Structure of the Atom

Neutron

Step 1: There are three main sub-atomic particles in an atom - electrons, protons, and neutrons. Step 2: Electrons carry a negative charge (-1), protons carry a positive charge (+1). Step 3: Neutrons, discovered by James Chadwick in 1932, have no electric charge (neutral). Step 4: Neutrons are present in the nucleus along with protons and contribute to the atomic mass. Alpha particles are actually helium nuclei with +2 charge.

Protons only

Step 1: The atomic number (Z) is a fundamental property that defines each element. Step 2: It is specifically equal to the number of protons in the nucleus of an atom. Step 3: All atoms of the same element have the same number of protons, regardless of the number of neutrons or electrons. Step 4: For example, all carbon atoms have 6 protons (Z=6), all oxygen atoms have 8 protons (Z=8). The number of protons and neutrons together gives the mass number, not the atomic number.

2

Step 1: Valency is the combining capacity of an atom, determined by electrons in the outermost shell. Step 2: Magnesium has electronic configuration 2,8,2, so it has 2 electrons in its outermost shell. Step 3: Since it's easier to lose 2 electrons than gain 6 electrons to complete the octet, magnesium loses 2 electrons. Step 4: Therefore, the valency of magnesium is 2, meaning it can form 2 bonds. This is why magnesium forms MgO (not MgO₂) and MgCl₂ (not MgCl).

Different mass number

Step 1: Isotopes are atoms of the same element, so they have the same atomic number (same number of protons). Step 2: However, they have different numbers of neutrons in their nuclei. Step 3: Since mass number = protons + neutrons, isotopes have different mass numbers. Step 4: For example, carbon-12 and carbon-14 are isotopes - both have 6 protons but different neutrons (6 and 8 respectively). They have similar chemical properties but different physical properties due to mass difference.