Chemistry atomic structure

What is an atom?

The smallest part of an element that can exist

What is an element?

A substance made of only one type of atom — about 100 different elements exist

What is a compound?

Two or more elements chemically combined in fixed proportions — can only be separated by chemical reactions

What is a mixture?

Two or more elements or compounds not chemically combined — chemical properties of each substance are unchanged

What are the methods of separating mixtures?

Filtration, crystallisation, simple distillation, fractional distillation and chromatography — no new substances are made

What was the original model of the atom before the electron was discovered?

Atoms were thought to be tiny solid spheres that could not be divided

What is the plum pudding model?

A ball of positive charge with negative electrons embedded throughout — like plums in a pudding

What evidence led to the nuclear model replacing the plum pudding model?

Rutherford's gold foil experiment — most alpha particles passed straight through but some were deflected and a few bounced back

What were the conclusions of the gold foil experiment?

Most of the atom is empty space — there is a small dense positively charged nucleus — most of the mass is concentrated in the nucleus

What did Bohr add to the nuclear model?

Electrons orbit the nucleus at specific distances called energy levels or shells — his calculations agreed with experimental observations

What did Chadwick discover?

The neutron — providing evidence that the nucleus contained neutral particles as well as protons

What is the relative charge and mass of a proton?

Charge +1 — relative mass 1

What is the relative charge and mass of a neutron?

Charge 0 — relative mass 1

What is the relative charge and mass of an electron?

Charge -1 — relative mass negligible (approximately 1/2000)

What is the atomic number?

The number of protons in an atom — all atoms of the same element have the same atomic number

What is the mass number?

The total number of protons and neutrons in an atom

How do you calculate the number of neutrons?

Mass number minus atomic number

Why do atoms have no overall charge?

The number of electrons equals the number of protons — positive and negative charges cancel out

What are isotopes?

Atoms of the same element with the same number of protons but different numbers of neutrons

What is relative atomic mass?

An average value that takes account of the abundance of the isotopes of the element

How do you calculate relative atomic mass from isotope abundances?

Multiply each isotope's mass by its percentage abundance — add together — divide by 100

What is the approximate radius of an atom?

0.1 nm (1 x 10⁻¹⁰ m)

How does the radius of the nucleus compare to the atom?

The nucleus is less than 1/10000 of the radius of the atom — about 1 x 10⁻¹⁴ m

What is the electronic structure of sodium (atomic number 11)?

2,8,1

What is the electronic structure of oxygen (atomic number 8)?

2,6

What is the electronic structure of calcium (atomic number 20)?

2,8,8,2

What is the maximum number of electrons in the first three shells?

First shell — 2. Second shell — 8. Third shell — 8

How is the periodic table arranged?

Elements arranged in order of increasing atomic number — elements with similar properties are in the same vertical column called a group

Why do elements in the same group have similar properties?

They have the same number of electrons in their outer shell

What does the period number tell you?

The number of electron shells an atom has

How did early scientists try to classify elements before subatomic particles were discovered?

They arranged elements in order of atomic weights — but this led to some elements being placed in inappropriate groups

What did Mendeleev do differently?

He left gaps for undiscovered elements and predicted their properties — he also changed the order of some elements where atomic weight gave the wrong group

Why was Mendeleev's table eventually modified to order by atomic number?

When protons were discovered atomic number was found to determine chemical properties — this resolved anomalies like argon and potassium

What are the Group 0 elements called and why are they unreactive?

Noble gases — they have a full outer shell of electrons so do not need to gain or lose electrons

What is the trend in boiling point going down Group 0?

Increases — stronger intermolecular forces as atoms get larger

What are Group 1 elements called?

Alkali metals — they have one electron in their outer shell

What is the trend in reactivity going down Group 1?

Increases — outer electron is further from nucleus and more shielded so less energy needed to remove it

What is the trend in melting and boiling point going down Group 1?

Decreases — weaker metallic bonding as atomic radius increases

What happens when Group 1 metals react with water?

Metal + water → metal hydroxide + hydrogen — reaction becomes more vigorous down the group

What are Group 7 elements called?

Halogens — they have seven electrons in their outer shell and exist as diatomic molecules

What is the trend in reactivity going down Group 7?

Decreases — outer shell is further from nucleus and more shielded so harder to attract an extra electron

What is the trend in melting and boiling point going down Group 7?

Increases — stronger intermolecular forces as molecules get larger

What are the colours and states of the halogens at room temperature?

Fluorine — pale yellow gas. Chlorine — green gas. Bromine — orange/brown liquid. Iodine — grey/black solid

Can a more reactive halogen displace a less reactive one?

Yes — a more reactive halogen displaces a less reactive halogen from a solution of its salt

What is the colour change when chlorine water is added to potassium bromide solution?

Colourless to orange/brown — chlorine displaces bromine

What is the difference between metals and non-metals?

Metals form positive ions — non-metals do not. Metals are found left and bottom of periodic table — non-metals top right