Chemistry Revision CH1

Topics: Atom, Atomic mass, Electron Pages: 125 (13359 words) Published: June 1, 2014
WJEC
CBAC

AS/A LEVEL
GCE in Chemistry
REVISION AID
UNIT 1

AS

UNIT CH1 – Controlling and Using Chemical Changes (in order to make things, produce energy and solve environmental problems)

Preamble
This unit begins with some important fundamental ideas about atoms and the use of the mole concept in calculations.
Three key principles governing chemical change are then studied, viz. the position of equilibrium between reactants and products, the energy changes associated with a chemical reaction and the rate at which reactions take place. These principles are then applied to some important problems in the fields of chemical synthesis, obtaining energy and the maintenance of the environment.

UNIT CH1

Controlling and Using Chemical Changes (in order to make things, produce energy and solve environmental problems)

TOPIC 1
1.1

Basic ideas about atoms

1.2

Chemical calculations

TOPIC 2
2.1

Chemical eqilibria and acid-base reactions

2.2

Energetics

2.3

Kinetics

TOPIC 3

1.1

Basic ideas about atoms

Topic 1.1 (a)
Learning Outcome: Describe electrons, protons and neutrons in terms of their relative charges and masses, and the distribution of charges and masses within atoms.
Atomic Structure.
Atoms are made up of three fundamental particles, the proton, the neutron and the electron.
electrons surrounding
the nucleus

most of the
volume of the
atom is empty
space

the nucleus made up of
protons and neutrons.
This is where nearly all
the mass of the atom is.

A typical atom
The mass of a proton is almost exactly the same as the mass of a neutron. The mass of a proton is 0.0000000000000000000000017 g., which is very small. It has a positive charge of 0.00000000000000000016 coulomb.

These small numbers are very inconvenient.
So we call the mass of a proton 1 and its charge +1 and describe the other particles relative to these values, as shown below.

Particle

Relative mass

Relative charge

proton

1

+1

neutron

1

0

electron

1/1840

-1

Topic 1.1 (b)
Learning Outcome: understand the terms atomic number, mass number and isotope, and the connection between atomic numbers and mass numbers.

The number of protons in the nucleus of an atom is called the atomic number (Z). Each element has its own atomic number.
For example hydrogen has atomic number 1.
Lithium has atomic number 3.
Chlorine has atomic number 17.

The number of protons plus the number of neutrons is called the mass number (A).
An atom is electrically neutral because the number of negative electrons surrounding the nucleus equals the number of positive protons in the nucleus. A particle where the number of electrons does not equal the number of protons is no longer an atom but is called an ion and has an electrical charge. We can define an atom in terms of the symbol for the element, the atomic number of

A
the atom (Z) and the mass number (A). Z

E
23

For example, an atom of sodium may be represented as 11 Na
11 is the atomic number and 23 is the mass number.
Some elements exist as a mixture of atoms which are only different in their mass numbers. These atoms are called isotopes. Chlorine gas exists as a mixture of two isotopes, one having a mass number of 35 and one having a mass number of 37. These are represented as

35
17

Cl and

37
17

Cl

Topic 1.1 (c)
Learning Outcome: deduce, given atomic and mass numbers, the numbers of protons, neutrons and electrons in specified isotopes;

Taking the isotopes above,

35
17

Cl and

37
17

Cl , we can deduce the numbers of electrons,

protons and neutrons in each neutral atom.

isotope

atomic number(Z) =
number of protons

mass number(A) =
number of
protons plus
neutrons

number
of
neutrons

number of
electrons

35
17

Cl

17

35

18

17

37
17

Cl

17

37

20

17

Topic 1.1 (d)
Learning Outcome: explain the formation of ions...
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