An atom is the smallest particle of an element which is still recognisable as that element.

A molecule is the smallest particle of a substance that is capable of separate existence.

An ion is a positively or negatively charged particle.◦ cation = positive ion◦ anion = negative ion

Atoms, Molecules and Ions

We use words to name and describe substances, but often we need a simpler way to represent them on paper.

We use chemical symbols as a shorthand way to talk about different elements.

Symbols

Some symbols are simply the first letter of the name of the element (always a capital letter).

These symbols are generally used for very common elements.

Symbols

B boron N nitrogenC carbon O oxygenF fluorine P phosphorusH hydrogen S sulfurI iodine U uranium

Where there are several elements beginning with the same letter, a second letter is used.

This is frequently (though not always) the second letter of the name of the element.

The first letter is a capital (upper case) and the second a small (lower case) letter.

Symbols

Al aluminium Ar argon As arsenicBa barium Bi bismuth Br bromineCa calcium Cl chlorine Co cobalt Cr chromiumNi nickel N

eneon

Pt platinum Pu plutonium

For some elements we use a symbol based on a non-English name of the element (typically Latin).

Symbols

Na sodium natrum Au gold aurumCu copper cuprum K potassium kaliumAg silver argentum Fe iron ferrumHg mercury hydrargentu

mPb lead plumbum

We combine symbols to represent how a compound is put together - called a formula.

H2O

This formula tells us that:◦ water is made of the elements hydrogen and oxygen◦ the water molecule contains two atoms of hydrogen

bound to one atom of oxygen.

Formulae

Subscripts tell us how many of that atom or ion is in that compound

They refer to the element immediately preceding it (as in water above)

They cannot be changed, as these numbers are determined by the place of each element in the periodic table◦ If you change subscripts then you are actually

inventing new substances! ◦ e.g. water (H2O) vs. hydrogen peroxide (H2O2)

Subscripts

Let’s further compare H2O (water) and H2O2 (hydrogen peroxide).

H2O H2O2

melting point 0°C 0.4°C

boiling point 100°C 151°C

density 1 g/mL 1.47 g/mL

stability to heat Stable to 2000° Can explode at room temp.

bleaching action none powerful

toxicity none highly toxic

The importance of subscripts

Brackets with a subscript outside mean that we need to multiply everything inside by that number.

Ca(OH)2 has 1 calcium ion (Ca2+) and 2 hydroxide ions (OH–). ◦ The brackets indicate that overall there are 2 hydrogen (H)

atoms and 2 oxygen (O) atoms.

Fe2(SO4)3 has 2 iron (Fe3+) ions and 3 sulfate ions (SO4

2–). ◦ The brackets indicate that overall there are 3 sulfur (S)

atoms and 12 oxygen (O) atoms.

Brackets

Some Examples

Atoms have a tiny, dense NUCLEUS surrounded by a cloud of ELECTRONS.

The nucleus is made up of two types of particle, PROTONS and NEUTRONS.

Electrons can be pictured in orbits around the nucleus.

Structure of an Atom

protonsneutronselectrons

symbol

pne

mass

11

electric charge

+1 011

1800

Atoms All atoms of a given element have the same

number of protons in their nucleus

The number of protons in an atom tells us WHICH ELEMENT it is:◦ 1 proton - the atom must be hydrogen ◦ 6 protons - the atom must be carbon

atomic number = number of protons

Atoms Atoms have equal numbers of protons &

electrons

Atoms are electrically neutral, whereas their protons and electrons are charged.◦ Protons have a positive charge (+1)◦ Electrons have a negative charge (-1)

like charges repel unlike charges attract

Atoms Atoms in a sample of an element would

repel each other if they had an electric charge.

This is why atoms must be electrically neutral.◦ There must be equal number of + and - charges.

number of protons = number of electrons

Around the outside of the atom, electrons are placed in shells that are like the planets orbiting around the Sun.

These ‘orbits’ start close to the nucleus and get further and further away.

Electrons fill the inside shells first.

Electrons and Shells

The outer shell of electrons is called the valence shell.

These valence shell electrons are the ONLY electrons that are involved in chemical reactions.

Electrons and Shells

However, the atoms of some elements are very unstable.

In order to become more stable, these atoms can lose or gain electrons to become ions.

e.g. the sodium atom (Na) is explosively reactive, but the sodium ion (Na+) is essential to our diet – such as in table salt.

Ions

A positive ion is an atom that has lost one or more electrons:◦ More positive protons than negative electrons

We write a positive charge for every electron lost, e.g.◦ Na+ means one electron lost (same as Na1+)◦ Ca2+ means two electrons lost◦ Al3+ means three electrons lost

Positive Ions

A negative ion is an atom that has gained one or more electrons◦ More negative electrons than positive protons

We write a negative charge for every electron lost, e.g.◦ F- means one electron gained (same as F1-)◦ O2- means two electrons gained◦ N3- means three electrons gained

Negative ions

Atoms want to have a full outer shell of electrons, and will lose or gain electrons to make this happen.

Based on this idea, we can explain:◦ why some molecules are stable, while others are

not ◦ The way elements in groups 1, 2, 7 and 8 form

ions

A Full Outer Shell

Group 1 elements have a strong tendency to GIVE UP ONE ELECTRON to form a +1 ion Na 2 8 1 Na+ 2 8 -1 electron K 2 8 8 1 K+ Group 2 elements, similarly, give up TWO ELECTRONS to form a +2 ion Mg 2 8 2 Mg2+ -2 electrons Ca 2 8 8 2 Ca2+

Explaining charges on ions

Group 1 elements have a strong tendency to GIVE UP ONE ELECTRON to form a +1 ion Na 2 8 1 Na+ 2 8 -1 electron K 2 8 8 1 K+ Group 2 elements, similarly, give up TWO ELECTRONS to form a +2 ion Mg 2 8 2 Mg2+ -2 electrons Ca 2 8 8 2 Ca2+

Explaining charges on ions

2 8 8

Explaining charges on ions

2 8 8

Group 1 elements have a strong tendency to GIVE UP ONE ELECTRON to form a +1 ion Na 2 8 1 Na+ 2 8 -1 electron K 2 8 8 1 K+ Group 2 elements, similarly, give up TWO ELECTRONS to form a +2 ion Mg 2 8 2 Mg2+ -2 electrons Ca 2 8 8 2 Ca2+

2 8

2 8 8

Explaining charges on ions

2 8F–

Cl–

F 2 7

Cl 2 8 7 + 1 electron

Group 7 elements can ACCEPT ONE ELECTRON to give a –1 ion

2 8 8

no ions formedNe 2 8

Ar 2 8 8 outer shell already full

Group 8 elements (the inert gases) already have FULL outer shells,so they have no tendency to lose or gain electrons.

Forming Ions1. Read off the number of electrons in the outer

shell of the atom (= group number)2. Add or subtract up to 3 electrons to give a full

outer shell (8 electrons for everything except H)◦ Since like charges repel, a charge of 3+ or 3- is normally

the maximum allowed Na 2 8 1 Na+ 2 8 11 protons 11 protons 11 electrons 10 electrons Cl 2 8 7 Cl- 2 8 8 17 protons __ protons 17 electrons __ electrons

1718

Forming IonsWork out the charge from the number of protons and electrons, or (easier) from the number of electrons added/subtracted.

Na Cl OElectrons in outer shell? 1

Add/subtract how many to make octet? subtract 1

Is the ion positive or negative?

7

add 1

+1 -1

6

add 2

-2

Ions form when electrons are donated and accepted

An atom of sodium, Na cannot form an ion alone. However, if an atom of Na were allowed to react

with an atom of Cl:

Cations can only form in combination with anions

Sodium donates one electron to chlorine so that each can have an inert gas configuration. This results in a compound, NaCl, consisting of Na+ and Cl- ions.

Na 2 8 1 Na+ 2 8 Cl 2 8 7 Cl- 2 8 8

sodium ion

chloride ion

Some ions are made of one atom (monatomic)

Other ions are made of more than one atom, which stay combined together and act as one unit – polyatomic (poly = many)◦ This unit has an overall charge, which can be either

positive or negative

e.g. nitrate (NO3-), sulfate (SO4

2-), phosphate (PO4

3-)

Polyatomic ions