UNDERSTANDING THE PROPERTIES OF ELEMENTS Chapter 5

5.1 Evolution of the Atomic Model An atom is the smallest particle of an element that retains the identity of the element

John Dalton (1808)

Dalton’s Atomic Theory All matter is made up of small particles called atoms. Atoms cannot be created nor destroyed. All atoms of the same element are identical in mass and size. The atoms of one element are different is mass and size from the atoms of other elements. Compounds are created when atoms of different elements link together to definite proportions.

1. Dalton’s Model of the Atom Dalton’s atomic model – an atom is a solid sphere The atom contains no protons, no electrons and no neutrons Analogy – compared to a billiard ball

JJ Thomson (1897)

2. Thomson’s Discovery of Electrons When electric current was applied to cathode (negatively charged), a ray was emitted and moved toward the anode (positively charged) Different elements in the gas discharge tube emitted the same kind of ray http://www.youtube.com/watch?v=IdTxGJjA4Jw

Thomson concluded that: Thomson’s model – the atom is a positive sphere with embedded electrons Net charge of zero No protons and no neutrons Identified as the raisin-bun model or plum pudding model

Ernest Rutherford (1914)

3. Rutherford’s Discovery of Nucleus Rutherford’s model: He aimed a type of radiation called alpha particles (positive) at a thin sheet of gold foil He predicted based on Thomson’s raisin-bun model that the particles would pass straight through the gold foil However, a small number of alpha particles bounced almost straight back from the gold foil

Nuclear model An atom has a tiny, dense, positive core called a nucleus The nucleus is surrounded mostly by empty space and there are no neutrons http://www.youtube.com/watch?v=5pZj0u_XMbc

Niels Bohr (1921)

4. Bohr’s Description of Energy Levels Bohr supported Rutherford’s theory but explained it further. Proposed that electrons could only move within fixed regions or energy levels around the nucleus

Bohr --- Explained that an electron must absorb a specific amount of energy (quantum) in order to move from one energy level to a higher one Compared the idea to the steps on a ladder which can only be at certain distances from the ground. It is not possible to stand in between the rungs of a ladder.

Bohr concluded that: Electrons could only occupy certain energy levels Absorption of a specific amount of energy will cause the electron to move to a higher energy level The atom has a central positive nucleus and negative electron in energy levels around the nucleus.

Bohr’s Theory electrons move around the nucleus in orbits like the planets orbiting the sun http://www.youtube.com/watch?v=wCCz20JOXXk

Other Discoveries: Proton – positively charged particle found in the nucleus of an atom Neutron – particle without a charge found in the nucleus of an atom (by Chadwick) Both these particles provided the evidence needed by Rutherford to account for the mass of the nucleus

In summary --- Dalton – The atom is the smallest particle of matter which is indestructible. Thomson – The atom has negatively charged electrons embedded in a mass of positively charged material. Rutherford – The atom has a very small dense positively charged nucleus with electrons moving around it. Bohr-Rutherford – The electrons in an atom occupy energy levels around the nucleus

Homework Do pg 186 # 1, 2, 3, 5, 6