Atomic Structure

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Self-study chapter What I learnt -   Atoms are made of 3 different particles – protons, neutrons and electrons. They are also called sub-atomic particles.  (taken from: agssciencetransition.wikispaces.com) Protons: ✓Carries one positive electric charge ✓ Relative mass of 1 ✓ Represented by symbol, p Neutrons: ✓ Carries no electric charge ✓ Relative mass of 1 ✓ Represented by symbol, n Electrons: ✓Carries one negative electric       charge ✓ Relative mass of 1/1840 (often negligible) ✓ Represented by symbol, e All atoms are electrically neutral as they contain an equal number of protons and electrons. The proton number is the number of protons in an atom. The proton number is also known as atomic number. It is represented by symbol, z. The proton number can also tell the number of electrons in an atom. The nucleon number is the total number of protons and neutrons in an atom. The nucleon number is also called mass number. For example: 24 Mg 12 Mg – chemical symbol for Magnesium   24 – Nucleon number = mass number = number of protons and neutrons 12 – Proton number = atomic number = number of protons/electrons Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Therefore, isotopes have the same proton number but different nucleon number. For example: 17 Cl        35      17 Cl 37   Some elements do not have isotopes (eg. fluorine) Isotopes have the same chemical properties but slightly different physical properties.  Radioisotopes are isotopes that emit high-energy radiation. They are radioactive. The radiation emitted by radioisotopes is dangerous as it can damage living cells and can cause cancer. The electron arrangement determines the atom’s chemical properties. Electrons move around the nucleus of an atom in electron shells. The way electrons are arranged in an atom is called electronic structure or electronic configuration.  Valence shell is the shell that is the furthest from the nucleus. The chemical properties of an element depend on the number of valence electrons. Question(s) after reading: How and who found that an atom was the smallest particle of an element that posses the chemical properties of the element?

1) History of the model of an atom: People involved: Thales of Miletus (600 B.C.), Greek philosopher, Democritus (460 B.C), Aristotle, John Dalton (1800s), Ernest Rutherford (1911) Roles: Democritus – named the smallest particle of an element as an atom. Aristotle – disagreed with Democritus and dismissed the idea of atoms. (there was a contradiction of opinions) John Dalton – Performed experiments to prove that atoms did exist. Ernest Rutherford – Conducted experiments to find out more about the structure of atoms. They managed to find out about the arrangement of electrons in atoms. ^ Rutherford’s experiment (assisted by Geiger and Marsden) (taken from: http://www.daviddarling.info/ encyclopedia/R/Rutherfords_experiment_ and_atomic_model.html) 2) The first electron shell must be filled up first. It can hold a maximum of 2 electrons. The second and subsequent shells can hold a maximum of 8 electrons.