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Revision - Atomic Structure - Test 2

Dalton’s model of the atom

John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls. Dalton's model included these ideas.

Atoms cannot be broken down into anything simpler.

The atoms of a given element are identical to each other.

The atoms of different elements are different from one another.

During chemical reactions atoms rearrange to make different substances.

The Dalton model has changed overtime because of the discovery of subatomic particles.

JJ Thomson’s model of the atom

In 1897 physicist J.J.Thomson discovered the electron and using a cathode-ray tube conducted an experiment which identified the electron as a negatively charged subatomic particle, hence proving that atoms are divisible.

Based on his investigations Thomson proposed a model of the atom known as the plum pudding model which depicted negative electrons spread throughout soft globules of positively charged material.

Diagram showing The Plum Pudding Model of the atom:

Rutherford’s model of the atom

In 1909 Ernest Rutherford presented his model of the atom based on the gold foil experiment.

Rutherford shot a beam of positively charged particles at a thin sheet of gold foil and, based on the plum pudding model, expected the particles to pass through the foil which most of them did.

However a few were deflected and bounced directly back, which led to him to postulate that most of an atom’s mass is concentrated in a region of space at the centre of the atom called the nucleus.

In Rutherford’s model the atom consists mainly of empty space with the nucleus at the centre and the electrons orbiting in paths around the nucleus.

This model was known as the planetary model of the atom.

Diagram showing Rutherford’s Planetary Model of the atom:

The Modern Periodic Table

The elements in the modern periodic table are arranged in order of increasing atomic number, when this is done:

Elements in a row or period are in order of increasing atomic number.

Elements with similar properties are in the same column or group.

Non-metals are on the right of the table (the other elements are metals).

Atomic Structure

The central nucleus contains protons and neutrons only which are packed close together in a small region of space.

Electrons move in the space outside the nucleus in orbits.

Diagram showing the atom and subatomic particles:

Subatomic particles

The table below indicates the relative charge and mass of the subatomic particles:

Particle

Relative charge

Relative mass

Proton

+1

1

Neutron

0

1

Electron

-1

1/1836

Atoms are neutral and although they contain charged subatomic particles atoms are uncharged.

Since the charge on a proton is +1 and on an electron is -1, the number of protons must be the same as the number of electrons for the charges to cancel each other out and produce a neutral atom.

Nucleus and Isotopes

Mass number = number of protons + number of neutrons

Atomic number = number of protons (same as electrons)

Number of neutrons = mass number - number of protons

The number above the element symbol on the periodic table is the mass number and the number below the element symbol is the atomic number.

Isotopes

Isotopes are different atoms of the same element that contain the same number of protons and a different number of neutrons.

Isotopes of the same element have the same number of electrons. So, their chemical properties are identical.

Relative atomic mass

Relative atomic mass (Ar) tells us the average mass of 1 atom compared to 1/12 of 1 atom of carbon-12

Carbon is given an Ar of 12 (as it has 6 protons and 6 neutrons).

Electronic configuration

Elements are made of only one type of atom.

Compounds are two or more elements chemically bound together to form compounds. E.g. carbon dioxide.

Electrons move around the nucleus in different energy levels. The energy levels are called electron shells.

Electronic configuration: the arrangement of electrons into shells for an atom. This can be figured out by following some rules:

Electrons always occupy shells that are closest to the nucleus first.

When a shell becomes full, electrons will fill the next shell.

Each electron shell can accommodate a fixed number of electrons:

First shell: 2 electrons.

Second shell: 8 electrons.

Third shell: 18 electrons (but only fill it with 8).

The number of notations in the electronic configuration will show the number of shells of electrons the atom has, showing the period.

The last notation shows the number of outer electrons the atom has, showing the group.

Periodic table

Group 1 is alkali metals.

Group 7 is halogens.

Group 0 (8) is noble gases.

On the periodic table there are 8 groups and 7 periods.

Groups are the columns.

Periods are the rows.

Most of the elements are metals. A few are non-metals.

Metals are on the left of the periodic table.

Non-metals are on the right of the periodic table.

They are separated by a zig-zag line.

Mendeleev’s periodic table

There were only 63 elements known in 1869.

Mendeleev organised these elements into a table with rows and columns.

He did 3 things:

Grouped together elements with similar properties.

Put elements in order of relative atomic mass.

Left gaps for unknown elements and predicted their properties.

Group 1 - Alkali metals

Ions have charge.

Atoms are neutral.

Protons and neutrons do not take part in reactions, only electrons do.

If losing electrons = positive charge.

If gaining electrons = negative charge.

Metals lose electrons.

Non-metals gain electrons.

Are all malleable and conduct electricity.

Low melting points.

Are easily oxidised and burn brightly in air.

All of the group 1 metals have 1 valence electron (1 electron in the outer shell).

Group 1 metals are stored in oil, so they don’t start oxidising.

The reaction of Group 1 metals with water:

Group 1 metal being added to water

Observations

Lithium

When put into water, it starts fizzing.

The water turned from green to blue.

Floats on water, metal disappears.

Moves around

Sodium

Turned into a ball.

Fizzing, water turns blue.

Floats on water.

Orange flame.

Fizzing even more vigorously.

Potassium

Easiest to cut.

Catches fire immediately, lilac flame.

Most reactive.

Floats on water, dissolves.

Fizzes the most vigorously.

The reactivity increases as you move from lithium to sodium to potassium. (The reactivity increases down the group.) This is because as the outer electron gets further from the nucleus, it becomes easier to remove.

Lithium + water

Bubbles fiercely on the surface.

Sodium + water

Melts into a ball and fizzes about the surface.

Potassium + water

Bursts into flames and flies about the surface.

Group 7 - Halogens

All halogens are diatomic molecules, with two atoms held together by a single covalent bond.

They are poor conductors of heat and electricity.

Toxic and corrosive.

As you go down the group, the melting points, boiling points and densities increase.

Chlorine is a green gas, bromine is a brown liquid, and iodine is a purple/black solid.

All halogens can be used as disinfectants and bleaches, as they can kill microorganisms and remove the colour from materials.

In general, the halogens become less reactive as you go down the group.

All of the group 7 atoms have 7 valence electrons (7 electrons in the outer shell).

The order of reactivity of halogens can also be worked out using displacement reactions. In a displacement reaction, a more reactive element takes the place of a less reactive element in an ionic compound. So, a more reactive halogen displaces a less reactive halogen from a halide compound.

Group 0 (8) - Noble gases

Colourless.

Have very low melting and boiling points, and are poor conductors of heat and electricity.

They are inert - they do not react easily with anything.

They all exist as single atoms as they do not form bonds easily with other atoms.

All noble gases have a complete outer shell of electrons.

Noble gases are unreactive because their atoms already have a stable electronic configuration with a complete outer shell.

All of the group 8 atoms have a full valence electron shell (8 electrons in the outer shell).

There are only very small amounts of each noble gas in our atmosphere.

Word equation:

Lithium + water → Lithium hydroxide + hydrogen.

Symbol equation:

Li + H2O → LiOH + H2

(s) (l) (aq) (g)

Word equation:

Sodium + water → Sodium hydroxide + hydrogen.

Symbol equation:

Na + H2O → NaOH + H2

Word equation:

Potassium + water → Potassium hydroxide + hydrogen.

Symbol equation:

K + H2O → KHO + H2