Atomic Structure and the Periodic Table

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.

What is a compound?

Two or more different elements chemically combined in fixed proportions.

How are compounds formed?

By chemical reactions between elements.

Can a compound be separated by physical methods?

No - its elements can only be separated by chemical reactions.

What is a chemical symbol?

One or two letters representing an element - first letter is always capital.

What does a chemical formula show?

Elements present in a compound + number/ratio of their atoms.

What happens during a chemical reaction?

Atoms are rearranged to form one or more new substances.

What must be conserved in a chemical reaction?

Number of atoms of each element - atoms are not created or destroyed.

Why must chemical equations be balanced?

There must be the same number of each type of atom on both sides.

What do the large numbers before formulae in balanced equations represent?

Relative numbers of particles/moles reacting.

Can you change the small numbers within a chemical formula to balance an equation?

No - this would change the substance.

What is a mixture?

Two or more substances not chemically combined.

Can substances in a mixture be separated physically?

Yes - using differences in physical properties.

What is filtration used for?

Separating an insoluble solid from a liquid.

What is crystallisation used for?

Producing a soluble solid from a solution.

What is simple distillation used for?

Separating a solvent from a solution.

What is fractional distillation used for?

Separating a mixture of liquids with different boiling points.

What is chromatography used for?

Separating substances in a mixture because they move at different rates through a medium.

What was the earliest model of the atom?

Tiny solid spheres that could not be divided.

What discovery led to the plum pudding model?

Discovery of the electron.

What was the plum pudding model?

Sphere of positive charge with negative electrons embedded throughout it.

What experiment challenged the plum pudding model?

Alpha-particle scattering experiment.

What happened to most alpha particles in the scattering experiment?

Passed straight through the thin gold foil.

What did most alpha particles passing straight through show?

Most of the atom is empty space.

What happened to some alpha particles?

They were deflected.

What did alpha-particle deflection show?

Positive charge is concentrated in a small central nucleus.

What happened to a very small number of alpha particles?

They were deflected backwards.

What did the rare large deflections show?

The nucleus is very small, dense + contains most of the atom's mass.

What model replaced the plum pudding model?

Nuclear model - tiny positive nucleus surrounded by electrons.

How did Bohr improve the nuclear model?

Electrons orbit the nucleus at specific distances/energy levels.

What was discovered after the proton?

The neutron - explaining additional mass in the nucleus.

What particles are found in the nucleus?

Protons + neutrons.

Where are electrons found?

In energy levels/shells around the nucleus.

What is the relative charge of a proton?

+1.

What is the relative charge of a neutron?

0.

What is the relative charge of an electron?

-1.

What is the relative mass of a proton?

1.

What is the relative mass of a neutron?

1.

What is the relative mass of an electron?

Very small - about 1/2000.

Why is an atom electrically neutral?

Number of protons = number of electrons - charges cancel.

What is atomic number?

Number of protons in the nucleus.

What does atomic number determine?

The element - each element has a unique number of protons.

What is mass number?

Total number of protons + neutrons.

How do you calculate number of neutrons?

Neutrons = mass number - atomic number.

What is an isotope?

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

Why are isotopes of an element chemically similar?

They have the same electronic structure.

What is relative atomic mass, Ar?

Weighted mean mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom.

Why is relative atomic mass often not a whole number?

It is a weighted mean of the masses of the element's isotopes.

How do you calculate relative atomic mass from isotope abundances?

Ar = sum of (isotope mass x percentage abundance) / 100.

How do you calculate Ar if isotope abundances are given as ratios?

Ar = sum of (isotope mass x abundance) / total abundance.

What is the electronic structure of an atom?

Arrangement of its electrons in energy levels/shells.

Which energy levels do electrons occupy first?

Lowest available energy levels - innermost shells first.

How many electrons can the first shell contain?

2.

How many electrons can the second shell contain for the first 20 elements?

8.

How many electrons can the third shell contain for the first 20 elements?

8.

What is the electronic structure of sodium, atomic number 11?

2,8,1.

What is the electronic structure of chlorine, atomic number 17?

2,8,7.

What is the electronic structure of calcium, atomic number 20?

2,8,8,2.

How can you find the total number of electrons from an electronic structure?

Add the electrons in all shells - equals atomic number in a neutral atom.

How are elements arranged in the modern periodic table?

In order of increasing atomic number.

What is a group in the periodic table?

Vertical column.

What is a period in the periodic table?

Horizontal row.

What does the group number tell you for Groups 1-7?

Number of electrons in the outer shell.

What does the period number tell you?

Number of occupied electron shells.

Why do elements in the same group have similar chemical properties?

They have the same number of outer-shell electrons.

Why is the table called periodic?

Similar properties occur at regular intervals.

How can an element's electronic structure be predicted from its periodic-table position?

Period gives occupied shells - group gives outer-shell electrons for Groups 1-7.

How were elements originally arranged before atomic structure was understood?

Mainly in order of atomic weight.

What were problems with early periodic tables?

They were incomplete + some elements were placed in inappropriate groups if strict atomic-weight order was followed.

How did Mendeleev improve the periodic table?

Left gaps for undiscovered elements + changed the order when needed to keep elements with similar properties together.

Why did Mendeleev leave gaps?

He predicted that undiscovered elements should exist there.

What evidence supported Mendeleev's periodic table?

New elements were discovered with properties similar to those he predicted.

Why is atomic number better than atomic weight for arranging the periodic table?

Atomic number gives the number of protons and correctly determines the element's position.

How did discovery of isotopes explain problems with ordering by atomic weight?

Isotopes change average atomic masses - so atomic-weight order does not always match atomic-number order.

Where are metals mainly found in the periodic table?

Left + towards the bottom.

Where are non-metals mainly found?

Right + towards the top.

What ions do metals form?

Positive ions.

Why do metals form positive ions?

They lose outer-shell electrons.

What are Group 0 elements called?

Noble gases.

Why are Group 0 elements very unreactive?

They have stable full outer electron shells.

How many outer-shell electrons do Group 0 elements have?

8 - except helium, which has 2.

Do noble gases usually form molecules?

No - they exist as individual atoms because they are very stable.

What happens to noble-gas boiling points down Group 0?

Boiling points increase.

Why do Group 0 boiling points increase down the group?

Atoms become larger/heavier - intermolecular forces become stronger - more energy needed to overcome them.

What are Group 1 elements called?

Alkali metals.

How many outer-shell electrons do Group 1 atoms have?

1.

What ions do Group 1 metals form?

+1 ions - they lose their one outer-shell electron.

What happens to Group 1 reactivity down the group?

Reactivity increases.

Why does Group 1 reactivity increase down the group?

Outer electron is further from nucleus + more shielded - weaker attraction - electron is lost more easily.

How do Group 1 metals react with water?

Metal + water - metal hydroxide + hydrogen.

What are the observations when Group 1 metals react with water?

Fizzing as hydrogen forms - metal moves on surface and gradually disappears.

What happens to Group 1 reactions with water down the group?

They become more vigorous.

What is produced when a Group 1 metal reacts with oxygen?

A metal oxide.

What is produced when a Group 1 metal reacts with chlorine?

A metal chloride.

What type of compounds do Group 1 metals form with non-metals?

Ionic compounds.

What are Group 7 elements called?

Halogens.

Are halogens metals or non-metals?

Non-metals.

How many outer-shell electrons do Group 7 atoms have?

7.

What type of molecules do halogens form?

Diatomic molecules - pairs of atoms, e.g. Cl2, Br2 and I2.