Grade 10

Physical Science

Atoms

First ideas about atoms

• The ancient Greeks believed that everything was made from some combination of Earth, Air, Fire and Water. Chemists want more definite ideas and tried to find the basic materials that all things were made of.

Democritus

• 500BC – Democritus – Greek philosopher. He thought that everything was made up of tiny particles, which were too small to be broken into smaller parts, atomos, meaning “uncuttable”. Democritus’ ideas were not widely believed because he had no evidence to support this hypothesis.

Antoine Lavoisier

• The French chemist, Antoine Lavoisier recognised special chemicals called elements. Elements are very important because they are pure substances which cannot be split up into simpler substances by means of chemical reactions.

Each element is made of only one type of atom.

Law of Conservation of Mass

• In 1774, Antoine Lavoisier used a new chemical balance to weigh the chemicals he used in his experiments. He found that if the reactions take place in a sealed container, then the mass of the chemicals that were there before the reaction took place was exactly the same as the mass of the products of the reaction after it was finished. He called this discovery the Law of Conservation of Mass.

Question 1

• Antoine Lavoisier is famous for:

• A naming the atom

• B recognising elements

• C weighing atoms

Answer 1

Brecognising elements

Joseph Proust

• Joseph Proust, also a French scientist, had seen that no matter how you prepared a chemical compound, it always contains the same elements, joined in the same proportions by weight.

• If you heat copper (Cu) in oxygen (O2) the compound formed is copper oxide (CuO).

• If you heat copper carbonate (CuCO3), the new compound is also copper oxide.

Law of Constant Proportions

• Proust found that in both cases, the copper and the oxygen are always there in the ratio of 64:16 or 4:1. In 1799, Proust stated these findings in the Law of Constant Proportions.

• 2Cu + O2 2CuO

• CuCO3 CuO + CO2

John Dalton

• John Dalton published a book called A System of Chemical Philosophy in 1808. He suggested that the French chemists’ laws and other chemical behaviour could be explained by the idea of tiny, indestructible, solid particles, which he called atoms.

Dalton’s atomic theoryDalton’s atomic theory proposed that:•All matter consists of tiny particles called atoms, which cannot be broken up.•All the atoms of a particular element have the same mass and the same behaviour, but are different to the atoms of other elements.•When atoms combine, they do so in proportions of small whole numbers to form ‘compound atoms’ (what we call ‘molecules’). Atoms cannot be created or destroyed in a chemical reaction.

Question 2 Dalton's Atomic Theory states the following:

1. matter is made up of tiny, indestructible particles called atoms

2. the atoms of one element are all identical

3. that atoms of different elements have the same mass

A Statement 1 is true

B Statement 2 is true

C Statement 3 is true

D Statements 1, 2 and 3 are true

Answer 2

D Statements 1, 2 and 3 are true

Joseph Thomson

• Joseph Thomson (1856 – 1940), a British physicist, did many experiments to find out what happened when an electric field acted on gases. Thomson discovered that there were negatively charged particles present, which he called electrons.

Thomson’s plum pudding

model• Since atoms have no overall

charge, Thomson proposed that the atom was made up of uniform positive charge with the negative electrons stuck into positive matter, like currants in a currant bun. “Plum pudding theory.”

Models and facts

• A model like Thomson’s atomic model uses a simple idea to help other people to understand the way a scientist is thinking. A model should fit some of the known facts, but need not fit all of them. Thomson’s model is a good illustration of this.

Here are some atomic facts so far:

• All atoms contain electrons.

• All atoms contain positive charges.

• The mass of an atom is much more than the mass of the electrons.

• Different atoms have different sizes.

Question 3Which of the following is the best definition of a scientific model?

A any visual representation of scientific experimental findings

B a 3D object used to demonstrate key scientific theories and data

C a visual, verbal, or mathematical explanation of experimental data

D an explanation of any scientific idea, theory, or experimental results

Answer 3

C a visual, verbal, or mathematical explanation of experimental data

Question 4Thompson’s Plum Pudding model allowed scientists studying the atom to do which of the following:

A analyze and test Thompson’s findings.

B create an image that could be used in schools.

C increase interest and funding in atomic theory.

D disperse the information more quickly to the public.

Answer 4

A analyze and test Thompson’s findings.

Marie Curie

In France, Marie Curie discovered radioactivity. She showed that a new kind of radiation was given off by uranium, polonium and radium. These radiations could penetrate paper and caused their compounds to glow in the dark. Sadly, unknown to her, they also caused leukemia, from which she died.

Ernest Rutherford

Rutherford realised that what was happening with radioactivity was that the atoms were splitting and releasing energy in the form of three kinds of radiation:

alpha (α) particles

beta (β) particles

gamma (γ) rays

Alpha, beta and gamma

• Beta particles turned out to be electrons.

• Gamma rays were high energy x-rays.

• Alpha particles had a charge of +2 equal and opposite to the charge on two electrons. (helium atom)

Rutherford chose alpha particles to explore the inside of atoms.

As a matter of interest

In 1911, Rutherford started testing Thomson’s model of the atom. He placed very thin gold foil in the path of alpha particles. Most alpha particles deflected 1° to 4°, which would be expected with the plum pudding model.

Rutherford’s scattering experiment

The surprise!

Then, to the surprise of his two assistants, T. Marsden and Hans Geiger (who designed the Geiger counter for radioactive studies), some of the alpha particles were being deflected by much greater angles and one out of every 20 000 hit something and bounced back!

Cannonball & loo roll

This didn’t fit Thomson’s model. He said: ‘It was about as incredible as if you fired a 375 mm shell at a piece of toilet paper and it came back and hit you!

Rutherford had to come up with a new model.

Rutherford’s conclusion

Rutherford concluded that over 99,9% of the mass of an atom and all its positive charge was squeezed into a tiny nucleus at its centre. The rest of the atom’s volume was taken up by circling electrons which moved around the nucleus like planets circling around the Sun.

Rutherford’s model

Rutherford found that the nucleus was very small, one 10 000th the size of the atom itself. Atoms have diameters of 2 to 3 × 10−10 m and nuclei around 1 × 10−14 m. Imagine a marble at the centre of the Greenpoint stadium!

Question 5

Imagine a circle with a diameter of 2 km. If this represents an atom, which answer would best represent the nucleus? (Figures in brackets represent the diameters).

A A table tennis ball (2 cm)

B A tennis ball (7 cm)

C A volley ball (20 cm)

D A soccer ball (25 cm)