Atomic structure and the periodic table

What did people think atoms were before the discovery of the electron?

Tiny spheres that could not be divided

What was John Dalton's theory?

There are as many different kinds of atoms as there are chemical elements

What was J.J Thomson's model (1897)?

His discovery of electrons lead to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded randomly in it

What was Rutherford's experiment?

Firing positively charged alpha particles at a thin gold sheet

What was the results of Rutherford's experiment?

-Most of the alpha particles passed straight through without being deflected

-A small number were deflected

-An even smaller number were deflected back the way they came

What is the explanation for the results of Rutherford's experiment?

-Most of the alpha particles passed straight through the gold foil showing that the gaps between the nuclei are mostly empty space and that most of the mass of an atom is concentrated in a very small space in the middle of the atom called the nucleus

-The ones that were deflected experienced an electric force of repulsion showed that the nucleus was charged

-Since only a small number of alpha particles were deflected this showed that the nucleus was small

What was the conclusion of Rutherford alpha particle scattering experiment?

The mass of an atom was concentrated in the centre and that the nucleus was charged. This nuclear model replaced the plum pudding model. 1911

What was Niels Bohr's model of the atom (1913)?

Adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations

How was the proton discovered?

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge

What did James Chadwick discover?

-His experimental work provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea

-1932 James Chadwick discovered neutrons, and isotopes were identified

How were elements arranged before the discovery of protons, neutrons and electrons?

In order of atomic weight (relative atomic mass)

How did Dimitri Mendeleev arrange the elements in the periodic table (1868)?

-Left gaps for elements he thought had not been discovered yet

-Swapped the places of some elements so that elements with similar chemical properties were located under each other

How was Mendeleev proved to be correct?

-Elements with properties predicted by him were discovered and fit the gaps

-Knowledge of isotopes made it possible to explain why the order based on atomic weights was not correct

How are elements arranged in the periodic table today?

Elements in the periodic table are arranged in order of atomic (proton) number so elements with similar properties are in columns (groups)

Elements in the same group in the periodic table have the same number of electrons in their outer shell and it gives them similar chemical properties

What is the relative mass of a proton, neutron and electron?

Proton: 1

Neutron: 1

Electron: 1/1836 (negligible)

What is the relative charge of a proton, neutron and electron?

Proton: +1

Neutron: 0

Electron: -1

What is the radius of an atom?

0.1 nm (1×10‐¹⁰)

What is the radius of the nucleus of an atom?

Less than 1/10000 of that of the atom (about 1×10-¹⁴)

How are electrons arranged in atoms?

-Electrons move around the nucleus in shells

-The electron shell nearest the nucleus can hold a maximum of 2 electrons

-The second shell can hold a maximum of 8 electrons

How to find out how many protons, neutrons and electrons are in the nucleus?

-Each element in the periodic table is given two numbers. The top number is the mass number and the bottom is the atomic number

-Mass number: protons + neutrons

-Atomic number: protons (and electrons)

-Neutrons = mass number - atomic number

What does the group number and periodic number (row) say about an atom?

-Group number = number of electrons in the outer shell of the atom

-Period number (row) = number of electron shells in the atom

What are isotopes?

-Isotopes are atoms of the same elements, with the same number of protons and electrons but different numbers of neutrons

-Isotopes have identical chemical properties because they have identical electronic structures

What is electronic structures?

E.g electronic structure of sodium is

2,8,1 (number of electrons in each outer shell)

What is an element?

A substance made of one type of atom

What is relative atomic mass?

The average mass of all the isotopes of an element, taking into account the abundance and mass number of each isotope

What is the calculation for relative atomic mass?

Isotope abundance × isotope mass number / sum of abundance of all isotopes

What are the rows and columns in the periodic table called?

-Columns = groups

-Rows = periods

How do elements in the same group behave?

They behave in a similar way when they react with other substances. This is because elements in the same group have similar chemical properties because they have the same number of electrons in their outer shells

Why are the noble gases/group 0 unreactive?

They have a complete outer shell. They do not need to lose or gain any electrons because they already have a stable electronic structure

Why does the boiling point increase as you go down group 0?

-The reason why the boiling points increase is to do with the size of the atoms

-Correlation between how many electrons molecules have and the strength of attraction between any two molecules

-For noble gases, as you go down the group, the atoms get bigger (they have more electrons) and the strength of the attraction between the atoms increases. Therefore the boiling points increase

Why are group 1 called alkali metals?

They react vigorously with water forming an alkaline solution

Why do group 1 metals get more reactive as you go down the group?

As you go down group 1 the alkaline metals become more reactive. This is because the outer shell electron gets further away from the nucleus. Therefore the attraction from the nucleus is weaker and the atom loses its electron more easily

As you go down group 1 the atoms have more shells of electrons. The inner electron shells shield the outer electron from the electrostatic attraction of the nucleus and again make it easier for the outer electron to be lost

Why do Group 7 elements get less reactive as you go down the group?

Group 7 atoms need to gain 1 electron to complete their outer shell. It becomes more difficult to gain an electron because

As the atoms get bigger, the nucleus gets further away from the electron it’s trying to gain so the attraction for the electron is weaker

As the number of shells increase there is greater shielding (or repulsion) of the electron it’s trying to gain. Therefore, the attraction from the nucleus is weaker

Therefore the reactivity of the halogens decrease as you go down the group

How does the boiling point change as you go down group 7?

They get bigger as you go down group 7. Therefore the molecules have more electrons and this causes the intermolecular forces to become stronger, so the boiling points increase