Na (sodium) and F (fluorine) can bond because they have different electronegativities. Electronegativity is a measure of an atom's ability to attract electrons to itself. Fluorine is one of the most electronegative elements on the periodic table, while sodium is one of the least electronegative. This creates an electrostatic attraction between the two atoms, allowing them to form a chemical bond. The bond between Na and F is called an ionic bond.
The ionic bond between Na and F is formed when the sodium atom loses one electron to form a Na+ cation, and the fluorine atom gains one electron to form a F- anion. The resulting molecule is NaF.
Be (beryllium) and Br (bromine) can bond because they also have different electronegativities. Be is less electronegative than Br, and this creates an electrostatic attraction between the two atoms, allowing them to form a chemical bond. The bond between Be and Br is called a covalent bond.
The covalent bond between Be and Br is formed when both atoms share electrons. The resulting molecule is BeBr2 (Beryllium dibromide).
In general, it's important to note that the type of bond formed between two atoms depends on their electronegativities, if they are similar they tend to form covalent bond and if they are different they tend to form ionic bonds.