Page 52 - Science Class 07
P. 52
non-metals. In the periodic table, metalloids v Atoms of different elements combine to
form a jagged zone dividing elements that have form compounds.
clear metallic proper es from elements that COMPOUNDS AND MOLECULES
have clear non-metallic proper es.
A compound is a substance made from two or
Boron, silicon, germanium, arsenic, an mony, more elements that have reacted chemically
tellurium and polonium are metalloids. with each other. The elements in the compound
ATOMS cannot be separated by physical means.
An atom is a fundamental piece of ma er. Let us see this example below:
(Ma er is anything that can be touched Water is a compound. It is wri en as H₂O,
physically. Everything in the universe except symbol for water. This means it is a chemical
energy) is made of ma er and so everything in bond of two elements— Hydrogen and Oxygen.
the universe is made of atoms. H₂O is wri en as symbol for water because
An atom itself is made up of three ny kinds of there are two atoms of hydrogen, making it a
par cles called sub-atomic par cles — hydrogen molecule, bonding with one atom of
protons, neutrons and electrons. The protons oxygen.
and the neutrons make up the center of the Some atoms can also join up with other atoms
atom called the nucleus and the electrons fly of the same element. When two or more atoms
around above the nucleus in a small cloud. The join up, they form a molecule. Oxygen (O₂) is a
electrons carry a nega ve charge and the molecule because it has two atoms joined
protons carry a posi ve charge. In a normal together.
(neutral) atom, the number of protons and the
ELEMENTS AND MOLECULES
number of electrons are equal. O en, but not
always, the number of neutrons is the same, Some mes two or more atoms of an element
too. combine to form more stable molecules, e.g.,
oxygen gas occur in the form of oxygen
The word atom means ‘something that cannot
molecules (O ) in the nature. In this case also
be divided’. In 1808, Dalton also confirmed this 2
the proper es of elements differ from
theory by his experiments which is known as
molecules of compounds.
the Dalton’s atomic theory.
According to this theory:
v Each element (or other ma er) is made up
of very small and individual par cle called 1. How an element is different from a
atoms. ma er?
v Atoms of the same element are iden cal in 2. What is a pure substance?
every aspect having same mass, size and 3. How can you say that a given material is a
chemical proper es. metal or not?
v Atoms can neither be created nor can be 4. Sulphur is a non-metal. Why?
destroyed. 5. Differen ate between atoms and
molecules.
52 Science-7
