Download Structure of an Atom

L. R. & S. M. VISSANJI ACADEMY
SECONDARY SECTION - 2016-17
CHEMISTRY - GRADE: VII
Structure of an Atom
Key concepts: Dalton’s atomic theory, Subatomic particles, Structure of an atom, Atomic
number, Mass number of an element, Isotopes, Electronic configuration, Basic concept of
transfer and sharing of electrons, Cations, Anions, Valency- positive and negative.
Dalton’s Atomic theory (1808): He proposed an atomic theory known as Dalton’s atomic
theory. The main points of his theory are:
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Elements are made of small indivisible particles called atoms.
Atoms of a given element are identical but differ from the atoms of other elements.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple whole-number ratios to form chemical
compounds.
 Atoms take part in all chemical reactions.
According to Dalton’s atomic theory, the atom is indivisible. But in the early twentieth
century, it was found that an atom is made up of even smaller particles, namely, electrons,
protons and neutrons. These e particles are known as subatomic particles.
Electrons: The negatively charged particles in an atom are called electrons. An electron has
one unit negative charge. The mass of an electron is 1/1837 of the mass of hydrogen atom
(lightest atom). Thus, electrons have a negligible mass.
Protons: The positively charged particles in an atom are called protons. A proton has one
unit positive charge. Its mass is almost same as the mass of a hydrogen atom. The number
of protons is equal to the number of electrons present in an atom.
Neutrons: Particles with no charge are called neutrons. So, they are neutral. The mass of a
neutron is about the same as the mass of a proton.
Structure of an atom
An atom consists of two structural parts- nucleus and shells or orbits.
Nucleus: It is the central part of an atom which contains both protons and neutrons. The
protons and neutrons present in the nucleus are together called nucleons. Neutrons are
neutral and the protons are positively charged. Thus, the nucleus is also positively charged
due to the presence of protons.
The nucleus makes up most of the mass of the atom because both protons and neutrons are
present here and the mass of an electron is negligible. As compared to the overall size of an
atom, the nucleus is extremely small.
Orbits or Shells: The electrons revolve around the nucleus in certain definite circular paths,
called orbits or shells.
The electrons present in the outermost shell of an atom are called valence electrons. This
outermost shell is called valence shell. The valence electrons most readily participate in a
chemical reaction.
An atom is electrically neutral: In an atom, the number of protons is equal to the number of
electrons. That is, the positive charge of protons is balanced by the negative charge of
electrons. Thus, an atom as a whole is electrically neutral.
Atomic number and Mass number of an element:
The atomic number of an element is the number of protons present in an atom of an element.
It is denoted by the capital letter Z.
For example, Carbon has 6 protons, the atomic number of carbon is 6. Since the number of
protons is equal to the number of electrons in an atom, the atomic number is also numerically
equal to the number of electrons.
Atomic Number (Z)= Number of protons = Number of electrons
The mass number of an element is the total number of protons and neutrons present in an
atom of the element. It is denoted by the capital letter A.
Mass Number (A) = Number of protons + Number of neutrons
A Carbon atom has 6 protons and 6 neutrons. Thus, its mass number is 6+6 = 12.
In the notation for an atom of an element, atomic number is shown as subscript and mass
number is shown as superscript. The atomic number of carbon is 6 and its mass number is
.
12, thus Carbon is represented by
Number of Neutrons = Mass number – Atomic Number = A – Z
Isotopes: They are atoms of the same element having the same atomic number (Z) but
different mass numbers (A).
The element hydrogen has three isotopes- Protium, Deuterium and Tritium. All the three
have same atomic number, i.e., one but their mass numbers are one, two and three
respectively.
Electronic Configuration
The arrangement of electrons in different orbits around the nucleus of an atom of an element
is known as electronic configuration of the element.
There are certain rules which are followed for distribution of electrons in the shells:
 The maximum number of electrons in each shell of an atom is 2n 2, where n represents
the shell number. The maximum number of electrons that can be in each shell is
given below.
Shell K with n=1, contains number of electrons =2 * 12= 2
Shell L with n= 2, contains number of electrons =2 * 2 2 = 8
Shell M with n= 3, contains number of electrons =2 * 32 = 18
Shell M with n= 3, contains number of electrons =2 * 42 = 32
 The outermost shell of an atom can have a maximum of 8 electrons.
 If an atom contains 8 electrons in its outermost shell, then it is stable and inert. If the
number of electrons is less than 8, then it is chemically unstable and reactive.
However, helium is an exception. It has only 2 electrons as it has only one shell.
Formation of Chemical bonds:
We know that atoms of the same element combine together to form a molecule of that
element, and atoms of different elements combine together to form a molecule of a
compound.
For example, when two hydrogen atoms combine, they form a molecule of hydrogen
(element). When two hydrogen atoms and one oxygen atom combine, they form a molecule
of water (compound).
Why do atoms combine to form molecules?
Atoms combine to form molecules so as to become chemically stable. A stable atom has 8
electrons in its outermost shell. This condition is called octet. A stable atom does not
combine with atoms of other elements.
All the noble gases, except helium, have 8 electrons in their outermost shell. Helium has
only one shell which has 2 electrons. This condition is called duplet and is also stable. Thus,
all the noble gases are chemically stable. They do not combine with atoms of any other
element. However, there are many atoms which have less than 8 electrons in their
outermost shells. Such atoms are chemically unstable and they combine with other atoms to
attain an octet of electrons so as to become chemically stable. The atoms are held together
in a molecule with a force of attraction. This force of attraction is called chemical bond.
There are two ways in which atoms combine to form a molecule. They are:
 By transferring electrons or electrovalent bonding
 By sharing of electrons or covalent bonding
Ions: The charged atoms are called ions.
Cations: The positively charged ions are called cations.
Anions: The negatively charged ions are called anions.
Valency: It is defined as the number of electrons lost, gained, or shared by its atom in a
chemical combination so as to become chemically stable.
Positive valency: Atoms which donate electrons are said to have positive valency.
Examples: Monovalent (K+), bivalent (Ca2+), trivalent (Al3+)
Negative valency: Atoms which accept electrons are said to have negative valency.
Examples: Monovalent (Cl -), bivalent (O2-), trivalent (N3-)
Note: refer textbook and class notes for Elements (symbols), atomic structures (atomic no. 1
to 10) and learn atomic nos. and mass nos. of elements with atomic nos. 1 to 20.