Page 92 - ICSE Chemistry 8

• In 1911, Ernest Rutherford carried out gold-foil experiment and found that most of the mass of an
atom was located in the centre of the atom called nucleus.
• In 1932, James Chadwick discovered that atoms have neutrons as parts of their structure.
• According to the modern concept of the atom, there are three subatomic par cles—electrons, protons
and neutrons. Protons and neutrons together form the nucleus, while electrons revolve around the
nucleus in defi nite orbits or shells.
• Electrons are distributed in diff erent energy levels or shells according to Bohr-Bury scheme of electronic
confi gura on.
• Atoms of element combine or react to a ain the electronic confi gura on of the nearest noble or inert
gas and thus become stable.
• The bond formed between two atoms when they combine is called a chemical bond. Atoms can
combine either by transferring electrons or by sharing electrons.
• The chemical bond formed by the complete transfer of electrons from one atom to the other is called
an electrovalent or ionic bond. The chemical bond formed by the sharing of electrons is called a
covalent bond.

EXERCISES

BE PROMPT

A. Fill in the blanks.
1. ____________ are the building blocks of ma er and cannot be chemically subdivided by ordinary
means.

2. ____________ is the metal plate connected to the nega ve terminal of the voltage source.
3. The ____________ is posi vely charged central part of the atom.
4. Atoms of the same element having same atomic number but diff erent mass number are known as
____________.

5. ____________ shell is the outermost shell of an atom.
6. A ____________ bond is formed by the sharing of electrons.

B. Iden fy the error in the given statements. Circle the incorrect word and write the correct word in
the blank.

1. All ma er consists of ny par cles called elements. ____________________
2. Gases are good conductors of electricity under ordinary condi ons. ____________________

3. The number of electrons in an atom is equal to the number of protons
in the atom which makes the atom posi vely charged. ____________________
4. Elements with valency two are called bivalent elements. ____________________
5. Ca ons are nega vely charged ions. ____________________

6. Sodium chloride is a covalent compound. ____________________

78