cm 05a structure of atom

Atomic Structure

Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.

What are atoms made of?

Atoms are made of small particles called protons, electrons and neutrons.

These particles are called sub-atomic particles.


Where in the atoms are these sub-atomic particles found?

Protons and neutrons are known as nucleons.

Protons and neutrons are found in the nucleus.

Electrons circle the atoms in areas called shells.


Structure of an Atom - Electron

Carries a charge of -1

Found in shells which surround the atom

Represented by the symbol, eRepresented by the symbol, e

Has negligible mass or relative mass of

1840

1


Structure of an Atom - Neutron

Carries no charge (charge = 0)

Has relative mass of 1

Represented by the symbol, nRepresented by the symbol, n

Found in the nucleus which is in the centre of an atom


Structure of an Atom - Proton

Carries a charge of +1

Has relative mass of 1

Represented by the symbol, pRepresented by the symbol, p

Found in the nucleus which is in the centre of an atom


The Proton Number and Nucleon Number

An atom contains an equal number of positively charged protons and negatively charged electrons.

All atoms are electrically neutral.


The Proton Number and Nucleon Number

The proton number is represented by the symbol Z.

The proton number can also tell us the number of electrons in the atom (since atoms are neutral).

It is also known as the atomic number.

The number of protons in an atom is called the proton number.


An example

For example, the proton number of nitrogen is 7.

Hence, we can deduce that a nitrogen atom has seven protons and seven electrons.


Different atoms have different proton numbers

Atoms of different elements have different numbers of protons.

Carbon, for example, has a proton number of 6. Any atom with 6 protons must be a carbon atom.

Each element has a unique proton number.


Nucleon Number

It is represented by the letter A.

Nucleon number is the total number of protons and neutrons in an atom.

Nucleon number (A) Nucleon number (A) = number of protons = number of protons ++ number of neutrons number of neutrons


Nucleon Number

The nucleon number is also called the mass number.

This is because the mass of an atom depends on the number of protons and neutrons in an atom.


Representing Proton and Nucleon Numbers

The nucleon and proton numbers can be included when representing an element in symbols.

An element is represented using only the nucleon number e.g. sodium-23 or 23-Na.

Nucleon number in superscript, left of symbol

Na23

11

Proton number in subscript, left of symbol

Chemical symbol

cm 05a structure of atom

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