide.
A
z
X
A = Z + N (N = neutron number)
N = A Z
For neutral atoms with no overall charge, the number of electrons is the same as the number of
protons.
Negative ions (anions) have more electrons than protons. Anions are larger compared to the original
atom. Non-metal atoms gain electrons when they form ions. So there is a bigger negative charge in
the electron cloud of the ion, which means there is a greater repulsion between the electrons and
the electron cloud expands a bit.
Positive ions (cations) have fewer electrons than protons. Cations are smaller compared to the
original atom. Metals lose electrons when they form ions. So the positive charge of the nucleus is
larger than the negative charge in the electron cloud. This means that in a cation, the electrons are
pulled closer.
Isotopes:
Atoms of the same element having the same number of protons but different number of neutrons. (In
terms of subatomic particles).
OR
Atoms of the same element having the same atomic number but different mass number. The difference
in mass number is due to the difference in the number of neutrons.
e.g. Carbon has three isotopes:
Carbon - 12 Carbon - 13 Carbon - 14
12
6C
13
6C
14
6C
12
C
13
C
14
C
C - 12 C - 13 C - 14
Carbon 14 is radioactive and is used in carbon dating.
Unit 1 Notes
Adnan Chowdhury 3
The number and arrangement of electrons decides the chemical properties of an element.
Isotopes have the same electronic configuration of electrons, so they have identical chemical
properties.
Physical properties depend on the mass of the atom. Therefore, isotopes of the same element
have different densities, boiling and melting points, rates of diffusion, etc.
Isoelectrons:
Isoelectrons are atoms or ions of different elements with the same number of electrons.
e.g. N
3-
, O
2-
, F
-
, Ne, Na
+
, Mg
2+
, Al
3+
( all containing 10 electrons).
The ionic radius of a set of isoelectronic ions decreases with increasing atomic number. This is
because number of electrons remain the same, but the number of protons increases. This means
the electrons are attracted to the nucleus more strongly, pulling them in a little, so the ionic
radius decreases.
Problem:
Element Number of protons Number of neutrons Number of electrons
A 19 20 18
B 17 18 17
C 12 12 10
D 17 20 16
E 35 44 36
F 18 22 18
Identify:
i. The neutral atoms
ii. Cations
iii. Anions
iv. Isotopes
v. Isoelectrons
Relative Atomic Mass:
The average mass of the masses of the atoms of the element relative to 1/12
th
of the mass of a
Carbon 12 atom.
Relative Isotopic Mass:
The mass of a particular isotope of an element, relative to 1/12
th
of the mass of a Carbon 12 atom.
Relative Molecular / Formula Mass:
The mass of a molecule relative to 1/12
th
of the mass of a Carbon 12 atom.
