Atomic Structure. Objectives. Describe the experiments of Thomson and Rutherford explain how they contributed to our present understanding of atomic structure . Describe the mass and charge differences among electrons, protons and neutrons .
Surrounding the nucleus is a region occupied by negatively charged particles called electrons.
Atomic Number (Z) – represents the number of protons in the nucleus of each atom of that element.
Atoms of the same element have the same number of protons.
Atoms of different elements have different number of protons.
On the periodic table, the elements are placed in order of increasing atomic number.
At the top left of the table is hydrogen, H, which has an atomic number of 1.
All atoms of the element hydrogen have one proton in the nucleus.
Next is helium, which has two protons, and therefore an atomic number of two.
Because all atoms are neutral (no charge), we know the atomic number is equal to the number of electrons.
# of protons = # of electrons
Example: Silver (Ag) has an atomic number of 47. Therefore, silver has 47 protons and 47 electrons in each silver atom.
The simplest atoms are those of hydrogen. All hydrogen atoms contain one proton.
However, hydrogen atoms can contain different number of neutrons.
Three types of hydrogen atoms are known with a neutron count of 0, 1 or 2.
The most common type of hydrogen atom is called protium. It accounts for 99.985% of the hydrogen atoms found on earth.
The nucleus of protium consist of one proton and zero neutrons.
Another type of hydrogen atom is deuterium. This accounts for 0.015% of hydrogen atoms.
The nucleus of deuterium consist of one proton and one neutron.
The third type of hydrogen atom is tritium. It exists in very small amounts in nature.
The nucleus of tritium consist of one proton and two neutrons.
Isotopes – are atoms of the same element that have different masses.
Protium, deuterium and tritium are all isotopes of hydrogen.
The isotopes of an element all have the same number of protons and electrons but different number of neutrons.
Mass Number – represents the total number of protons and neutrons in the nucleus of an isotope.
Identifying an isotope requires knowing both the atomic number and mass number of the element.
Calculating the mass number:
Mass number = atomic number + number of neutrons
Atomic # = 1 (1 proton)
# of neutrons = 2
Mass number = 3
Isotopes of an element are usually identified by specifying their mass number.
There are two common methods:
The mass number is written with a hyphen after the name of the element.
hydrogen-3 represents tritium
2) The second method shows the elements symbol written with the atomic and mass numbers.
hydrogen-3 represents tritium is written as:
Examples of hydrogen and helium:
How many protons, electrons and neutrons are there in an atom of chlorine-37?
Problems 1-3, page 78