###### Atomic structure

By its mass Oxygen is the most abundant chemical element in the Earth’s biosphere, air, sea and land. In nature oxygen occurs as composed of three stable isotopes $$^{16}O$$, $$^{17}O$$$$^{18}O$$, with $$^{16}O$$ as the most abundant.

Define the term “isotopes”

Isotopes are variants of particular chemical elements which differ in neutron number.

###### Atomic structure

Complete the table to show the number of sub-atomic particles in isotopes of oxygen.

 Proton Neutron Electron $$^{16}O$$ $$^{17}O$$ $$^{18}O$$

The number of protons is called the atomic number or proton number. The number of protons = the number of electrons = serial number of oxygen in Periodic Table of Elements = 8. The number of neutrons = relative atomic mass of the isotope – the number of protons. So the number of neutrons for isotope $$^{16}O$$ is 8, for isotope $$^{17}O$$ is 9, for isotope $$^{18}O$$ is 10.

###### Atomic structure

Which of these sub-atomic particles have zero charge?

The answer is 2 (neutrons). These particles have the same mass as protons and zero charge.

###### Atomic structure

Which of the proposed diagrams shows the electronic configuration of the oxygen atom?

The answer is 4 ($$1s^22s^22p^4$$). The electrons tend to occupy the position approximating the minimum value of the energy, and are located around the core according to the Pauli principle. The greatest number of electrons which can accommodate each electronic layer is determined by the formula $$N=2n^2$$. The total number of electrons of oxygen is 8. Two electrons will be located in the first energy level, two on the s-orbital of the second energy level and the remaining four on the p-orbital of the same level.

###### Relative molecular mass

Which relative molecular mass, $$M_r$$, is not correct for the molecule given?

Calculate the molecular masses of all the compounds.

$$M_r(CaCO_3)=M_r(Ca)+M_r(C)+3*M_r(O)=40+12+3*16=100$$

$$M_r(Mg(OH)_2)=M_r(Mg)+2*M_r(O)+2*M_r(H)=24+2*16+2*1=58$$

$$M_r(CuSO_4*5H_2O)=M_r(CuSO_4)+5*M_r(H_2O)=$$$$=M_r(Cu)+M_r(S)+4*M_r(O)+5*(2*M_r(H)+M_r(O))=64+32+4*16+5*(2*1+16)=250$$

$$M_r(HClO_4)=M_r(H)+M_r(Cl)+4*M_r(O)=1+35.5+4*16=100.5$$

$$M_r(CuSO_4*5H_2O)$$ is not correct.

###### Electron orbitals

The maximum number of electrons that can be located at s-/p-/d-orbitals:

One orbital may be presented up to two electrons. At one sublevel is one s-orbital, three p-orbitals and five d-orbitals. Thus, the number of electrons will be 2s, 6p, and 10d.

###### Atomic structure

Position proposed by the ions in order of increasing radius: $$Cs^+, K^+, Li^+, Na^+, Rb^+$$

The correct answer to this question is: $$Li^+, Na^+, K^+, Rb^+, Cs^+$$

###### Periodic Table

An element has a melting point of 1555°C and a density of 12.0 $$g/cm^3$$. It can be used as a catalyst. In which position in the Periodic Table is the element found?

This element is palladium.

###### Inert gases

Argon is the cheapest among inert gases (its content in the atmosphere is about 1%); it is widely used in gas welding and cutting, and other applications. Why is argon used in gas welding and cutting?

Argon insulates metal from the air.

###### Periodic Table

Some properties of four elements W, X, Y and Z are listed.

A) W melts at 1410°C and forms an acidic oxide.

B) X has a high density and is easily drawn into wires.

C) Y acts as a catalyst and its oxide reacts with acids.

D) Z is a red-brown solid used to make alloys.

Which of the elements are transition metals?

Only A

###### Periodic Table

M is a red-brown metal. It has a melting point of 1085°C. Many of its compounds are blue.

This element is cooper.