The ancient Greeks argued that all matter was made up of just four 'elements': earth, air, fire and water - Leaving Cert Chemistry - Question 5 - 2017

The Russian chemist is Dmitri Mendeleev.

The atomic number is defined as the number of protons in the nucleus of an atom, which also determines the element's identity and its position in the periodic table.

The two isotopes $^{12}_6$X and $^{14}_6$X differ in the number of neutrons present in their nuclei. Specifically, $^{12}_6$X has 6 neutrons, while $^{14}_6$X has 8 neutrons.

These different forms of the same element are called isotopes.

Both $^{12}_6$X and $^{14}_6$X have 6 electrons, as the number of electrons in a neutral atom matches the atomic number.

Element X is carbon (C).

In a neon atom, the electron arrangement is 2 electrons in the first shell and 8 electrons in the second shell, represented as follows:

  1st shell:  ↑↓
  2nd shell: ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓

In a fluorine atom, the electron arrangement is 2 electrons in the first shell and 7 electrons in the second shell, represented as follows:

  1st shell:  ↑↓
  2nd shell: ↑↓ ↑↓ ↑↓ ↑

Fluorine is more reactive than neon because it has seven electrons in its outer shell, indicating that it is not filled to capacity. This makes fluorine eager to gain one more electron to achieve a full outer shell configuration, making it reactive. In contrast, neon has a full outer shell with eight electrons, making it stable and less reactive.