1.2 The atom

What is an atom?

Is the smallest particle of an element that can take part in a chemical reaction

What did John Dalton discover?

1. All matter is composed of tiny indivisible particles called atoms

2. Atoms of the same element are identical

3. Atoms of different elements are different

4. Cannot be created nor destroyed

5. Compounds are formed when atoms of different elements combine in simple fixed ratios

Limitations on Dalton’s atomic model

1. Atoms are not indivisible- sub atomic particles

2. Atoms of the same element are not always identical- isotopes (same element, diff masses)

3. Atoms of different elements can have the same mass- isobars

What did Ernest Rutherford propose?

1. Empty space in the atom

2. Space occupied by the nucleus is very small

3. Mass is concentrated in the nucleus

4. Electrons orbit the nucleus

Ernest Rutherford’s limitations

1. Didn’t explain electron arrangement

2. Didn’t explain why negatively charged electrons were not drawn into the positively charged nucleus

Niels Bohr’s proposals

1. Electrons occupy specific orbits

2. Electrons can jump from one energy level to another- absorption from lower to higher, emission from higher to lower

3. Each orbit has a fixed amount of energy

4. Energy is emitted or absorbed in fixed amounts called quanta(photons)

What are isotopes?

Atoms of the same element with the same atomic number but different mass numbers

What is relative atomic mass?

The weighted average mass of an element’s isotopes compared with carbon-12

Why is relative atomic mass usually a decimal?

It is an average of isotopic masses

Why is carbon-12 used as a standard?

It is stable and internationally accepted

Mass spectrometer

Instrument used to accurately determine the ram of substances and also obtain the relative abundance of the isotopes of a given element

Determining ram

What is electronic arrangement?

The arrangement of electrons in energy levels around the nucleus

What are electrons shells?

Energy levels occupied by electrons

What are valence electrons?

Electrons in the outermost energy levels

Why are valence electrons important?

They determine chemical properties