Atoms

• All elements are made up of very small particles called atoms (look back to previous notes).

• All atoms in an element are the same.

• Atoms aren’t EMPTY SPACE. They are actually made up of even smaller particles called sub-atomic particles.

• Three sub-atomic particles make up an atom. They are called:-

1. PROTONS 2. NEUTRONS

3. ELECTRONS

Sub-Atomic Particles in an Atom (1)

• The structure of an atom is shown below.

The centre of the atom is hard and is called the NUCLEUS. It containsPROTONS and NEUTRONS.

The ELECTRONS are very light and are found MOVING around

the outside of the nucleus.

The simplistic shape of an atom is represented by a sphere (3-D), or a circle (2-D). However, the diagram above is more accurate. The outline shape of the atom is actually the flight path of the electrons.

Sub-Atomic Particles in an Atom (2)

• The 3 sub-atomic particles have different properties.

Particle Mass (amu) Charge Location Symbol

Proton 1 1+ Nucleus p+

Neutron 1 0 Nucleus n0

Electron 0 1- Moving e- outside nucleus ***amu = atomic mass units***

Sub-Atomic Particles in an Atom (3)

• A simplified diagram of an atom.

+

-

-

The charge on the nucleus isPOSITIVE due to the PROTONSpresent.

The ELECTRONS are

NEGATIVELY charged.

Atoms are said to be ELECTRICALLY NEUTRAL, because the POSITIVE charges of the PROTONS are EQUAL to the NEGATIVE charges of the ELECTRONS, and CANCEL OUT.

Important Numbers

• Atoms of different elements vary in size and mass due to them containing different number of sub-atomic particles.

• How do we know the numbers of sub-atomic particles different atoms contain??????

• We use two very important numbers called the:-

» 1. ATOMIC NUMBER

» 2. MASS NUMBER

1. The Atomic Number

• The ATOMIC NUMBER of an atom is equal to the number of PROTONS in the nucleus of the atom, which is the same as the POSITIVE charge on the nucleus.

• In a NEUTRAL ATOM, the ATOMIC NUMBER is also the same as the number of ELECTRONS.

• The ATOMIC NUMBER, is the number an atom has on the periodic table.

17ClThe Atomic Number can be

represented like this.

2. The Mass Number

• The MASS NUMBER of an atom is the same as the TOTAL number of PROTONS and NEUTRONS in the NUCLEUS of an atom.

35

Cl17

• Mass number - Atomic number = Number of

neutrons

Mass number always goes here.PROTON + NEUTRONS

Atomic numberPROTONS = ELECTRONS

Using this information

Element AtomicNumber

MassNumber

Numberofprotons

Numberofneutrons

Numberofelectrons

Carbon 12

7 3

Calcium 20

Neon 10 20

80 45

Argon 18 22

2. The Mass Number (cont)

7

Li This atom has:- 3 protons (3+ charge on nucleus)

3 3 electrons 4 neutrons39

K This atom has:- 19 protons (19+ charge on nucleus)

19 19 electrons 20 neutrons

All atoms except 1 have both protons and neutrons in the nucleus. H

1

Isotopes

• ISOTOPES are atoms of the SAME element, with the SAME ATOMIC NUMBER but have a DIFFERENT MASS NUMBER ( This means a DIFFERENT NUMBER of NEUTRONS in the NUCLEUS).

• A machine called a MASS SPECTROMETER is used to show that most elements contain a mixture of isotopes.

• For example, CHLORINE is made up of 2 isotopes and HYDROGEN is made up of 3

isotopes.

35 37 1 2 3

Cl and Cl H , H and H17 17 1 1 1

Electron Arrangement (how electrons are arranged in atoms) (1)

• Electrons move around (orbit) the outside of a nucleus. • The electrons do NOT randomly do this. They orbit in

neatly arranged in ENERGY LEVELS or SHELLS.

+

Third Energy Level has the highest energy

Second Energy Level

First energy level has thelowest energy

Nucleus

Electron Arrangement (2)• Imagine the energy levels/shells of an atom to be like the

layers of an onion, with each layer only holding a MAXIMUM number of electrons. The diagram below is called a TARGET PICTURE.

+

FIRST SHELL - Can hold a maximum of 2 electrons

SECOND SHELL - Can holda maximum of 8 electrons

THIRD SHELL - Can hold a maximum of 8 electrons

Electron Arrangement (3)• Electrons don’t fill energy levels in a random way.

• They fill the first energy level, then the second and so on, until all the electrons are inside a level.

• Electrons are put into a shell singly at first (at the 4 points of a clock face), then paired up. This is shown below.

7+

This atom is nitrogen, because itsatomic number 7 (7 positive charges).

The shorthand way of writing thiselectron arrangement is:-

2, 5

Electron Arrangement (4)

Element AtomicNumber

Number ofelectrons

Electronarrangement

Lithium 3 3 2, 1

Oxygen 8 8 2, 6

Magnesium 12 12 2, 8, 2

Chlorine 17 17 2, 8, 7

Potassium 19 19 2, 8, 8, 1

Electron Arrangement (5)

• A notation exists for writing electron arrangement, this is shown on page 1 of the data book.

14+

This is a SILICON atom, because the number of + charges is equal tothe ATOMIC NUMBER.

This means the number of - charges(electrons) are also 14.

14 electrons are arranged:- 2 in thefirst energy level, then 8 in the second, and lastly 4 in the third (adds up to 14.)

Shorthand notation :- 2, 8, 4

Electron Arrangement (5)

• Chemists have shown that the electron arrangement of an atom is related to its CHEMICAL PROPERTIES (how it reacts).

• It has been found that elements in the same group of the periodic table have very similar properties.

• Looking at the electron arrangement of elements within a group it can be seen that they ALL have the SAME NUMBER OF OUTER SHELL ELECTRONS. It is these electrons which are involved in chemical reactions.

• The number of outer electrons is the SAME as the GROUP NUMBER of the ELEMENT.

Electron Arrangement (6)

Groupnumber

1 2 3 4 5 6 7 8 or 0

No. ofouterelectrons

1 2 3 4 5 6 7 8

Example Sodium2,8,1

Calcium2,8,8,2

Aluminium 2,8,3

Carbon2,4

Nitrogen2,5

Sulphur2,8,6

Chlorine2,8,7

Argon2,8,8

All react violently with water

Very reactive, poisonousnon-metals

All unreactivegases

The Noble Gases

• All the noble gases have 8 outer electrons - a full outer shell (octet). For neon this is 2.

• Atoms with a full outer shell are unreactive and this explains the chemical properties of the noble gases.

• Atoms only react to get a full outer shell of electrons, thus becoming stable.