Atoms and Isotopes

What are atoms?

• Very small, radius of 1 × 10^-10 metres

• Contain positively charged nucleus made up of protons & neutrons

  • Surrounded by negatively charged electrons

What electrical charge do atoms have?

• Proton - +1

• Electron - -1

• Equal number of proton & electrons so no overall charge

Why may the energy level of an electron change?

• When the atom emits or absorbs electromagnetic radiation:

• Absorbing electromagnetic radiation moves electrons to higher energy level, further from nucleus

• Electromagnetic radiation is emitted when an electron drops to lower energy level

How do atoms become positive and negative ions?

• Atom loses one outer electron- +ion

• Atom gains extra electron - -ion

What do all atoms of a particular element have?

The same number of protons

What is the atomic number?

The numbers of protons in an atom

What is the mass number?

The number of protons and neutrons in an atom

What is an isotope?

Atoms of the same element that has a different number of neutrons

What are two common carbon isotopes?

• Carbon-12- 6 protons & 6 neutrons

• Carbon-13- 6 protons & 8 neutrons

What is the Plum Pudding Model?

• J.J Thompson 1897

• Depicts the atom as a ball of positive charge with electrons embedded in it

What is the Rutherford, Geiger and Marsden Experiment?

• Bombarded thin gold foil with alpha particles: if plum pudding model correct alpha particles pass through

• Most passed through, but not all.

• Some were reflected back

  • Must’ve been repelled by same charge alpha particles carried

  • Repelling charge heavier than alpha particles

• Conclusion:

• Mass of atom concentrated in central nucleus, positively charged

• Electrons surrounded nucleus

What did Niels Bohr do?

Adapted nuclear model, suggested that electrons orbit nucleus at specific distances

What did later experiments lead to?

Idea that positive charge of nucleus can be divided into number of smaller particles, each with same amount of positive charge. Given name proton.

What did James Chadwick do in 1932?

• Carried out experiments

• Gave evidence that there was another particle within nucleus called ‘neutron,

• Led to further refinement of nuclear model for structure of the atom