Chemistry AS Unit 1 Module 1 Atoms and Reactions

ATOMS ARE:

• Made up of Protons, Neutrons and Electrons

Electrons have -1 charge - move around the nucleus in orbitals

Nucleus made up of Protons and Neutrons

Mass and Charge of these Subatomic Particles are very small, so relative mass and relative charge are used instead.

Mass Number (A) - the total number of Protons and Neutrons in the nucleus

Atomic (Proton) Number (Z) - number of protons in the nucleus - identifies the element

ALL atoms of the same element have the same number of protons

For neutral atoms with no overall charge, the number of electrons is the same as the number of protons.

Number of neutrons - A minus Z

Negative ions have more electrons than protons

Br-   The negative charge means there's 1 more electron than there are protons. 

Br has 35 protons, so Br- must have 36 electrons.

The overall charge = 35 - 36 = -1

Positive Ions have fewer electrons than protons

Mg2+  The 2+ charge means there's 2 fewer electrons than there are protons

Mg has 12 protons so Mg2+ must have 10 electrons

The overall charge = 12 - 10 = 2+

Different mass numbers mean different numbers of neutrons

The atomic numbers are the same - Both isotopes have 17 protons and 17 electrons

Number and arrangement of electrons decides the chemical properties of an element - Isotopes have the same configuration of electrons so have the same chemical properties

Isotopes have slightly different physical properties - different densities, rates of diffusion etc.

Physical properties depend more on the mass.

Relative atomic mass, Ar: The weighted mean mass of an atom of an element compared to one twelfth of the mass of an atom of carbon-12: an average of the isotopes of the element, so not usually a whole number

Relative isotopic mass: The mass of an atom of an isotope compared with one twelfth of the mass of an atom of carbon-12: Always a whole number

Formula mass: The weighted mean mass of a formula unit compared to one twelfth of the mass of an atom of carbon-12: Used for compounds that are ionic or giant covalent - add up all the Ar of all the ions

Relative molecular mass, Mr: The weighted mean mass of a molecule compared to one twelfth of the mass of an atom of carbon-12: Add up all the Ar of all the atoms in the molecule