The Atom

the ‘building blocks’ of all substances atoms cannot be split chemically into anything smaller the smallest particles

atoms chemically bounded together

contain only one sort of atom, alone or grouped into molecules, but only one type of atom

contain more than one type of atom from one type of molecule

contain more than one type of molecule

the smallest particles and consists of A Nucleus containing Protons + and Neutrons no charge. Electrons - charge, surround the nucleus Electrons are arranged in energy levels or shells Number of Protons is the atomic number

the number of Protons ( contained in the Nucleus )

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions: if an atom loses one or more electrons, it becomes a positively charged ion if an atom gains one or more electrons, it becomes a negatively charged ion

Electrons are arranged in different shells around the nucleus. The innermost shell - or lowest energy level - is filled first. Each succeeding shell can only hold a certain number of electrons before it becomes full. The innermost shell can hold a maximum of two electrons, the second shell a maximum of eight, and so on.

the relative capacity of an atom to undergo a chemical reaction with another atom, . as atoms become larger and the outer electron is further from the nucleus. The force of attraction between the positively-charged nucleus and the negatively-charged outer electron becomes weaker, which is why the outer electron is more easily lost

the total number of protons and neutrons in an atom.

The full chemical symbol for an element shows its mass number at the top and atomic number at the bottom It tells us that a carbon atom has: Six protons (because its proton number, at the bottom, is 6) Six electrons (because the number of protons and electrons in an atom is the same) The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12.

An isotope has the same number of protons, but a different number of neutrons

molecular mass is the mass of one molecule of the substance compared to the mass of one atom of hydrogen

The elements having a completely filled outermost orbit (or shell) will be chemically inert (non-reactive). For example, the outermost shell in the case of helium (He), neon (Ne) and argon (Ar) are completely filled as shown below. As a result, helium, neon, and argon are chemically inactive (or inert). These elements do not form compounds with other elements. Because of this chemical inactivity, these gases are called as noble gases

The elements containing only one or seven electrons in their outermost shell show greater chemical reactivity, i.e., such elements react very fast with other elements. For examples, sodium and chlorine having the follows electronic configurations are highly reactive. Sodium 2, 8, 1 —- Here, the outermost shell has been one one electron: one more than the completely filled shell. Chlorine 2, 8, 7 —– Here, the outermost shell has seven electrons: one less than that required to fill the shell completely.